Irrisept Versus Standard of Care in the Prevention of Surgical Site Infections: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Irrisept Versus Standard of Care in the Prevention of Surgical Site Infections (NCT02255487), a randomized na study. The primary outcome is Percent of Subjects With Surgical Site Infections (SSI).

2. Clinical Question

The clinical question is about the comparative frequency of a postoperative event, not about a change in infection severity or the time until infection. The registry’s brief summary identifies patients randomized after open abdominal laparotomy for abdominal trauma or an acute surgical abdomen. The study’s stated purpose was to compare SSI rates between the irrigation-device group and the SoC group. The record also lists hospital readmission, hospital costs, and length of hospital stay as secondary outcomes. The results available for interpretation include counts for the primary endpoint and posted comparisons, but the registry record does not report a detailed protocol, the reason the study was terminated, or a definition of the mITT population.

3. Trial Design

This was a randomized, parallel-group, prospective, controlled clinical study described as Phase IV and multicenter. Participants were assigned to Irrisept or standard of care. The registry record lists no masking, so the study was open-label in the sense that masking was not used. It describes routine institutional surgical preparation for the SoC group and use of the IrriSept device for surgical irrigation in the intervention group. The record does not report the method used to generate or conceal the random allocation sequence.

Enrollment is listed as 627. The posted primary outcome results use denominators of 293 for Irrisept and 291 for SoC. The primary outcome population is described as mITT, but the registry record does not explain the population definition, how participants entered it, or why its denominators differ from the enrollment figure. Those details should not be inferred from the totals alone.

The study began on 2015-02-18, and primary completion is listed as 2017-03-13. Its status is terminated. The registry record does not report why it was terminated, whether the planned enrollment was reached, or whether termination affected follow-up or outcome availability. The stated primary follow-up window was 30 (+/- 3) days after the surgical procedure. The record also lists secondary outcomes for readmission, hospital costs, and length of stay; the posted secondary results described for comparison include readmission and length of stay.

The registry record does not report the trial’s original sample-size justification, planned recruitment target, or detailed statistical analysis plan. It also does not report randomization strata, site counts, or prespecified rules for handling missing outcomes. These omissions limit how fully the design and analysis can be reconstructed from the registry alone. The randomized design is relevant to interpretation, but design features that are not described cannot be assumed.

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: The planned sample size was 1100 screened, 650 completed \& 600 evaluated. However, the study was terminated early after the interim analysis due, in part, to issues with the accuracy and completeness of the data collected. There were 627 consented and 617 randomized subjects. The intent-to-treat (ITT) population was defined as all randomized subjects. Of these, 309 were randomized to Irrisept and 308 to Standard of Care.

Overall Study

Analysis population for the primary outcome, as defined in the registry: Data is from the percentage of subjects that reported an SSI for the mITT subject populations.

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 Percent of Subjects With Surgical Site Infections (SSI), measured over 30 (+/- 3) days post-surgical procedure.

The primary endpoint was the percent of subjects with surgical-site infections, with the outcome recorded as a count of participants. The timeframe was 30 (+/- 3) days after the surgical procedure. The endpoint description says the primary measure was the rate of SSIs observed 30 days from the date of the index operation and identifies the mITT populations as the basis for the outcome.

An event-count endpoint answers a direct clinical-trial question: how many participants in each group experienced at least one event during the defined follow-up period? It is used to compare the frequency of an outcome between randomized groups. Unlike a time-to-event measure, this posted endpoint does not describe when within follow-up an infection occurred. It also does not distinguish infections by severity in the reported primary result.

The registry lists 48 events among 293 Irrisept participants and 32 among 291 SoC participants. Its posted comparison expresses the between-group contrast as a risk difference, with Irrisept as the comparison group and SoC as the reference. This helps describe the size and uncertainty of the observed contrast, but the endpoint definition and counts alone do not establish why any difference arose. Interpretation also depends on the outcome population, completeness of follow-up, and methods used to identify and classify infections. The registry record does not report those operational details or how missing primary outcomes were handled.

7. Results Posted in the Registry

• Irrisept: 16.4% (48/293) versus 11.0% (32/291) on control; risk difference 5.4 percentage points (95% CI -0.2 to 10.9), risk ratio 1.49 (0.98–2.26); unadjusted two-proportion z-test p = 0.058.

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.

Hospital Readmission Rates

• 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:
  • Irrisept: 4.4% (13/293) versus 4.5% (13/291) on control; risk difference -0.0 percentage points (95% CI -3.4 to 3.3), risk ratio 0.99 (0.47–2.11); unadjusted two-proportion z-test p = 0.986.
• Interpretation:
  • The 95% confidence interval includes no difference, so these data do not show a statistically significant difference between Irrisept and the control. The interval gives the range of effects that remain compatible with the data.

Length of Hospital Stay

• Endpoint type: Continuous: a mean value per participant, compared as a difference in means.
• Test: Unadjusted two-sample z-test for the difference in means, using the posted standard deviations.
• Result:
  • Irrisept: mean 11.60 versus 12.00 on control; difference -0.40 (95% CI -4.31 to 3.51), standardized effect d = -0.02; z-test p = 0.841.
• Interpretation:
  • The 95% confidence interval includes no difference, so these data do not show a statistically significant difference between Irrisept and the control. The interval gives the range of effects that remain compatible with the data.

The secondary outcomes offer context beyond the primary SSI endpoint: they address hospital readmission, hospital costs, and length of hospital stay. The posted results described for comparison cover readmission and length of stay. Across these available comparisons, the estimates are centered close to no difference rather than displaying a clear, consistent pattern of separation between groups. They do not provide a strong independent confirmation of the primary endpoint’s observed contrast. Hospital costs are listed as a secondary outcome, but a result for that outcome is not reported in the registry record.

Secondary outcomes can help show whether a primary result is accompanied by differences in related aspects of care, but they answer distinct questions and may use different outcome scales. A readmission event and a stay-duration measure, for example, are not interchangeable with an SSI event. Similarity or contrast across them cannot establish the cause of the primary result. The available secondary comparisons are also subject to uncertainty, and the registry record does not report sponsor analyses for these endpoints.

Unadjusted secondary results generally carry less confirmatory weight when several outcomes are examined. General statistical explanation: as the number of statistical tests increases, the chance of at least one apparently noteworthy result can rise unless the analysis accounts for multiplicity or the outcomes are interpreted under a prespecified hierarchy. The registry record does not report whether such a plan existed or how these secondary outcomes were prioritized. Their results are therefore most appropriately treated as descriptive and exploratory in this registry record, rather than as separate definitive tests supporting a claim.

The secondary outcomes can inform a broader account of the trial, but they do not remove the uncertainty in the primary comparison. Nor should the absence of a clear secondary pattern be taken as proof that the groups are identical on those outcomes. Their interpretation depends on the unreported analysis details and on the precision of each estimate.

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

• Hospital Readmission Rates
• Hospital Costs
• Length of Hospital Stay

9. Statistical Interpretation

10. Statistical Methods Explained

What does a risk difference express?

A risk difference compares the proportion of participants with an event in one group with the corresponding proportion in another group. It is an absolute contrast on the risk scale: zero represents equal event risks, a positive value means the event risk was higher in the first group named in the comparison, and a negative value means it was lower. For the primary outcome, the posted risk difference is 0.054, defined for Irrisept relative to SoC. The comparison summary lists the associated group values as 0.164 for Irrisept and 0.11 for SoC. The risk difference communicates the contrast in the same units as those proportions.

How do risk ratios and odds ratios differ?

A risk ratio compares the event risks as a ratio, while an odds ratio compares the odds of an event. Both are relative measures, unlike the absolute risk difference. A ratio of one represents no relative contrast; values greater than one indicate a larger measure in the first group relative to the reference, and values less than one indicate a smaller measure. The primary comparison reports a risk ratio of 1.49 and an odds ratio of 1.586. Their confidence intervals are broad enough to include values near one, so neither should be interpreted without its uncertainty. Relative measures can appear more or less striking depending on the underlying event frequency; they complement rather than replace the absolute contrast.

What does a confidence interval convey?

General statistical explanation: a confidence interval is a range of estimates generated by a method designed to have a specified long-run coverage under its assumptions. It describes statistical uncertainty around an estimate; it is not a range guaranteed to contain the true value, nor does it represent the range of outcomes for individual participants. For a difference, an interval spanning zero indicates that the data and method do not rule out no difference at the stated confidence level. For a ratio, the corresponding no-contrast value is one. Interval width gives a sense of precision: wider intervals leave more plausible values unresolved.

For the primary outcome, the reported risk-difference interval is -0.002 to 0.109. Because it spans zero, the posted estimate does not exclude no difference under the interval’s method. The reported relative-measure intervals also include one. Those features are consistent with the unadjusted p-value of 0.058, while still leaving uncertainty about the size and direction of the underlying contrast.

What does the p-value tell the reader?

General statistical explanation: a p-value measures how incompatible the observed data are with a specified null model, assuming that model and the analysis assumptions hold. It is not the probability that the null hypothesis is true, and it does not give the probability that a result occurred “by chance.” The registry comparison reports an unadjusted p-value of 0.058 for the primary contrast. Because it is unadjusted, it does not account for testing multiple outcomes or comparisons. The registry record does not report a multiplicity plan, so the intended interpretation of multiple tests cannot be recovered from the entry.

Why is the population definition important?

The primary outcome description refers to the mITT subject populations. Such labels can refer to analysis populations defined by trial-specific rules; the meaning of the label should not be assumed without those rules. The registry record does not define mITT, explain participant exclusions, or describe missing-outcome handling. The primary denominators are 293 and 291, while the registry lists overall enrollment as 627. Those figures refer to different reported quantities, and the record does not explain the relationship between them. As a result, the completeness and exact composition of the analysis population cannot be evaluated from the entry alone.

Why can over-dispersion matter?

General statistical explanation: for count outcomes, over-dispersion means that variability is greater than a model’s assumed distribution allows. If a count model ignores this extra variability, its standard errors and intervals can be too small, making the evidence seem more precise than it is. The primary outcome is reported as participants with an event, rather than as a recurrent-event count. The registry record does not describe a count-model analysis or an over-dispersion assessment, so no such method should be attributed to this trial. This concept is relevant generally when interpreting analyses of event counts, but it is not a reported feature of the posted primary analysis.

The registry comparison is a simple unadjusted recalculation from posted summary statistics. The record does not report a more detailed model, covariate adjustment, or a sponsor analysis to compare against. Accordingly, the reported contrast should be read as a summary comparison, not as evidence that other potential sources of variation were explicitly modeled. The trial’s original statistical plan and assumptions are not reported in the registry record.

11. Confidence Intervals and Precision

Precision is the degree to which an estimate is concentrated around a particular value. The primary risk-difference estimate is 0.054, with a 95% confidence interval from -0.002 to 0.109. The interval extends from a value just below zero to a positive value that is appreciably larger than the point estimate. Thus, the posted data leave uncertainty about whether the underlying contrast is close to no difference or more clearly positive. The point estimate should not be read as an exact or settled effect.

The interval includes zero, which is the no-difference value for a risk difference. The risk-ratio interval, 0.982 to 2.26, and the odds-ratio interval, 0.981 to 2.563, include one, the no-relative-contrast value for those measures. These intervals do not establish equivalence. Rather, they show that the registry results do not narrowly constrain the possible contrast to a small range around the estimate.

General statistical explanation: confidence intervals reflect sampling uncertainty under the method’s assumptions. They do not account automatically for every possible source of uncertainty, such as outcome misclassification, incomplete follow-up, or unreported differences in analysis decisions. The registry record does not give enough detail to assess those issues fully. Nor does the interval quantify how well the trial’s results apply to patients beyond the population and setting described in the record.

The unadjusted p-value of 0.058 is close to, but above, 0.05. That single threshold comparison should not replace consideration of the estimate, the interval, the outcome definition, and the analysis population. In particular, a result that does not cross a conventional significance threshold is not proof of no effect; a result that does cross one would not by itself establish clinical importance. The primary result is best described as an observed positive contrast with uncertainty that includes no difference.

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 planning values are participants per arm for 80% power and participants per arm for 90% power. They are based on a two-sided significance level of 0.05, equal allocation between two arms, and a normal approximation. For each illustration, the effect to detect is a stated fraction of the observed effect. The outcomes represented are not time-to-event outcomes, so the figures refer to participants per arm rather than total events.

The 80% power column gives the illustrated number of participants per arm for a design that would have an 80% chance of detecting the specified effect under the stated assumptions, if that effect were the true one and the analysis assumptions held. The 90% column uses a higher target chance of detection and therefore requires more participants for the same effect. The effect fractions describe planning scenarios, not separate observed results: the illustrations consider 100% of the observed effect, 75% of the observed effect, and 50% of the observed effect. Smaller effects need more participants to detect with the same power and significance level.

These are planning illustrations calculated from the observed effect, not observed power. The trial’s own original sample-size assumptions and justification are not reported in the registry record. Therefore, the illustrations should not be taken as evidence that the study was designed around these assumptions, or that its enrollment was adequate or inadequate under the original protocol.

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 = 0.110; treatment proportion p₁ = 0.164; average p̄ = 0.137.
2. First term: z_1−α/2 × √[2p̄(1−p̄)] = 1.96 × 0.4861 = 0.9528.
3. Second term: z_1−β × √[p₀(1−p₀) + p₁(1−p₁)] = 0.842 × 0.4846 = 0.4079.
4. n = (0.9528 + 0.4079)² / (0.0539)² = 638.3, rounded up to 639 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 <- 0.109965635738832                          # control proportion
p1 <- 0.16382252559727                          # 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(2255487)
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 = 639 (80% power), 854 (90% power)
 75% of observed effect: n per arm = 1,089 (80% power), 1,457 (90% power)
 50% of observed effect: n per arm = 2,342 (80% power), 3,134 (90% power)
power.prop.test: 638.3 
Simulated power at n = 639 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. Multiplicity, Missing Data and Analysis Populations

Multiplicity arises when a study evaluates more than one outcome, comparison, or analysis. If each test is interpreted on its own without adjustment, the chance of at least one false-positive finding can increase across the set of tests. General statistical explanation: a prespecified hierarchy, adjusted testing procedure, or clearly exploratory framing can help readers understand how multiple results should be weighed. The registry record lists a primary outcome and several secondary outcomes, but does not report a multiplicity strategy, a hierarchy for testing, or whether secondary outcomes were intended to support confirmatory conclusions.

The primary outcome population is described as mITT. The registry record does not define mITT for this trial, state the inclusion and exclusion rules, or explain how deviations from assigned treatment were handled. It does not report how missing primary or secondary outcomes were addressed, whether imputation was used, or whether sensitivity analyses were performed. These details matter because a participant without an observed outcome may be treated differently under different analysis rules, and those choices can affect estimates and uncertainty.

The record lists enrollment as 627 and reports primary outcome denominators of 293 and 291. It does not explain the relationship between these figures or report the amount or reasons for missingness. No inference about the number of participants with missing outcomes should be made from the totals alone. The record also does not provide a participant-flow account sufficient to trace screening, randomization, follow-up, exclusions, and analysis inclusion.

There are no reported analyses listed in the registry entry. The posted group comparisons are available, but the record does not identify a sponsor analysis plan or describe adjustment for site or other covariates. These omissions limit evaluation of whether all analyses were prespecified and how robust the results are to alternative reasonable decisions. They do not, by themselves, show that any particular analysis choice was inappropriate; they mean that the relevant information is not reported in the registry record.

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

The posted primary result describes a higher observed SSI frequency in the Irrisept group than in the SoC group: 48 of 293 compared with 32 of 291. The estimated risk difference is 0.054, and its 95% confidence interval extends from -0.002 to 0.109. This is evidence about the observed groups and the uncertainty around their contrast, not a complete explanation of why the outcomes differed.

The interval includes no difference, and the unadjusted p-value is 0.058. These findings do not establish that the groups had the same underlying event risk. They also do not establish a definite difference. The estimates remain compatible with no difference and with a positive difference of greater size. Statistical uncertainty should be distinguished from clinical importance, which cannot be determined from a p-value alone and is not defined in the registry record by a clinical threshold.

The study addresses SSI within 30 (+/- 3) days of the procedure among the reported mITT populations. It does not answer whether there are differences beyond that follow-up window, in the severity of infections, or in other patient outcomes. The registry record lists additional outcomes, but those do not substitute for the primary endpoint or resolve its uncertainty. Generalizing the findings also requires attention to the described population—patients having open abdominal laparotomy for abdominal trauma or an acute surgical abdomen—and to the multicenter setting, for which further detail is not reported.

The randomized allocation is relevant to comparing groups, but the registry entry does not provide all details needed to assess allocation implementation, follow-up completeness, or the analysis population. Its status is terminated, and the reason is not reported in the registry record. These limits mean the posted results should be read as a bounded account of the recorded trial data, rather than as a universal conclusion about all patients or settings. No treatment recommendation follows from these statistics alone.

16. Why This Trial Matters Statistically

This trial illustrates why an estimate, its confidence interval, and the underlying outcome counts should be read together. The primary risk-difference estimate is positive, but its interval includes zero and extends to a larger positive contrast. A single threshold-based reading of the unadjusted p-value would omit that range of uncertainty. Reporting both absolute and relative measures also helps distinguish the size of a group contrast on different scales, while keeping in view that the measures arise from the same outcome data.

The record also highlights how registry detail shapes statistical interpretation. Randomization and a defined follow-up window are reported, but the mITT population is not defined, missing-data handling is not described, and the original sample-size justification and multiplicity strategy are not reported. These are not minor technicalities: they affect what can be established about the analysis and how strongly its results can support wider conclusions. A careful reader must separate what the posted comparisons show from details that the record does not provide.

The study’s termination status makes transparent reporting particularly useful, though the reason for termination is not reported in the registry record. The planning illustrations based on observed effects also demonstrate the difference between retrospective planning scenarios and the trial’s original design assumptions. Together, these points make the record useful for statistical learning: it shows how an apparently simple two-group event comparison depends on population definitions, precision, multiple outcomes, and reporting completeness.

17. 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.

Several trial-specific limits constrain interpretation. The study was not masked, according to the registry record. Lack of masking can matter for outcomes that involve clinical judgment or depend on how events are detected and recorded; however, the record does not describe SSI adjudication or whether outcome assessors were blinded. It is therefore not possible to determine how outcome ascertainment was organized.

The study is listed as terminated, but the reason for termination, timing of that decision, and any effect on planned enrollment or follow-up are not reported in the registry record. Enrollment is 627, while the primary outcome is reported for mITT groups with denominators of 293 and 291. The record does not explain the relationship between these figures or provide enough participant-flow detail to evaluate exclusions and completeness. The mITT label is not defined, and missing-data procedures are not described.

The registry identifies the study as multicenter but does not report the number of centers or explain whether site-level differences were considered in analysis. It also does not provide the original sample-size rationale, detailed analysis plan, or multiplicity procedures. No sponsor analyses are listed. Consequently, the available unadjusted comparisons cannot be checked against a fully documented prespecified analysis from the registry record.

The follow-up window is 30 (+/- 3) days after surgery, so the primary result concerns a limited postoperative period. The record does not provide detailed SSI definitions, severity categories, or longer-term outcomes. It describes a particular surgical population—patients with abdominal trauma or an acute surgical abdomen undergoing open abdominal laparotomy—which limits how directly the results can be extended to other procedures or populations. These are constraints on interpretation, not proof that the findings are invalid.

18. Related Tutorials

19. Related Statistical Calculators

20. Sources

• ClinicalTrials.gov: Irrisept Versus Standard of Care in the Prevention of Surgical Site Infections — NCT02255487.
• Registry record last updated 2022-05-24; status last verified 2022-04.
• Results first posted 2022-05-24.
• 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.

21. Record Summary

NCT02255487 is a randomized, parallel-group, multicenter study comparing the IrriSept System with standard of care in patients with abdominal trauma or an acute surgical abdomen undergoing open abdominal laparotomy. It is described as Phase IV, prospective, controlled, and unmasked. Enrollment is listed as 627; the study status is terminated. The registry lists a start date of 2015-02-18 and primary completion on 2017-03-13.

The primary endpoint was SSI within 30 (+/- 3) days after surgery, reported for mITT populations. The posted counts were 48 of 293 for Irrisept and 32 of 291 for SoC. The risk difference was 0.054, with a 95% confidence interval from -0.002 to 0.109 and an unadjusted p-value of 0.058. The registry also lists secondary outcomes for readmission, hospital costs, and length of stay. The original sample-size rationale, analysis plan, missing-data handling, reason for termination, and definition of the mITT population are not reported in the registry record.