The Skin Prep Study: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for The Skin Prep Study (NCT01472549), a randomized na study. The primary outcome is Number of Participants With Surgical Site Infection.

2. Clinical Question

The study addressed a comparative-effectiveness question: whether the choice between two antiseptic skin preparations was associated with different observed counts of surgical-site infection after cesarean delivery. The endpoint definition used National Healthcare Safety Network definitions of the Centers for Disease Control and Prevention. The registry record describes the trial as randomized and parallel, and the posted results include 572 participants in the chlorhexidine-alcohol group and 575 in the iodine-alcohol group.

This question is specific to skin antisepsis at cesarean delivery. The registry summary describes the rationale as a need for evidence to guide the choice of antiseptic for this procedure. It also notes that evidence from other types of surgery may not necessarily answer the same question for cesarean delivery. The study compares the two named preparations; it does not, in the posted results, compare either preparation with no skin antisepsis.

The primary comparison concerns an event count over a stated follow-up window, not a measure of infection severity or duration. The registry record does not report a separate primary question about costs, length of stay, or patient experience. Those topics appear among secondary outcomes or background rationale, but the primary outcome is the count of participants with surgical-site infection within 30 days.

3. Trial Design

The Skin Prep Study is recorded as a randomized, parallel-group trial with no masking. Its official title describes a randomized comparative-effectiveness trial of antiseptic skin preparation for preventing surgical-site infection at cesarean delivery. The registry status is completed. The recorded study start is 2011-09, and primary completion is 2015-06. The record lists an enrollment of 1147.

Participants were assigned to chlorhexidine-alcohol or iodine-alcohol preparation. The chlorhexidine-alcohol formulation is listed as 2% chlorhexidine gluconate with 70% alcohol; the iodine-alcohol formulation is 8.3% povidone-iodine with 72.5% alcohol. The reported outcome groups contain 572 participants assigned to chlorhexidine-alcohol and 575 assigned to iodine-alcohol. The registry identifies Washington University School of Medicine as sponsor.

The primary outcome was the number of participants with superficial or deep surgical-site infection within 30 days after cesarean delivery. The definition is based on National Healthcare Safety Network definitions of the Centers for Disease Control and Prevention. The record also lists secondary outcomes concerning wound-related readmissions or office visits, endometritis, skin irritation, allergic reaction, skin contamination after preparation, cost savings, and length of hospital stay.

Random allocation supports a between-group comparison, while the parallel structure means participants received one assigned preparation rather than being compared with themselves across treatments. The trial was unmasked, so the record does not describe masking of participants or study personnel. Whether outcome assessors were masked is not reported in the registry record. The original design assumptions, including the planned sample-size justification and any prespecified interim analyses, are not reported in the registry record.

4. Participant Flow and Accounting

The registry reports how participants moved through each stage of the study. All counts below are exactly as posted; missing counts are not inferred from other stages.

Overall Study

5. Baseline Characteristics

The table reproduces the baseline characteristics posted in the registry, which describe the participants analysed at baseline and help show whether the randomized groups were comparable.

6. Primary Endpoint

The registry lists the primary outcome as Number of Participants With Surgical Site Infection, measured over 30 days.

The primary endpoint counts participants with a superficial or deep surgical-site infection within 30 days after cesarean delivery. The registry definition is based on National Healthcare Safety Network definitions of the Centers for Disease Control and Prevention. The outcome is expressed as a count of participants experiencing an event, rather than as a continuous measure such as time in hospital or a score of symptom severity.

A binary outcome of this kind makes it possible to compare the proportion of participants with an infection in each randomized group. The posted group results are 23 participants in the chlorhexidine-alcohol group and 42 in the iodine-alcohol group. A risk difference expresses the contrast as a difference between the groups' observed risks; a risk ratio expresses it as a relative comparison. These measures describe the same event using different scales, so their numerical values should not be read as interchangeable.

Using a defined follow-up window and an established infection definition helps make the event criterion explicit. It also limits the primary question to infection identified within the stated 30-day period; infections outside that timeframe are not part of this endpoint as described. The registry record does not report additional details about adjudication, surveillance procedures, or handling of participants without complete follow-up. The endpoint therefore describes the outcome as defined in the record, while those operational details remain unreported.

The endpoint is clinically relevant to the study's stated purpose, but its count alone does not describe the severity of each infection, the treatment required, or its consequences for an individual participant. Those are separate questions and should not be inferred from the primary event counts.

7. Results Posted in the Registry

• Iodine-alcohol: 7.3% (42/575) versus 4.0% (23/572) on control; risk difference 3.3 percentage points (95% CI 0.6 to 6.0), risk ratio 1.82 (1.11–2.98); unadjusted two-proportion z-test p = 0.016.

8. Secondary Efficacy Endpoints

The registry also posts results for secondary outcomes. For each outcome below, the endpoint type, the test applied to the posted summary statistics, the result and its interpretation are given in turn. Secondary endpoints are usually not adjusted for multiple testing, so they support rather than confirm the primary finding.

Number of Participants With Re-admissions or Office Visits for Wound-related Problems

• Endpoint type: Binary: the number of participants with the event, compared as proportions.
• Test: Unadjusted two-proportion z-test, with a Wald 95% confidence interval for the risk difference.
• Result:
  • Iodine-alcohol: 4.3% (25/575) versus 3.3% (19/572) on control; risk difference 1.0 percentage points (95% CI -1.2 to 3.2), risk ratio 1.31 (0.73–2.35); unadjusted two-proportion z-test p = 0.366.
• Interpretation:
  • The 95% confidence interval includes no difference, so these data do not show a statistically significant difference between Iodine-alcohol and the control. The interval gives the range of effects that remain compatible with the data.

Number of Participants With Endometritis

• Endpoint type: Binary: the number of participants with the event, compared as proportions.
• Test: Unadjusted two-proportion z-test, with a Wald 95% confidence interval for the risk difference.
• Result:
  • Iodine-alcohol: 1.9% (11/575) versus 1.4% (8/572) on control; risk difference 0.5 percentage points (95% CI -1.0 to 2.0), risk ratio 1.37 (0.55–3.38); unadjusted two-proportion z-test p = 0.495.
• Interpretation:
  • The 95% confidence interval includes no difference, so these data do not show a statistically significant difference between Iodine-alcohol and the control. The interval gives the range of effects that remain compatible with the data.

Number of Participants With Skin Irritation

• Endpoint type: Binary: the number of participants with the event, compared as proportions.
• Test: Unadjusted two-proportion z-test, with a Wald 95% confidence interval for the risk difference.
• Result:
  • Iodine-alcohol: 0.5% (3/575) versus 0.0% (0/572) on control; risk difference 0.5 percentage points (95% CI -0.1 to 1.1); unadjusted two-proportion z-test p = 0.084.
• Interpretation:
  • The 95% confidence interval includes no difference, so these data do not show a statistically significant difference between Iodine-alcohol and the control. The interval gives the range of effects that remain compatible with the data.

The secondary outcomes with numerical results cover wound-related readmissions or office visits, endometritis, and skin irritation. Their overall pattern is less conclusive than the primary result: the reported uncertainty for these comparisons includes values compatible with no difference, and the event counts are limited. This does not establish that the groups were equivalent for these outcomes. It means the posted comparisons do not estimate their differences with the same clarity as the primary comparison.

These outcomes address related but distinct aspects of postoperative care and safety. Wound-related visits, endometritis, and skin irritation are not interchangeable with the primary surgical-site infection definition. They can help contextualize the primary result, but they do not by themselves confirm or refute it. The registry lists additional outcomes, including length of hospital stay, allergic reaction, skin contamination after preparation, and cost savings. Numerical results for those outcomes are not reported in the registry record. The record lists 7 secondary outcomes, while the detailed numerical secondary results available in the posted data cover three.

Secondary findings carry less weight when many outcomes are examined without a reported adjustment for multiplicity. In general, repeated testing increases the opportunity for chance findings, so a nominal p-value from a secondary comparison is less persuasive as confirmatory evidence unless the testing plan and multiplicity control are clear. The registry record does not report a multiplicity adjustment or a hierarchy for interpreting secondary endpoints. These results are therefore best treated as supporting context and potential leads for further study, rather than as independent proof of a treatment effect or absence of one.

The secondary pattern should be read alongside the primary result, not substituted for it. Different endpoint definitions, event frequencies, and statistical precision can lead to different degrees of evidence across outcomes. The record does not report enough information about missing observations or outcome ascertainment to assess whether those factors influenced the secondary comparisons.

Secondary outcomes registered for this trial, for reference (only those above were recalculated):

• Length of Hospital Stay
• Number of Participants With Re-admissions or Office Visits for Wound-related Problems
• Number of Participants With Endometritis
• Number of Participants With Skin Irritation
• Number of Participants With Allergic Reaction
• Number of Participants With Skin Contamination After Skin Prep
• Cost Savings

9. Statistical Interpretation

10. Statistical Methods Explained

For the primary endpoint, each participant is classified according to whether a qualifying surgical-site infection was recorded within 30 days. This is a binary outcome: a participant either meets the event definition or does not. Group counts can be expressed as risks, and treatment groups can then be compared using absolute or relative measures. The posted results report the event counts and group sizes, along with both the registry's analysis and an unadjusted re-calculation.

What does a risk difference describe?

A risk difference subtracts one group's risk from the other group's risk. It is an absolute-scale contrast: it describes how far apart the observed risks are, rather than how many times one risk is relative to the other. The re-calculated risk difference is 0.033 for iodine-alcohol relative to chlorhexidine-alcohol, with a 95% confidence interval from 0.006 to 0.059. The comparison is stated in that direction, so a positive estimate corresponds to a higher observed risk in the iodine-alcohol group. A risk-difference interval that includes zero is compatible with no absolute difference at the confidence level used; an interval entirely on one side of zero indicates that the data are less compatible with zero under that interval procedure.

How does a risk ratio differ?

A risk ratio divides one group's risk by the other's. A value of 1 represents equal risks on the relative scale; values above or below 1 indicate which group's observed risk is larger, depending on the numerator and denominator. The registry's primary analysis reports a risk ratio of 0.55 for its stated group order, with a 95% confidence interval from 0.34 to 0.90. The re-calculation reports the reverse comparison, iodine-alcohol relative to chlorhexidine-alcohol, with a risk ratio of 1.817 and confidence limits of 1.107 and 2.98. Since the direction of comparison is reversed, these estimates should not be compared as though they use the same numerator and denominator.

What does the confidence interval add?

A confidence interval is a range of estimates compatible with the data under a specified statistical procedure. It gives information about precision as well as direction. A narrow interval indicates more precision than a wide interval on the same scale; an interval that spans the no-difference value leaves more uncertainty about whether the comparison could be null. A 95% confidence interval is not a statement that there is a 95% probability that the fixed treatment effect lies in that particular interval. It describes the long-run coverage property of the method when its assumptions hold.

What does the p-value represent?

The registry reports a chi-squared analysis for the primary risk ratio, with p = 0.02. The unadjusted re-calculation reports p = 0.016. A p-value measures how incompatible the observed data, or more extreme data, are with a null hypothesis under a specified model and its assumptions. Smaller p-values indicate greater incompatibility with that null under those assumptions. They do not measure effect size, clinical value, or the probability that the result arose by chance in an everyday sense. The registry's own analysis is the reported trial analysis; the re-calculation is a simple, unadjusted comparison from the posted summary statistics.

Why distinguish the reported analysis from a re-calculation?

The registry reports its primary analysis as a chi-squared comparison of risk ratios. The re-calculation uses the group summaries to produce unadjusted contrasts. Such a calculation is useful for checking the direction and approximate scale of the comparison, but it cannot reproduce unreported adjustments or analytic details. The record does not report covariate adjustment, the precise confidence-interval construction, or the analysis population. Where the registry's own result differs from a simple re-calculation, the sponsor's own analysis takes precedence. The discrepancy indicator is false for this record, and the two approaches give a consistent directional reading.

Why do secondary comparisons require caution?

Several outcomes are listed beyond the primary endpoint. The record includes numerical results for wound-related readmissions or office visits, endometritis, and skin irritation. Their reported methods include chi-squared analyses for the first two and Fisher's exact test for skin irritation. Fisher's exact test is a method for comparing categorical event data that can be useful when counts are sparse. Each outcome has its own effect estimate and uncertainty. Testing multiple outcomes increases the chance that at least one result may appear noteworthy even when there is no corresponding underlying effect. No multiplicity adjustment is reported, so secondary findings should be read as less definitive than the designated primary comparison.

General statistical context: randomization helps make treatment groups comparable on average, but it does not remove sampling uncertainty. A trial result remains an estimate from the enrolled participants, and the confidence intervals convey part of that uncertainty. A statistical test cannot address unreported design and data-handling choices, such as how missing outcomes were treated. Those details are not reported in the registry record.

11. Confidence Intervals and Precision

The primary estimate is accompanied by a 95% confidence interval, which communicates the range of values compatible with the data under the statistical method used. The registry's reported risk ratio is 0.55, with limits from 0.34 to 0.90. The interval is on one side of the relative no-difference value of 1. The unadjusted comparison in the reverse direction gives a risk ratio of 1.817, with limits from 1.107 to 2.98, and a risk difference of 0.033, with limits from 0.006 to 0.059. The absolute-scale interval is on one side of zero, its no-difference value.

These intervals indicate uncertainty about the effect's magnitude even though their limits exclude the corresponding no-difference value. They are not guarantees that the true effect falls within the displayed limits, and they do not account for every possible source of uncertainty, such as unreported missing-data procedures or deviations from the analysis plan. The registry record does not report the method used to construct the sponsor's confidence interval.

For secondary outcomes, the confidence intervals are less decisive. The intervals for wound-related readmissions or office visits and endometritis include the no-difference values on their respective scales. The skin-irritation risk-difference interval also includes zero. The event counts for those outcomes are comparatively sparse in the posted results, so their estimates are correspondingly imprecise. Confidence intervals should be considered with the outcome counts and the analysis method, rather than interpreted only by whether they cross a conventional threshold.

Precision is distinct from importance. An interval can be statistically compatible with a difference without establishing whether its size matters in practice; conversely, a wide interval may leave a range of potentially relevant effects unresolved. The study's posted estimates describe the observed comparison in this trial, while broader applicability depends on how closely another setting resembles the study population and procedures.

12. Planning a Similar Trial: Sample Size

This is a planning illustration, not a re-evaluation of the original trial. It asks: if a new study wanted to detect a difference like the one observed, or a smaller one, how large would it need to be? It is deliberately not “observed power” of the finished trial, which is not a meaningful quantity.

The sample-size table and curve are planning illustrations based on the observed primary effect, not the original trial's design calculation. The stated assumptions are a two-sided significance level of 0.05, equal allocation between the two arms, a normal approximation, and an effect to detect set to the stated fraction of the observed effect. The trial's own original sample-size assumptions and justification are not reported in the registry record.

For this binary outcome, the figures are participants per arm, not total events. The 80% power column represents the number of participants per arm estimated to give an 80% chance of detecting the specified effect under the stated assumptions, if that effect is the true one. The 90% column represents a higher chance of detecting that same specified effect, and therefore requires a larger planned group size. These are design-planning quantities, not descriptions of what happened in the trial.

The rows for 100% of the observed effect, 75% of the observed effect, and 50% of the observed effect ask what group size would be needed if a future study aimed to detect the full observed difference or a smaller fraction of it. Smaller effects are harder to distinguish from random variation, so detecting them with the same power generally requires more participants. The displayed values rise as the target effect is reduced. For example, the planning figures for 100% of the observed effect are 777 participants per arm for 80% power and 1,040 participants per arm for 90% power; for 50% of the observed effect, they are 2,683 and 3,591, respectively.

Because the observed effect is itself uncertain, using it as the planning target can produce optimistic or unstable illustrations. The calculations do not establish that the original trial was adequately powered, and they are not observed power. They show how sample-size requirements change under the listed assumptions and effect targets; they do not predict the result of another study.

13. How the Sample Size Is Derived

Power is the probability that a study of a given size rejects the null hypothesis when a specific true effect exists. It depends on four things: the effect to detect, the variability of the outcome, the significance level (here a two-sided α of 0.05, so z_1−α/2 = 1.960) and the desired power (z_1−β = 0.842 for 80% and 1.282 for 90%).

Formula

n is the number of participants in each arm, p₀ and p₁ are the control and treatment proportions, and p̄ is their average.

Worked calculation for this trial

1. Control proportion p₀ = 0.040; treatment proportion p₁ = 0.073; average p̄ = 0.057.
2. First term: z_1−α/2 × √[2p̄(1−p̄)] = 1.96 × 0.3269 = 0.6406.
3. Second term: z_1−β × √[p₀(1−p₀) + p₁(1−p₁)] = 0.842 × 0.3260 = 0.2744.
4. n = (0.6406 + 0.2744)² / (0.0328)² = 776.7, rounded up to 777 participants per arm for 80% power.

The smaller-effect rows in the table above repeat the same calculation with the effect multiplied by 0.75 and 0.5 (for ratios, applied to the logarithm of the ratio).

R code

alpha <- 0.05
z_a   <- qnorm(1 - alpha / 2)          # two-sided
p0 <- 0.0402097902097902                          # control proportion
p1 <- 0.0730434782608696                          # treatment proportion
n_per_arm <- function(power, frac = 1) {
  q1 <- p0 + (p1 - p0) * frac               # smaller effect = fraction of the observed difference
  pbar <- (p0 + q1) / 2
  z_b <- qnorm(power)
  ceiling(round((z_a * sqrt(2 * pbar * (1 - pbar)) + z_b * sqrt(p0 * (1 - p0) + q1 * (1 - q1)))^2 / (q1 - p0)^2, 6))
}
for (f in c(1, 0.75, 0.5))
  cat(sprintf("%3.0f%% of observed effect: n per arm = %s (80%% power), %s (90%% power)\n", 100 * f,
              format(n_per_arm(0.80, f), big.mark = ","), format(n_per_arm(0.90, f), big.mark = ",")))

# Check against R's built-in function (unrounded n per arm)
cat("power.prop.test:", round(power.prop.test(p1 = p0, p2 = p1, power = 0.80)$n, 1), "\n")

# Simulation: how often does the z-test reject at the 80%-power sample size?
set.seed(1472549)
n    <- n_per_arm(0.80)
reps <- 20000
x0 <- rbinom(reps, n, p0); x1 <- rbinom(reps, n, p1)
ph <- (x0 + x1) / (2 * n)
z  <- ((x1 - x0) / n) / sqrt(ph * (1 - ph) * 2 / n)
cat(sprintf("Simulated power at n = %s per arm: %.3f (%d replicates)\n", format(n, big.mark = ","), mean(abs(z) > z_a, na.rm = TRUE), reps))

Output from running the code

100% of observed effect: n per arm = 777 (80% power), 1,040 (90% power)
 75% of observed effect: n per arm = 1,288 (80% power), 1,723 (90% power)
 50% of observed effect: n per arm = 2,683 (80% power), 3,591 (90% power)
power.prop.test: 776.7 
Simulated power at n = 777 per arm: 0.803 (20000 replicates)

The simulation draws many pairs of event counts at the planned sample size and counts how often the two-proportion z-test rejects the null. It should land close to the target power; at very small required sample sizes it can fall below it, because the normal approximation is rough there.

14. Analyses Reported by the Sponsor

The registry reports the sponsor's primary analysis as a chi-squared analysis of the risk ratio: 0.55, with a 95% confidence interval from 0.34 to 0.90 and p = 0.02. This is the trial's own reported analysis and should be the main reference for interpreting the sponsor's result.

The simple unadjusted re-calculation uses the posted group summaries and frames the comparison as iodine-alcohol relative to chlorhexidine-alcohol. In that direction, the recalculated risk difference is 0.033, with a 95% confidence interval from 0.006 to 0.059, and p = 0.016. The re-calculated risk ratio is 1.817, with a 95% confidence interval from 1.107 to 2.98. These values point in the same direction as the registry analysis once the reversed comparison order is recognized: the reported primary event count is lower in the chlorhexidine-alcohol group.

The two analyses need not have identical numerical estimates because they are presented on different scales and in opposite comparison directions, and the re-calculation is unadjusted. The registry record does not report enough analytic detail to explain every methodological distinction. The discrepancy indicator is false, so no material disagreement is flagged. If a discrepancy were present, the sponsor's own analysis would take precedence; in this record, the re-calculation is best read as a transparent summary-statistic check rather than a replacement for the reported analysis.

15. Multiplicity, Missing Data and Analysis Populations

The registry identifies a primary outcome and lists multiple secondary outcomes. This distinction matters because the primary outcome is the principal basis for the trial's main statistical interpretation, while secondary outcomes can provide additional context. When many outcomes are tested, the chance of at least one apparently notable result can increase. Statistical procedures for controlling that issue, or a prespecified order for interpreting outcomes, are not reported in the registry record. The secondary p-values should therefore not be treated as though each were a separately confirmed primary finding.

The registry record does not report whether any interim analyses were planned or conducted, whether stopping boundaries were used, or how any such analyses would have affected the final inference. It also does not report the original statistical analysis plan, the handling of protocol deviations, or whether the primary analysis was adjusted for baseline factors. Those details should not be inferred from the posted summary statistics.

Missing-data handling is another important part of interpreting a clinical trial. The record gives group sizes and event counts for the posted outcomes, but does not state how participants with unavailable outcome information were handled, whether follow-up was complete for everyone, or how missing outcomes were classified. It does not report an intention-to-treat or per-protocol analysis population. Consequently, the posted counts can be described as recorded, but the registry summary alone does not establish the analysis population or the sensitivity of the results to missing data.

The study is described as unmasked. The record does not report whether outcome assessors were masked or how infection events were identified and adjudicated. Since the primary endpoint depends on classifying infections within a defined period, those process details can be relevant to risk-of-bias assessment. Their absence from the registry record is a reporting limitation, not evidence that a particular procedure was or was not used.

These omissions do not invalidate the recorded results, but they limit how fully the statistical analysis can be audited. The reported sponsor analysis remains the primary registry result; the simpler re-calculation cannot fill in unreported design, follow-up, or analysis-plan details.

16. Safety Results

The registry reports serious adverse events as affected participants divided by participants at risk.

Deaths

Other (non-serious) adverse events

Participants with at least one non-serious event at or above the registry's 0% reporting threshold.

Most frequent non-serious adverse event terms

The adverse-event record reports zero serious adverse events in both groups and zero deaths in both groups. It lists 15 other adverse events among 572 participants at risk in the chlorhexidine-alcohol group and 15 among 575 at risk in the iodine-alcohol group. These are the recorded counts in the registry summary; they do not by themselves describe the nature, timing, duration, or severity of the other adverse events.

A recorded count of zero serious events means none were listed in these posted results. It does not establish that an event is impossible, nor does it rule out uncommon risks that a trial of this size may not characterize precisely. The registry lists skin irritation and allergic reaction as secondary outcomes, but the numerical outcome details are not complete for all listed safety-related outcomes. Skin irritation results are posted separately, while numerical results for allergic reaction are not reported in the registry record.

Safety counts should be read in the context of the groups at risk, the event definitions, and how events were collected. The record does not report detailed collection methods, attribution to the preparation, or the severity breakdown for the other adverse events. These figures are descriptive and are not, on their own, a comprehensive comparison of safety.

17. What the Result Does, and Does Not, Mean

The posted primary data show 23 surgical-site infections in the chlorhexidine-alcohol group and 42 in the iodine-alcohol group, with follow-up defined as 30 days after cesarean delivery. The registry's reported risk ratio is 0.55, with a 95% confidence interval from 0.34 to 0.90 and p = 0.02. The unadjusted re-calculation, expressed as iodine-alcohol relative to chlorhexidine-alcohol, gives a risk difference of 0.033 and a 95% confidence interval from 0.006 to 0.059. These estimates describe the observed comparison in this randomized trial.

The result does not establish that the same contrast would occur in every setting, or that every participant assigned to one preparation would have a particular outcome. It is an estimate with uncertainty, based on a specific population, procedure, pair of antiseptic formulations, endpoint definition, and follow-up period. The record does not report detailed participant characteristics that would allow a fuller assessment of how closely other clinical settings match the trial population.

The finding also does not answer every question about these preparations. The primary endpoint does not measure infection severity, the resources required to manage an infection, or outcomes beyond the stated follow-up. The secondary results address some additional outcomes, but the registry record does not provide numerical results for every listed secondary measure. Nor does the record report enough information to assess all aspects of missing data, outcome ascertainment, or analysis choices.

Statistical evidence and clinical importance are related but different. The p-value and confidence interval describe compatibility with a statistical null and the precision of the estimate under the methods used. They do not determine whether a difference is important to patients or clinicians, and they are not a recommendation about which preparation to use. The study's result should be interpreted as evidence from this comparison, with the stated limitations, rather than as a universal conclusion.

18. Why This Trial Matters Statistically

This trial poses a direct comparative question in a defined surgical setting: whether two antiseptic skin preparations are associated with different counts of surgical-site infection after cesarean delivery. Randomization and a parallel-group design make a between-group comparison possible, while a clearly specified 30-day endpoint gives the primary analysis a defined event and timeframe.

The results also illustrate why effect measures need context. The sponsor reports a risk ratio, while the simple unadjusted re-calculation reports an absolute risk difference and a risk ratio in the opposite comparison direction. Each measure answers a slightly different question. Stating the group order is essential: reversing the numerator and denominator changes the direction of a relative estimate, while an absolute difference also depends on which risk is subtracted from which.

Confidence intervals add information that a p-value alone cannot provide. Here, the primary interval excludes the no-difference value on both the reported relative scale and the recalculated absolute scale, yet the interval still spans multiple plausible magnitudes. The secondary results offer a contrasting example: limited event counts and intervals that include no difference leave greater uncertainty. These features demonstrate why statistical significance, effect size, and precision should be considered together.

The record also highlights the distinction between a trial's own analysis and a simple calculation from summary data. The latter can make the direction and scale of the observed comparison transparent, but it cannot recover unreported analytic decisions. Finally, planning calculations based on an observed effect are illustrations rather than evidence of the trial's original design quality or observed power. Interpreting a clinical trial requires both the numerical estimates and careful attention to what the registry does not report.

19. Limitations

• The posted summary statistics are the only data these results rest on; patient-level data, full protocols and statistical analysis plans are not available.
• Recalculated results are simple, unadjusted calculations from posted summaries. They do not replace the sponsor's prespecified analysis.

The registry record is concise and leaves several details unavailable for assessing the result. It does not report participant characteristics, eligibility criteria, recruitment procedures, or detailed information about the clinical setting. Those omissions make it difficult to judge how representative the enrolled participants were or how broadly the comparison may apply.

The trial is recorded as unmasked, but the record does not say whether outcome assessors were masked. It does not describe how infections were detected, adjudicated, or followed through the 30-day period. The primary endpoint uses a stated definition, but operational details about surveillance and classification are not reported in the registry record.

The record also does not report the original sample-size justification, the prespecified statistical analysis plan, interim analyses, covariate adjustment, or a multiplicity strategy. It does not describe missing-data handling or identify the analysis population as intention-to-treat or per-protocol. These gaps restrict independent evaluation of analytic choices and the effect of incomplete follow-up.

Numerical results are not reported for every listed secondary outcome. In particular, the registry lists outcomes concerning length of hospital stay, allergic reaction, skin contamination after preparation, and cost savings, but the detailed numerical results for these are not reported in the registry record. The secondary findings that are posted involve different event definitions and sparse counts, and no multiplicity adjustment is reported.

These are limitations of what can be assessed from the registry record, not proof that unreported procedures were inadequate. They support a cautious interpretation centered on the reported primary comparison and its uncertainty.

20. Related Tutorials

21. Related Statistical Calculators

22. Sources

• ClinicalTrials.gov: The Skin Prep Study — NCT01472549.
• Registry record last updated 2018-08-08; status last verified 2018-07.
• Results first posted 2018-08-08.
• Data retrieved from ClinicalTrials.gov on 2026-11-09.
• Group sizes, means, event counts and sponsor-reported p-values are reproduced as posted. Effect estimates, confidence intervals and p-values not attributed to the sponsor are independently calculated from the posted summary statistics.

23. Record Summary

The Skin Prep Study (NCT01472549) is recorded as a completed, randomized, unmasked, parallel-group trial sponsored by Washington University School of Medicine. The registry lists enrollment of 1147, a start date of 2011-09, and primary completion in 2015-06. It compared chlorhexidine-alcohol with iodine-alcohol skin preparation for cesarean delivery.

The primary outcome was the number of participants with superficial or deep surgical-site infection within 30 days, based on National Healthcare Safety Network definitions of the Centers for Disease Control and Prevention. The posted counts were 23 in the chlorhexidine-alcohol group and 42 in the iodine-alcohol group. The sponsor-reported risk ratio was 0.55, with a 95% confidence interval from 0.34 to 0.90 and p = 0.02.

The record lists secondary outcomes and adverse-event counts, but numerical results are not available for every listed outcome. Details about the original design assumptions, missing-data handling, analysis populations, and multiplicity procedures are not reported in the registry record.