Treatment Study of Vacuum Assisted Closure for Postsurgical Subcutaneous Abdominal Wound Healing Impairments: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Treatment Study of Vacuum Assisted Closure for Postsurgical Subcutaneous Abdominal Wound Healing Impairments (NCT01528033), a randomized na study. The primary outcome is Time to Complete Wound Closure.

2. Clinical Question

The study also examined whether closure was confirmed and verified, as well as recurrence and other clinical, safety, patient-reported, and economic parameters. The registry lists confirmed and verified closure and recurrence as secondary outcomes. The posted secondary closure result gives the number of participants meeting that outcome in each group and an unadjusted comparison.

For the primary endpoint, closure was not defined only as a wound appearing closed at a single assessment. It required complete epithelialization, no drainage, no need for adjuvant therapy or dressing, and no sutures, with confirmation after 14 consecutive days. The assessment was based on photo documentation and a blinded, computer-based wound quality assessment. These criteria make the question one about sustained, verified closure rather than a brief or subjective impression of healing.

The registry describes random assignment and a parallel-group design. Participants and attending doctors and nurses were informed of assignment; central analysts assessing photographs were not informed of patient details or treatment allocation. The result therefore concerns the recorded outcomes under the trial’s stated eligibility and assessment procedures, not every abdominal wound or every clinical setting.

3. Trial Design

The registry record describes a randomized, parallel-group study comparing Vacuum Assisted Closure® with standard conventional wound therapy. The study is listed as single-masked. The brief summary clarifies that participants and attending doctors and nurses were informed of treatment assignment, while central personnel assessing photographs were not informed of patient details or assignment. The exact meaning of the registry’s single-masking label is not further reported in the registry record.

The study was conducted in hospitals in Germany, the Netherlands, and Belgium. The listed enrollment is 539. The trial status is completed, with a start date of 2011-08-02 and a primary completion date of 2018-03-14. The phase is listed as NA. The registry does not report a sample-size justification or further details about how allocation was generated or concealed.

The posted primary outcome is time to complete wound closure, measured in days over a 42-day timeframe and described for the intention-to-treat population. The record reports a mean and a 95% confidence interval for each treatment group. The posted group sizes for this outcome are 256 for NPWT and 251 for SCWT. The primary outcome description says closure was verified through photo documentation and blinded, computer-based wound quality assessment, with confirmation after 14 consecutive days.

The registry also lists confirmed and verified wound closure and recurrence as secondary outcomes. For confirmed and verified closure, the posted counts are 92 of 256 in the NPWT group and 54 of 251 in the SCWT group. These are outcome-specific counts and should not be treated as a complete account of every participant’s follow-up or disposition.

Although the brief summary refers to equal assignment by chance, the posted outcome group sizes are not identical. Reasons for this difference, including any exclusions, missing observations, or analysis-specific denominators, are not reported in the registry record. The study’s own design assumptions and any formal statistical analysis plan are also 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.

Primary Endpoint Assessment ITT

Reasons for not completing this period:

Follow Up

Reasons for not completing this period:

Analysis population for the primary outcome, as defined in the registry: Intention To Treat Population

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.

Population: Intention To Treat Population

6. Primary Endpoint

The registry lists the primary outcome as Time to Complete Wound Closure, measured over 42 days.

The primary endpoint is time to complete wound closure, expressed in days. Its timeframe is 42 days, and the registry describes it as a mean outcome for the intention-to-treat population. The reported group means are 36.1 days for NPWT, with a 95% confidence interval from 35.0 to 37.2, and 39.1 days for SCWT, with a 95% confidence interval from 38.3 to 40.0.

Closure required more than a visual appearance of healing. The registry defines it as 100% epithelialization, no drainage, no need for adjuvant therapy or dressing, and no presence of sutures. Closure also had to be confirmed after 14 consecutive days, with the stated allowance of 14 days -0 / + 3. Photo documentation and a blinded, computer-based wound quality assessment were used in verification.

This endpoint is useful because it represents both the timing and durability of closure under the stated definition. A time measure can preserve information about when closure occurred, rather than recording only whether closure was achieved by a particular assessment. However, the registry does not report the distribution of individual closure times, how participants without confirmed closure by the end of follow-up were handled in calculating the mean, or a formal between-group estimate and test for the primary outcome. Those omissions limit interpretation of the mean values as a comparative statistical result.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

Statistical methods help distinguish what the posted estimates describe from what they can establish. The registry reports a continuous primary endpoint and a binary secondary endpoint. Each requires a different summary, and their estimates should not be treated as interchangeable.

What does a confidence interval describe?

General explanation: a confidence interval is a range of values generated by a statistical procedure to reflect uncertainty in an estimate. For the primary endpoint, the reported 95% confidence intervals accompany each group’s mean time to closure. They characterize uncertainty around the group means; they are not the same as an interval for the difference between group means. A direct comparison would ordinarily require an estimate of the between-group contrast, its uncertainty, and the method used to account for the outcome’s distribution and follow-up. Such a primary comparative estimate is not reported in the registry record.

The two group intervals should not be used as a substitute for a formal between-group test. Visual overlap or non-overlap is not the stated primary analysis and does not reveal the exact result of a comparison procedure. In addition, the registry does not report standard deviations, the distribution of closure times, or details of censoring and non-closure handling. Those details matter because time measurements may be skewed or include participants who do not meet the closure definition during the observation period.

How should the binary secondary result be read?

General explanation: a risk difference compares event probabilities by subtraction, while a risk ratio compares them by division. An odds ratio compares odds rather than probabilities. These measures express a binary outcome on different scales; none is a second, independent trial result. For confirmed and verified closure, the registry posts all three measures for SCWT relative to NPWT. The negative risk difference indicates that the observed event proportion in the SCWT group was lower under the direction used for the comparison. Its unadjusted interval does not include zero, and the posted unadjusted p-value is p < 0.001. The estimate remains subject to the analysis choices and limitations noted in the record.

The reported counts are 92 of 256 for NPWT and 54 of 251 for SCWT. A proportion is the count divided by its group denominator; the posted proportions are 0.359 and 0.215. The risk difference, risk ratio, and odds ratio give complementary descriptions of the same recorded comparison. Their values depend on the outcome definition, observation window, and included participants. They should not be generalized beyond those conditions without supporting evidence.

What does “unadjusted” mean?

General explanation: an unadjusted comparison is reported without an adjustment for other factors or for the number of statistical comparisons. The registry’s p-value for confirmed and verified closure is unadjusted. The record lists several outcomes and describes additional clinical, safety, patient-reported, and economic parameters, but no multiplicity method is reported in the registry record. An unadjusted p-value addresses the specified comparison under the model or test used; it does not control the chance of false-positive findings across a collection of outcomes.

The registry lists no reported analyses in its analysis listing. That makes it difficult to determine whether the posted secondary comparison was prespecified, whether other analyses were conducted, or which model and assumptions produced the estimates. The primary outcome is presented as group means with confidence intervals, but the test or model used to derive them is not reported in the registry record. Randomization supports a comparative design, but it does not remove the need to understand the analysis population, missing data, and outcome ascertainment.

How should the planning illustrations be interpreted?

General explanation: sample-size planning estimates describe how many participants per arm might be needed under stated assumptions to detect a chosen effect with a target power. They are not estimates of what the completed trial’s power was, and they do not show that the observed results are true or clinically important. The registry explicitly identifies its illustrations as calculations from the observed effect, not as the trial’s own design.

For the confirmed-closure comparison, the illustration uses two-sided significance level 0.05, equal allocation, and a normal approximation. Planning for 100% of the observed effect gives 154 participants per arm for 80% power and 205 for 90% power. Planning for 75% of the observed effect gives 284 participants per arm for 80% power and 379 for 90% power. Planning for 50% of the observed effect gives 660 participants per arm for 80% power and 882 for 90% power. These are hypothetical planning outputs conditional on the stated assumptions and effect fractions, not observed power and not evidence that the original study was designed around these values.

General explanation: over-dispersion matters when observed variation is greater than a model assumes, because standard errors and intervals may then be too narrow if the extra variation is ignored. The registry’s primary endpoint is continuous, but it does not give enough distributional detail to assess whether a normal approximation or another model was used. The secondary outcome is binary, but the exact test or model is not reported. No additional modeling assumptions should be inferred from the posted estimates alone.

10. Multiplicity, Missing Data and Analysis Populations

The registry record does not report a statistical analysis plan, a hierarchy for outcomes, or a method for controlling multiplicity. The secondary confirmed-closure comparison is explicitly unadjusted. When multiple outcomes or analyses are considered, examining each at a conventional threshold without adjustment can increase the chance of at least one apparently positive result arising by chance. The record does not establish how many analyses were considered by investigators, and no multiplicity correction should be assumed.

The registry lists an intention-to-treat population for the primary endpoint. That label indicates the stated population for the reported primary outcome, but the record does not explain how missing outcome data, treatment discontinuation, protocol deviations, or participants without confirmed closure by the end of the timeframe were handled. Nor does it report whether every randomized participant contributed to every posted measure. Outcome-specific denominators differ in the safety reporting, and the reason for those denominators is not reported in the registry record.

For the primary outcome, the group-specific means and confidence intervals are posted, but the method for calculating the means and intervals is not described. The registry does not report whether time to closure was analyzed with a method that accounts for participants who had not achieved closure during follow-up. It also does not report the distribution of times or a primary between-group test and estimate. For the binary secondary outcome, the record posts denominators and an unadjusted comparison, but does not describe the exact test or model.

The registry’s analysis listing contains no reported analyses. This absence leaves unclear which comparisons were planned in advance and which were exploratory. The missing information does not itself show that an analysis was inappropriate; it limits independent evaluation of how robustly the results answer the stated question. A complete interpretation would need transparent analysis specifications, missing-data handling, and the relationship between the posted outcomes and any prespecified testing strategy.

11. 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 serious adverse event terms

Most frequent non-serious adverse event terms

Serious events with the highest arm-level rate: wound-related (9.4%), adverse device events (npwt-related) (0.0%), conventional wound treatment-related (0.0%).

The safety record reports serious adverse events and other adverse events for separate group denominators, along with deaths. For NPWT, serious adverse events affected 22 participants among 234 at risk, other adverse events affected 41 among 234 at risk, and 24 deaths are listed. For SCWT, serious adverse events affected 12 among 201 at risk, other adverse events affected 22 among 201 at risk, and 15 deaths are listed.

These are counts, not adjusted comparisons. The registry does not report a statistical comparison of safety outcomes, the timing of events, their individual severity beyond the serious-event classification, or whether the deaths were considered related to treatment. The at-risk denominators differ from the primary outcome group sizes, and the reasons for that difference are not reported in the registry record. The counts should not be converted into comparative risk estimates without further information.

The listed top serious adverse-event terms include wound-related events at 9.4%, adverse device events (NPWT-related) at 0.0%, and conventional wound treatment-related events at 0.0%. The record does not explain all coding, attribution, or denominator details behind these terms. A zero listed percentage should not be interpreted as proof that an event is impossible or that every type of harm was absent. Safety counts provide descriptive context, but they do not by themselves establish comparative safety.

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

The posted findings describe outcomes recorded in a randomized comparison of NPWT and standard conventional wound therapy for postsurgical subcutaneous abdominal wound healing impairment. For the primary time-to-closure endpoint, the registry reports a mean and 95% confidence interval for each group. The means are 36.1 days for NPWT and 39.1 days for SCWT. Because a direct comparative estimate and test are not reported, those group summaries do not quantify the statistical uncertainty of the between-group contrast.

The secondary confirmed-closure result is more directly comparative: it includes an unadjusted risk difference, interval, and p-value, along with risk ratio and odds ratio estimates. It describes the recorded binary outcome under the registry’s definition and observation timeframe. It does not answer every question about time to closure, recurrence, quality of healing, patient experience, economic outcomes, or safety. Those outcomes either require separate interpretation or are not reported in the registry record.

These statistics do not establish that either treatment works or does not work. They do not determine which option is appropriate for an individual patient. The estimates are specific to the trial’s eligibility criteria, treatment descriptions, outcome definitions, follow-up, and analysis reporting. Generalization to other wound types, settings, or treatment procedures is not established by the posted results alone.

Precision is also outcome-specific. Confidence intervals around the group means describe uncertainty in those means, while the interval for the risk difference describes uncertainty on a different scale for the binary outcome. Statistical significance of an unadjusted secondary comparison does not establish clinical importance, and it does not account for multiple outcomes. The record lacks details needed to judge the handling of missing data and non-closure, as well as the full analysis plan.

The most defensible reading is therefore bounded: the registry records the stated group summaries and one unadjusted secondary comparison. These data offer information about the outcomes as defined and assessed, but limitations in comparative primary reporting and analysis details prevent stronger conclusions from the posted statistics alone.

13. Why This Trial Matters Statistically

This trial raises several statistical issues common in clinical research. Its primary outcome is a time measure, but the registry reports group means rather than a full description of the distribution or a direct comparison. Means can be useful summaries, yet their interpretation depends on the distribution of times and on how participants who do not achieve the event during follow-up are handled. Without those details, readers cannot fully evaluate the primary analysis.

The distinction between a group estimate and a treatment contrast is important. The two group-specific confidence intervals quantify uncertainty around separate means, not the uncertainty around their difference. A randomized design is intended to support comparisons between groups, but transparent reporting of the contrast and its analysis remains essential to assess the evidence for that comparison.

The secondary confirmed-closure outcome illustrates why scale and multiplicity matter. Risk difference, risk ratio, and odds ratio describe the same binary outcome in different ways. An unadjusted p-value can be informative for its specific comparison, but its interpretation is weaker when the record does not explain how the outcome fits into a prespecified testing strategy or how multiple outcomes were handled.

The study also emphasizes outcome measurement. Sustained closure was defined with explicit wound criteria and photographic assessment by personnel blinded to treatment assignment. Blinded assessment can limit some forms of assessment bias, while the registry does not report enough detail to evaluate all features of measurement and analysis. The primary closure definition includes confirmation over time, so missing or incomplete follow-up could affect classification, yet the handling of such cases is not described.

Finally, the planning illustrations are based on the observed effect and are not the original trial’s design justification. They demonstrate that required sample size is sensitive to the assumed effect size, but they cannot be used as observed power. Clear separation of observed results, uncertainty, analysis assumptions, and planning scenarios is crucial for statistical interpretation.

14. 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 primary comparative result is incompletely reported. Group means and confidence intervals are available, but a between-group estimate and its test are not reported in the registry record. The treatment effect on the primary time measure therefore cannot be directly assessed from a posted contrast. The record also does not state how the mean was calculated when closure was not observed within the 42-day timeframe, or whether time-to-event methods were used.

The registry gives limited information on the analysis. It does not report a formal statistical analysis plan, model details, diagnostic checks, or procedures for missing data. The listed intention-to-treat population does not by itself explain how participants with incomplete outcome information were included. Reasons for differing outcome-specific denominators are not reported. These gaps make it difficult to assess possible bias from incomplete follow-up or departures from assigned treatment.

Multiplicity is another limitation. The confirmed-closure comparison is unadjusted, and the record does not explain whether outcomes were prioritized or whether any correction was used. The registry lists recurrence and describes other categories of outcomes, but their numerical results are not reported in the registry record. The absence of these results prevents a broad assessment of whether secondary outcomes are consistent.

Generalizability is bounded by the stated population: patients with postsurgical subcutaneous abdominal wound healing impairment without opening of the underlying fibrous tissue layer. The intervention and comparator are described, but standard conventional therapy followed institutional clinical standards, which may allow variation across settings. The record does not provide details sufficient to assess how that variation affected outcomes.

Finally, safety results are descriptive and include denominators that differ from those for the primary endpoint. The registry does not report comparative safety analyses or event attribution details. These limitations do not negate the posted observations, but they mean the available record is not a complete account of efficacy, safety, or the methods used to reach the results.

15. Related Tutorials

16. Related Statistical Calculators

17. Sources

• ClinicalTrials.gov: Treatment Study of Vacuum Assisted Closure for Postsurgical Subcutaneous Abdominal Wound Healing Impairments — NCT01528033.
• Registry record last updated 2020-06-25; status last verified 2020-06.
• Results first posted 2020-06-25.
• 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.

18. Record Summary

The registry describes a completed randomized, parallel-group study of Vacuum Assisted Closure® (negative pressure wound therapy) versus standard conventional wound therapy for postsurgical subcutaneous abdominal wound healing impairment without fascial dehiscence. The study was conducted in hospitals in Germany, the Netherlands, and Belgium, with listed enrollment of 539. Its phase is listed as NA.

The primary outcome was mean time to complete wound closure during a 42-day timeframe, with closure requiring specified wound criteria and confirmation after 14 consecutive days. The record reports group means and 95% confidence intervals, but no primary comparative estimate or test. A secondary confirmed-closure outcome has an unadjusted comparison; recurrence is listed but its results are not reported in the registry record.

The record gives descriptive adverse-event and death counts. It does not report the full statistical analysis plan, multiplicity handling, missing-data procedures, or details needed to interpret all analysis denominators. These omissions limit the strength and scope of conclusions that can be drawn from the posted results.