STRIPES Study: Study To Reduce Infection Post cEsarean Section: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for STRIPES Study: Study To Reduce Infection Post cEsarean Section (NCT02402907), a randomized na study. The primary outcome is Number of Participants With Incisional SSI.

2. Clinical Question

The STRIPES study asks whether using a 2% chlorhexidine gluconate (CHG) cloth before cesarean section is associated with fewer participants experiencing an incisional surgical site infection (SSI) by 6 weeks than using a placebo cloth. The registry describes scheduled primary or repeat cesarean sections at Mount Sinai Medical Center. The comparison is randomized and parallel, and the registry records triple masking.

The registry’s stated objective also refers to infectious morbidity, including surgical site infections and endometritis. The posted primary outcome is narrower: it counts participants with an incisional SSI. The result therefore addresses that specified outcome and follow-up time, not every possible infection or maternal outcome.

3. Trial Design

The registry describes a randomized, triple-masked, parallel-group trial. Participants were assigned to either a 2% CHG cloth or a placebo cloth. The placebo is described as a fragrance-free cleansing cloth. The study enrollment is 1356, and the registry status is completed. The sponsor is Icahn School of Medicine at Mount Sinai.

The study’s brief description says that eligible women presenting for a scheduled primary or repeat cesarean section at Mount Sinai Medical Center would use the assigned cloth the night before and the morning of the scheduled procedure. It also says participants would be blinded to their assigned arm. The registry records triple masking, but it does not identify which other groups were masked or provide further masking procedures.

The registry lists the study start as 2015-04 and primary completion as 2019-10-11. It identifies the interventions as devices and lists no phase beyond NA. The posted primary outcome is a count of participants with an incisional SSI at 6 weeks. Its group counts use 662 participants in the CHG cloth arm and 647 in the placebo cloth arm. Those outcome denominators are distinct from the enrollment figure; the registry record does not explain the difference.

The original protocol, the trial’s own sample-size justification, the detailed allocation procedure, and the prespecified statistical analysis plan are not reported in the registry record. The posted results can therefore be interpreted as recorded, but the exact design rationale and analysis procedures cannot be reconstructed from the registry information.

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.

Before assignment: 1356 were enrolled, 14 did not undergo procedure at Sinai, leaving 1342 participants (672 in experimental group and 670 in placebo group) that continued on the study.

Overall Study

Reasons for not completing this period:

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 Incisional SSI, measured over at 6 weeks.

The primary endpoint counts participants who experienced an incisional surgical site infection after cesarean section, with assessment at 6 weeks. It is a binary outcome at the participant level: each participant is counted as having or not having the specified event for this endpoint. The registry reports 17 participants with an incisional SSI in the CHG cloth arm and 24 in the placebo cloth arm.

An event count is useful because it makes the clinical outcome directly interpretable as the number of participants affected in each randomized group. Comparing those counts relative to their group sizes gives an estimate of the difference in event occurrence between the arms. The registry also posts a risk-difference estimate and an unadjusted p-value, so the comparison is not limited to looking at raw counts alone.

The endpoint is specific to incisional SSI. It is not a count of all infections, all infectious morbidity, or all complications. Although the study objective mentions endometritis as part of infectious morbidity, the primary endpoint described in the results is incisional SSI. The assessment time is also important: this endpoint concerns the result at 6 weeks, not events outside that stated period.

The registry does not describe how an SSI was diagnosed, how outcome assessors collected the information, or how participants with incomplete follow-up were handled. Those details are not reported in the registry record.

7. Results Posted in the Registry

• CHG Cloth: 2.6% (17/662) versus 3.7% (24/647) on control; risk difference -1.1 percentage points (95% CI -3.0 to 0.7), risk ratio 0.69 (0.38–1.28); unadjusted two-proportion z-test p = 0.236.

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 Maternal Readmissions

• 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:
  • CHG Cloth: 0.7% (5/672) versus 0.4% (3/668) on control; risk difference 0.3 percentage points (95% CI -0.5 to 1.1), risk ratio 1.66 (0.40–6.90); unadjusted two-proportion z-test p = 0.483.
• Interpretation:
  • The 95% confidence interval includes no difference, so these data do not show a statistically significant difference between CHG Cloth and the control. The interval gives the range of effects that remain compatible with the data.

The registry lists secondary outcomes covering maternal complications, neonatal intensive care unit admissions, maternal length of stay, and maternal readmissions. These outcomes address different aspects of maternal and neonatal care and do not measure the same event as the primary incisional SSI endpoint. Their relationship to the primary result is therefore complementary rather than interchangeable.

The detailed registry results include a comparison for maternal readmissions up to 6 weeks. Detailed numerical results for the other listed secondary outcomes are not reported in the registry record. As a result, the overall pattern across all secondary outcomes cannot be assessed from the available registry results. It would be inappropriate to characterize the secondary findings as consistently favoring either group without the corresponding outcome-level estimates.

The readmission comparison does not resolve the uncertainty in the primary SSI result. A readmission is a different outcome, with a different event definition and timeframe, and may reflect several causes. Secondary outcomes can help describe the broader set of recorded outcomes, but they cannot substitute for the primary endpoint or independently establish its result.

In general, testing several outcomes increases the chance that at least one result will appear noteworthy by chance. The registry does not report a multiplicity adjustment or a hierarchy governing interpretation of the secondary outcomes. Any unadjusted secondary p-values or estimates should therefore be regarded as exploratory unless the prespecified analysis plan says otherwise; that plan is not reported in the registry record. The available information supports describing the posted outcomes, but not treating isolated secondary findings as confirmatory evidence.

The record reports seven outcome measures and four secondary outcomes, while the detailed secondary results available in the registry entry cover only maternal readmissions. This difference in reporting coverage should be kept in mind when interpreting the overall evidence. Missing detailed results do not establish that an outcome was not measured; they mean its numerical findings are not reported in the registry record.

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

• Number of Participants With Maternal Complications
• Incidence of Neonatal ICU Admissions
• Maternal Length of Stay
• Number of Participants With Maternal Readmissions

9. Statistical Interpretation

10. Statistical Methods Explained

The primary outcome is binary: a participant either had an incisional SSI recorded by the 6-week assessment or did not. The posted results give event counts and group sizes, followed by a risk difference, relative-effect estimates, confidence intervals, and an unadjusted p-value. The registry record does not identify the exact statistical test or model used to produce the posted comparison.

What does a risk difference describe?

In general, a risk difference compares the event proportions in two groups by subtracting one group’s proportion from the other’s. Here, the comparison is oriented as CHG cloth minus placebo cloth. A negative estimate therefore corresponds to a lower observed event proportion in the CHG cloth group. The risk difference is an absolute effect measure: it expresses the separation between the group event proportions on the same proportion scale, rather than as a ratio.

The posted risk-difference estimate is negative, but its confidence interval spans zero. Zero represents no difference on this scale. An interval extending to both sides of zero indicates that the data do not pin down the direction of the underlying difference with precision. The posted p-value is unadjusted, meaning it is not described as corrected for considering multiple outcomes or tests.

What do relative measures add?

In general, a risk ratio compares the event proportion in one group with the event proportion in another, while an odds ratio compares their odds. These are relative rather than absolute measures. A ratio of one corresponds to no relative difference. The registry posts both types of relative estimate for the primary comparison, with confidence intervals that include one. This is consistent with substantial uncertainty about the relative effect. The risk difference and relative measures are alternative descriptions of the same event data, not independent experiments.

How should a confidence interval be read?

In general, a confidence interval describes a range of values that are compatible with the data under the method used to calculate it. It conveys uncertainty around an estimate; it does not guarantee that the true effect lies within the interval, and it does not give the probability that a particular effect is true. The registry posts confidence intervals for its effect estimates, but the confidence level and calculation method are not reported in the registry record. The interval’s inclusion of the null value is informative about the uncertainty in the comparison, but it does not establish equivalence.

What does the p-value mean?

In general, a p-value measures how unusual data at least as discordant with a specified null hypothesis would be if that hypothesis and the test assumptions held. It is not the probability that the null hypothesis is true, nor does it measure the size or clinical importance of an effect. The posted primary p-value is unadjusted. The registry’s sample-size planning illustration lists a two-sided significance level of 0.05, but that planning assumption does not establish the exact testing procedure used for the primary result.

What analysis details are unavailable?

The registry does not report whether the primary comparison was adjusted for any participant characteristics, whether the analysis followed an intention-to-treat approach, or how missing outcomes were treated. It also does not provide the exact statistical test, a model specification, or a prespecified hierarchy for outcomes. These details are not reported in the registry record. Consequently, the posted estimates can be described, but the methods behind them cannot be fully audited from the registry entry alone.

For binary outcomes, the amount of information depends partly on the number of events, not just the overall number of participants. In general, sparse events can make estimates less stable and confidence intervals wider. This is relevant when interpreting a comparison based on relatively few recorded events, but it does not justify assuming a particular model or correcting the posted estimates by hand.

11. Confidence Intervals and Precision

The primary risk-difference confidence interval extends across zero. That means the posted data do not sharply distinguish a lower event proportion in the CHG cloth arm from no difference or a higher event proportion. The interval is a direct indication that the estimated direction is not precise enough to support a firm directional conclusion.

The confidence intervals for the reported relative measures also include their null value of one. On a ratio scale, values on either side of one represent effects in opposite directions, while one represents no relative difference. The intervals therefore reinforce the interpretation that the relative effect is uncertain. The registry record does not report the confidence level or the method used to calculate these intervals.

Precision is not the same as statistical significance or clinical importance. A narrow interval would indicate greater precision, while a broad interval would permit a wider range of effect sizes. The posted primary interval covers the null and values on both sides, so the estimate should not be treated as a precise measurement of the treatment difference. The event counts are also modest relative to the total group sizes, and in general a small number of events can constrain precision for a binary outcome.

The confidence interval does not prove that there is no effect simply because it includes zero. Nor does it prove that a clinically important effect exists merely because some nonzero values remain compatible with the data. The range has to be considered alongside the outcome, study design, and uncertainty about the analysis details. The registry does not state a threshold for a clinically important difference, so the clinical importance of the compatible effect range is not reported.

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 values are planning illustrations based on the observed effect, not the trial’s original design calculation. For these binary outcomes, the values are participants per arm, rather than total events. The stated assumptions are a two-sided significance level of 0.05, equal allocation between the two arms, and a normal approximation; the effect to detect is the stated fraction of the observed effect. The trial’s own original design assumptions are not reported in the registry record.

For the primary comparison, planning for 100% of the observed effect gives 3,662 participants per arm for 80% power and 4,902 per arm for 90% power. Planning for 75% of the observed effect gives 6,797 per arm for 80% power and 9,099 per arm for 90% power. Planning for 50% of the observed effect gives 15,936 per arm for 80% power and 21,333 per arm for 90% power.

The 80% power column describes the modeled number of participants per arm needed for an 80% chance of detecting the specified effect under the listed assumptions; the 90% column describes the corresponding number for a 90% chance. These are prospective planning interpretations, not properties measured after the trial. The displayed figures are not observed power.

Smaller effects require more participants to detect under the same assumptions, which is why the planning values increase when the target is a smaller fraction of the observed effect. Because the observed effect is uncertain, planning around it can produce unstable or overly optimistic illustrations. These values should not be read as the enrollment target chosen by the investigators or as evidence that the completed trial had a particular probability of success.

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 = 0.037; treatment proportion p₁ = 0.026; average p̄ = 0.031.
2. First term: z_1−α/2 × √[2p̄(1−p̄)] = 1.96 × 0.2466 = 0.4833.
3. Second term: z_1−β × √[p₀(1−p₀) + p₁(1−p₁)] = 0.842 × 0.2465 = 0.2074.
4. n = (0.4833 + 0.2074)² / (-0.0114)² = 3,661.7, rounded up to 3,662 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 <- 0.0370942812982998                          # control proportion
p1 <- 0.0256797583081571                          # 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(2402907)
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 = 3,662 (80% power), 4,902 (90% power)
 75% of observed effect: n per arm = 6,797 (80% power), 9,099 (90% power)
 50% of observed effect: n per arm = 15,936 (80% power), 21,333 (90% power)
power.prop.test: 3661.7 
Simulated power at n = 3,662 per arm: 0.801 (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. Multiplicity, Missing Data and Analysis Populations

The registry includes a primary outcome and several secondary outcomes, creating the general issue of multiplicity: when multiple hypotheses are tested, the chance of at least one apparently unusual result can increase. In general, a prespecified testing hierarchy or an adjustment for multiple comparisons can help control that risk. The registry record does not report a multiplicity strategy, an outcome hierarchy, or whether secondary comparisons were intended to be confirmatory or exploratory.

The primary comparison is described with an unadjusted p-value. The registry also lists no reported analyses in its analysis-results listing, although endpoint counts and comparison estimates are posted. The exact analytic procedures, including whether any additional analyses were performed outside the registry listing, are not reported in the registry record. No multiplicity-adjusted results are posted.

Missing-data handling is also not described. The registry does not say how participants without an observed 6-week outcome were treated, whether follow-up was complete, whether missing outcomes were imputed, or whether any sensitivity analyses were conducted. The endpoint-specific group sizes are 662 for the CHG cloth arm and 647 for the placebo cloth arm, while enrollment is 1356; the registry does not explain the difference. It should not be assumed that the difference represents missing follow-up or any particular exclusion process.

The analysis population is not identified. The record does not state whether the primary comparison used all randomized participants, participants who received the assigned cloth, or another defined group. Although the trial is randomized, the registry information does not establish that the posted estimate is an intention-to-treat estimate. The methods for handling protocol deviations and crossovers are also not reported in the registry record.

These omissions limit the ability to evaluate robustness and reproduce the inferential analysis. They do not invalidate the recorded counts, but they mean the reader cannot determine how missing information, multiple outcomes, or analysis-population choices affected the reported estimates.

15. 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 5% reporting threshold.

The registry reports zero serious adverse events and zero other adverse events in the CHG cloth group, with 672 participants at risk for each category. It reports zero serious adverse events and zero other adverse events in the placebo cloth group, with 670 participants at risk for each category. Deaths are reported as zero in both groups, and no top serious adverse event terms are listed.

These figures describe the adverse-event information posted in the registry. A zero count means that no events in the corresponding category were recorded in this results entry; it does not establish that no adverse events occurred outside the definitions, observation period, or reporting process used for the registry. The record does not provide further detail on event collection, surveillance period, severity definitions, or attribution to the assigned cloth.

The at-risk group sizes differ from the group denominators reported for the primary SSI endpoint. The registry does not explain why. Safety counts should therefore be read using their own stated at-risk denominators rather than assumed to use the primary endpoint population. These entries are descriptive and do not, by themselves, establish comparative safety.

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

The registry result answers a narrow question: how many participants had an incisional SSI at 6 weeks in each randomized arm, and what comparison estimate was posted for those groups? The recorded event count was lower in the CHG cloth arm than in the placebo cloth arm. The estimated difference is uncertain, however, and the posted confidence interval spans the null. The unadjusted p-value does not provide strong evidence of a clear difference under the usual interpretation of a test against no difference.

This does not mean the two cloths have been shown to be equivalent. Establishing equivalence or non-inferiority requires a prespecified margin and an analysis designed to test that question; such a margin and analysis are not reported in the registry record. A non-significant result is not a demonstration that the true effect is zero.

Conversely, the lower observed event count in the CHG arm does not prove that using the CHG cloth caused fewer infections. Randomization supports a structured comparison, but the uncertainty in the estimate remains, and important analysis details are not reported. The registry result should not be extended beyond its endpoint and follow-up period to all infections, maternal complications, or longer-term outcomes.

The result also does not establish how the intervention compares in other populations or settings. The registry description concerns women presenting for scheduled primary or repeat cesarean section at Mount Sinai Medical Center. Results for unscheduled procedures or other settings are not reported in the registry record. The record also does not provide detailed outcomes for every listed secondary measure.

In general, statistical interpretation combines the estimated effect, its precision, the outcome definition, the design, and the quality of reporting. For this trial, the event counts and direction of the estimate can be described, but the uncertainty does not support a definitive causal conclusion from the posted result alone.

17. Why This Trial Matters Statistically

This trial illustrates why event counts, effect estimates, and uncertainty need to be read together. The group counts show what was recorded, while the risk difference expresses the comparison on an absolute scale and the relative measures express it on ratio scales. None of these summaries should be interpreted in isolation. The posted estimates point in a direction, but their confidence intervals include the relevant null values.

It also shows that a large total enrollment does not necessarily imply high precision for every binary endpoint. In general, precision depends on how many events occur as well as the number of participants. The primary outcome has fewer recorded events than participants, so the information about the event difference is more limited than the enrollment figure alone might suggest. This is a statistical observation, not a judgment about the trial’s conduct.

The planning illustration is another important distinction. Sample-size figures based on the observed effect are not the same as the investigators’ original design rationale, and they are not observed power. The registry’s planning assumptions are stated, but the original sample-size justification is not reported. Keeping these separate prevents a post hoc illustration from being mistaken for the trial’s prospective plan.

Finally, the registry entry demonstrates how incomplete reporting can limit interpretation even when outcome counts and estimates are available. Analysis populations, missing-data handling, multiplicity procedures, and several method details are not reported. Those gaps matter because they determine how confidently readers can assess the estimates and compare the primary and secondary findings. Clear reporting of both results and analysis choices is necessary for a fully reproducible statistical interpretation.

18. 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 does not report the exact statistical test or model used for the primary comparison, the confidence-interval level, or a complete analysis plan. It also does not describe the analysis population or say whether the comparison followed an intention-to-treat approach. These omissions make it difficult to reproduce the inferential results or assess the effect of analysis choices.

The outcome denominators are not the same as the overall enrollment, and the record does not explain the difference. Missing outcome data, exclusions, and follow-up completeness are not described. The record also does not give the SSI diagnostic criteria, the method of outcome ascertainment, or whether outcome assessment was blinded. Triple masking is recorded, but the masked groups and practical masking procedures are not specified.

The registry lists several secondary outcomes, but detailed results are available for maternal readmissions only. The other listed secondary outcome results are not reported in the registry record, so the overall secondary-outcome pattern cannot be assessed. No multiplicity strategy is described, which further limits the weight that can be placed on isolated secondary comparisons.

The trial’s original sample-size justification and design assumptions are not reported. The planning figures are illustrations based on the observed effect and cannot fill that gap. The registry also does not provide detailed information about protocol deviations, adherence to cloth use, or the handling of participants who did not follow the assigned regimen. These points are not evidence that such issues occurred; they are details unavailable in the record.

Finally, the posted primary endpoint is incisional SSI at 6 weeks. It does not by itself describe all infectious morbidity or longer-term outcomes. The result should be interpreted within that endpoint, population, and timeframe.

19. Related Tutorials

20. Related Statistical Calculators

21. Sources

• ClinicalTrials.gov: STRIPES Study: Study To Reduce Infection Post cEsarean Section — NCT02402907.
• Registry record last updated 2020-11-09; status last verified 2020-10.
• Results first posted 2020-11-09.
• 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.

22. Record Summary

NCT02402907, the STRIPES Study, is a completed randomized, triple-masked, parallel-group trial sponsored by Icahn School of Medicine at Mount Sinai. It compared a 2% CHG cloth with a fragrance-free placebo cloth before scheduled cesarean section. The registry lists enrollment of 1356 and primary completion on 2019-10-11.

The primary outcome was the number of participants with incisional SSI at 6 weeks. The posted counts were 17 of 662 in the CHG cloth arm and 24 of 647 in the placebo cloth arm. The risk-difference estimate was negative, but its confidence interval included zero; the unadjusted p-value was 0.236. The result is uncertain and does not establish a clear difference or equivalence.

The registry lists maternal complications, neonatal ICU admissions, maternal length of stay, and maternal readmissions as secondary outcomes, but detailed secondary results are not reported for all of them. Adverse-event entries list zero serious events, zero other events, and zero deaths in both groups. Important analysis and missing-data details are not reported in the registry record.