Ulcer Prevention Study in Post Gastric Bypass Patients: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Ulcer Prevention Study in Post Gastric Bypass Patients (NCT00557349), a randomized phase 4 study. The primary outcome is Number of Participants With Complaints, Specifically About Pain, Vomiting, Dyspepsia, and/or Dysphagia..

2. Clinical Question

The study’s stated aim was to compare the medications in relation to ulcer prevention after gastric bypass surgery. The posted primary outcome, however, counts participants with specified complaints; it is not itself a count of confirmed marginal ulcers. The secondary outcome concerns whether complaints led to an indication for upper endoscopy. These outcomes capture symptoms and a subsequent clinical evaluation decision, rather than directly reporting the occurrence of an ulcer.

The trial record describes a randomized, double-blind comparison. Random assignment is intended to make treatment groups comparable on average, while masking is intended to limit the influence of treatment knowledge on care or outcome assessment. Those design features do not remove uncertainty in a small study or turn symptom counts into a direct measure of ulcer occurrence. The registry record does not report results for a marginal-ulcer endpoint.

3. Trial Design

The registry record describes a randomized, double-blind, parallel-group, phase 4 clinical trial sponsored by the University of Missouri-Columbia. The listed enrollment is 40. The record lists a start of 2006-11, primary completion in 2007-11, and a completed status. The comparison was between daily omeprazole and daily famotidine, with each arm description specifying 40 mg.

In a randomized trial, assignment to treatment is determined by a chance-based process rather than by participant or clinician choice. A parallel-group design means participants are assigned to one of the compared treatments, rather than being described as receiving each treatment in sequence. Double masking indicates that the registry classifies the trial as masked at two levels, but the identities of the people masked are not reported in the registry record.

The treatment comparison was intended to address medication use after gastric bypass surgery. The primary outcome was the number of participants with complaints specifically involving pain, vomiting, dyspepsia, and/or dysphagia during the first 14 weeks after surgery. The secondary outcome was the number of participants with upper endoscopy indicated because of complaints over that same timeframe. The primary outcome description states that patients lost to follow-up were not included in the data analysis.

The registry record does not report the randomization method, allocation concealment procedures, masking procedures, treatment adherence, or handling of treatment discontinuation. It also does not report a detailed protocol or a sample-size justification. The listed enrollment should therefore be read as the registry’s enrollment figure, not as an explanation of how the number of participants for analysis was determined. Details about the number of participants randomized, follow-up completeness, and analysis populations beyond the stated exclusion of patients lost to follow-up 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.

Before assignment: While study participants were enrolled prior to their RYGB, they did not begin the study drug until after discharge from the hospital post-RYGB. During that time some patients were excluded, whether it was because the patient's surgery was cancelled or because the patient no longer met the inclusion/exclusion criteria.

Overall Study

Analysis population for the primary outcome, as defined in the registry: Patients lost to follow-up were not included in the data analysis.

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 Complaints, Specifically About Pain, Vomiting, Dyspepsia, and/or Dysphagia., measured over during first 14 weeks after surgery.

The primary endpoint counts participants who had complaints specifically about pain, vomiting, dyspepsia, and/or dysphagia during the first 14 weeks after surgery. The registry lists the outcome unit as participants and describes the data as a number of participants with an event. It is therefore a binary-type outcome at the participant level: whether a participant met the complaint definition is counted, rather than how often or how severely a complaint occurred.

Symptom complaints can be relevant to a trial about post-surgical complications because they may prompt clinical attention. They do not, by themselves, establish that an ulcer is present. The endpoint description does not specify how complaints were elicited, whether each symptom was analyzed separately, how multiple complaints in one participant were handled, or whether symptoms were confirmed by a diagnostic procedure. Those details are not reported in the registry record.

The registry’s stated study aim concerns ulcer prevention, but the posted primary endpoint is a symptom count rather than an ulcer count. The secondary outcome tracks upper endoscopy indicated due to complaints. This may provide information about a clinical response to symptoms, but it is not equivalent to reporting endoscopy findings. The registry record does not report a primary result for confirmed marginal ulcers.

The outcome is useful for describing whether participants experienced the specified complaints within the stated timeframe. It cannot, on its own, distinguish differences in symptom severity, duration, or cause. It also does not show whether complaints were attributable to ulcers or to another post-surgical or medication-related cause.

7. Results Posted in the Registry

• Famotidine: 100.0% (13/13) versus 100.0% (17/17) on control; risk difference 0.0 percentage points (95% CI 0.0 to 0.0), risk ratio 1.00 (1.00–1.00); unadjusted two-proportion z-test p = n/a.

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 Upper Endoscopy Indicated Due to Complaints

• 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:
  • Famotidine: 100.0% (13/13) versus 5.9% (1/17) on control; risk difference 94.1 percentage points (95% CI 82.9 to 105.3), risk ratio 17.00 (2.54–113.82); unadjusted two-proportion z-test p = <0.001.
• Interpretation:
  • The 95% confidence interval excludes no difference, so the proportion with the event is statistically higher for Famotidine than for the control. This is an unadjusted, secondary comparison, so it is suggestive rather than confirmatory.

The registry reports one secondary outcome: participants with upper endoscopy indicated due to complaints. Its overall pattern differs from the primary result, which shows equal observed proportions for the specified symptoms. This contrast does not necessarily mean the outcomes conflict. The primary endpoint counts complaints, while the secondary endpoint reflects a clinical decision to indicate an endoscopy. A symptom can occur without prompting that procedure, and a decision to investigate does not establish what the procedure found.

The secondary comparison is reported with an unadjusted p-value. Unadjusted means that the statistical test has not been corrected for the possibility of examining multiple outcomes or analyses. When multiple tests are performed, the chance of obtaining at least one apparently unusual result can increase, even when no true differences exist. The registry record does not report a multiplicity strategy, so the result should be treated as exploratory rather than as a definitive confirmatory finding.

The registry does not report a prespecified hierarchy for primary and secondary outcomes, a detailed endpoint analysis plan, or results from additional secondary measures beyond the posted outcome. That makes it difficult to determine how the secondary result fits into the original inferential plan. Its interpretation should therefore remain tied to the endpoint as recorded, without inferring a difference in ulcer incidence or a mechanism for the observed pattern.

For a broader reading, the secondary finding is one component of the trial record, not a replacement for the primary outcome. The distinction between an event being reported and an investigation being indicated matters because the latter may reflect clinical judgment as well as symptoms. The registry record does not describe criteria for indicating endoscopy, nor does it report endoscopic findings. Those omissions limit the clinical meaning that can be assigned to the secondary pattern.

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

• Number of Participants With Upper Endoscopy Indicated Due to Complaints

9. Statistical Interpretation

10. Statistical Methods Explained

The outcomes in the registry are counts of participants with an event. For statistical analysis, an individual either meets the endpoint definition or does not. A group proportion describes the fraction of analyzed participants who meet that definition. This type of outcome differs from a continuous measure such as a laboratory value, where each participant can have a value across a range.

What does a risk difference describe?

A risk difference compares the event proportions in two groups by subtraction. It is expressed on an absolute scale: a value of zero corresponds to equal observed proportions, while positive or negative values indicate the direction of the contrast according to the order in which groups are compared. The posted primary risk difference is 0.0. For the secondary comparison, the registry reports a risk difference of 0.941, with the comparison organized from the omeprazole group to famotidine in the arm listings. The sign and interpretation depend on that ordering, so the named groups should remain explicit when describing it.

A risk difference is often easier to relate to absolute event frequency than a relative measure. However, it can be unstable when group sizes are limited or when event counts are very low or very high. The secondary interval extends beyond the allowable range for a difference between proportions. A general reason this can happen is that a normal-approximation interval is not constrained to remain within the parameter’s possible range. Such an interval should not be read as implying a physically possible risk difference above the maximum.

What does a risk ratio describe?

A risk ratio compares event proportions by division, expressing one group’s proportion relative to the other’s. A value of one represents equal proportions; values above or below one indicate a relative difference based on the chosen group order. The secondary result reports a risk ratio of 17.0, with an interval from 2.539 to 113.8. The wide range indicates substantial uncertainty about the size of the relative contrast. A large relative estimate can arise when one group has a very low observed event proportion, so it should be considered alongside the absolute proportions and the risk-difference interval.

What does a p-value contribute?

A p-value evaluates how compatible the observed data are with a specified null model, under the assumptions of the test. It is not the probability that the null hypothesis is true, and it does not measure the magnitude or clinical importance of a difference. The secondary p-value is reported as <0.001 and is explicitly unadjusted. The registry record does not report the test method, its assumptions, or a multiplicity adjustment. No primary-outcome p-value is reported.

Small or sparse binary datasets can challenge standard approximations. When event proportions are near the extremes, estimated uncertainty can be especially sensitive to the method used. Exact methods, continuity corrections, or other approaches may behave differently from a simple normal approximation. The registry record does not identify which approach generated the posted intervals and secondary p-value, so the displayed results should be read as the registry’s reported comparisons rather than as a fully documented statistical analysis.

How should the reported intervals be read?

A confidence interval describes a range of values compatible with the data under a particular statistical procedure and its assumptions. It is not a guarantee that the true effect lies inside the interval, nor does a narrower interval automatically establish a more reliable study. Interpretation depends on the quality of measurement, the sample and analysis population, and the method used to calculate the interval. The primary interval’s identical lower and upper limits are unusual as an expression of uncertainty. Without a reported method or explanation, those limits should not be taken as evidence of perfect precision.

The registry includes sample-size planning assumptions based on an observed effect, equal allocation, a two-sided significance level of 0.05, and a normal approximation. Such calculations are planning illustrations, not observed power and not evidence that the trial was designed prospectively around those assumptions. The registry record does not report the original design assumptions or sample-size justification, so the planning illustration should not be confused with the trial’s own design rationale.

What information is missing for a fuller analysis?

The record does not report an adjusted model, covariate set, prespecified analysis plan, handling of protocol deviations, or sensitivity analyses. It also does not report whether the endpoints were analyzed in all randomized participants or according to another defined analysis population. Patients lost to follow-up were excluded from the primary outcome analysis, but the number lost and their treatment assignments are not reported in the registry record. These omissions limit assessment of the assumptions behind the estimates and of how robust the results are to different analysis choices.

11. Confidence Intervals and Precision

The primary risk-difference interval is reported as 0.0 to 0.0. The estimate and both limits are identical, accompanying equal observed group proportions. An interval with no displayed width would ordinarily suggest no uncertainty in the estimated contrast, but that is not a reasonable conclusion from the registry’s limited description alone. The method that produced these limits is not reported in the registry record. The values may reflect how the posted comparison was calculated, rather than the full uncertainty relevant to a treatment effect.

The secondary risk-difference interval extends from 0.829 to 1.053. Its upper limit is outside the feasible range for a difference between proportions. In general, intervals based on an unconstrained normal approximation can cross a parameter boundary, particularly with limited or sparse binary data. That feature makes the interval difficult to interpret literally and reinforces the need to know which method was used. The registry record does not report that method.

The secondary risk-ratio interval, from 2.539 to 113.8 around the estimate of 17.0, is wide. It signals that the relative size of the observed difference is estimated imprecisely. The unadjusted p-value of <0.001 does not eliminate this uncertainty: a small p-value and a broad interval can coexist, because they address different questions. The p-value concerns compatibility with a null model, while the interval conveys a range of effect sizes compatible with the data under the interval procedure.

Precision is also affected by what was measured and who was analyzed. The primary endpoint counts reported complaints, and the secondary endpoint records an indication for endoscopy; neither result reports confirmed ulcer diagnoses. Patients lost to follow-up were excluded from primary outcome analysis, and the number and outcome status of those patients are not reported in the registry record. Thus, even the numerical intervals cannot capture uncertainty due to missing outcomes or possible differences in how endpoints were assessed.

12. Multiplicity, Missing Data and Analysis Populations

Multiplicity arises when researchers make several statistical comparisons. Even if each test is interpreted using the same nominal threshold, considering many tests can increase the chance that at least one result appears statistically unusual by chance. Procedures such as adjustment of significance thresholds or control of an error rate can address this issue, but the choice depends on the goals and prespecified analysis plan.

The registry identifies a primary endpoint and one posted secondary endpoint. It reports an unadjusted p-value for the secondary comparison, but no multiplicity procedure or endpoint hierarchy is reported in the registry record. The p-value therefore should not be read as accounting for the broader set of potential comparisons that may have been considered. The record also lists zero reported analyses for the trial record overall and no reported analyses for the secondary outcome. The displayed comparisons do not come with a fuller account of their planned inferential role.

For missing data, the primary outcome description says patients lost to follow-up were not included in the data analysis. The record does not report how many patients were lost, when they were lost, whether loss differed by arm, or what assumptions were used about the missing outcomes. It also does not report an imputation strategy or sensitivity analyses. Excluding participants with missing follow-up can affect a comparison if those participants differ in outcome risk from those who remain under observation.

The analysis population is not otherwise described in enough detail to establish whether all randomized participants were included or whether other exclusions occurred. The registry lists enrollment of 40 and reports arm-specific denominators of 17 and 13 for the primary outcome. These figures are not accompanied by a participant flow description, so the record does not explain how enrollment relates to the analyzed outcome data. The number randomized to each arm, withdrawals, protocol deviations, and analysis-population definitions are not reported in the registry record.

The registry record also does not report interim analyses, stopping rules, a detailed statistical analysis plan, or adjustments for baseline characteristics. Their absence from the record does not establish what was or was not done; it means those procedures cannot be assessed from the information recorded. As a result, the inferential results should be interpreted cautiously, with particular attention to the stated exclusion of patients lost to follow-up and the unadjusted status of the secondary p-value.

13. Safety Results

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

Other (non-serious) adverse events

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

The adverse-event record lists no serious adverse events and no other adverse events in either arm: the omeprazole group has 0 serious and 0 other events among 17 at risk, and the famotidine group has 0 serious and 0 other events among 13 at risk. No serious adverse-event terms are listed. Deaths are not reported in the registry record.

These are event counts in the listed groups, not proof that no adverse events occurred outside the recorded categories or observation period. The registry record does not provide detailed event definitions, collection procedures, follow-up duration for safety, or a narrative account of how events were assessed. With no events recorded, the data cannot precisely characterize the range of possible safety risks. The figures should therefore be read as a description of what the registry lists, not as a comprehensive comparison of safety.

The adverse-event counts also do not resolve whether symptoms in the primary endpoint were related to treatment, surgery, or another cause. The registry does not link complaint counts to adverse-event classifications. No causal interpretation of the symptom outcomes can be made from these safety counts alone.

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

The posted results describe symptom complaints and indications for upper endoscopy during the first 14 weeks after surgery. For the primary outcome, the registry reports the same observed complaint proportion in both arms. For the secondary outcome, it reports a marked separation in endoscopy indications, with an unadjusted p-value of <0.001. These are observations from the analyzed participants and should not be converted into a claim that one medication works better or worse.

The primary endpoint is not a direct count of marginal ulcers, and the secondary endpoint does not report the results of upper endoscopy. The record therefore does not establish how many participants developed confirmed ulcers or whether either medication prevented them. It also does not show the severity, duration, or cause of the reported complaints. The trial’s stated aim and the measured endpoints are related, but they are not interchangeable.

Statistical significance in the secondary comparison does not establish clinical importance, causation, or generalizability. Its interval is wide for the relative estimate, and the risk-difference interval extends beyond the feasible parameter range. The comparison is unadjusted, and no multiplicity strategy is reported. These features limit how strongly the secondary result can support a broader conclusion.

The record also states that patients lost to follow-up were excluded from primary outcome analysis, without describing the extent or consequences of missing follow-up. The registry does not report detailed methods for outcome ascertainment, a full analysis plan, or confirmed ulcer findings. As a result, the most defensible reading is descriptive: the recorded primary counts are equal in the analyzed groups, while the secondary procedural-indication outcome shows a different pattern that requires cautious interpretation.

These findings do not determine what an individual patient should take or establish a general treatment recommendation. They do not demonstrate equivalence, because an absence of an observed primary difference is not the same as evidence that all plausible differences are negligible. They also do not establish that any observed secondary difference is caused by the assigned medication. Those conclusions would require information and analyses not reported in the registry record.

15. Why This Trial Matters Statistically

This trial is statistically instructive because its primary and secondary outcomes measure different stages of a clinical process. The primary endpoint records whether specified complaints were reported. The secondary endpoint records whether an upper endoscopy was indicated due to complaints. A symptom outcome and a decision to investigate symptoms are related, but the second can also depend on clinical judgment and local procedures. Different patterns across these outcomes therefore need not be contradictory.

The primary comparison also illustrates why identical observed proportions should not be confused with demonstrated equivalence. An equivalence conclusion requires a prespecified margin and an analysis designed to assess whether plausible differences fall within that margin. The registry record does not report such a margin or an equivalence analysis. The zero-width primary interval is not a substitute for this design information, particularly because its calculation method is not reported.

The secondary result illustrates the importance of looking beyond a p-value. A small unadjusted p-value can coexist with substantial uncertainty about the size of a relative effect, and a normal-approximation interval can extend beyond a parameter’s feasible range. Interpretation benefits from considering absolute group proportions, interval behavior, study size, endpoint meaning, missingness, and multiplicity together rather than treating a test result as a complete summary.

Finally, the registry distinguishes the study’s ulcer-prevention aim from the outcomes for which results are posted. That distinction is central to evidence interpretation: statistical analysis can only answer the question represented by the measured endpoint. If the desired question is whether ulcers occurred or were prevented, symptom counts and indications for endoscopy do not by themselves provide that answer. The record’s limitations make this trial a useful example of matching conclusions to the actual outcome definitions and reported analyses.

16. 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 limitations specific to this record constrain interpretation. The primary outcome is a count of participants with symptoms, while the study aim concerns ulcer prevention. The registry does not post a primary result for confirmed marginal ulcers, and it does not report findings from the endoscopies that were indicated. Thus the endpoints do not directly document ulcer occurrence or prevention.

The primary counts show every analyzed participant in each arm meeting the complaint definition, leaving no observed variation within either group. The primary risk-difference interval is reported with identical bounds, but the registry does not report how it was calculated. The result therefore offers limited information about the range of plausible treatment effects beyond the observed equality.

The secondary comparison has an unadjusted p-value and no reported multiplicity strategy. Its risk-difference interval extends beyond the possible range for a difference between proportions, while its relative-effect interval is wide. The registry does not provide enough methodological detail to assess the calculations or determine whether alternative methods would give different inference.

Outcome ascertainment and clinical decision criteria are not described. The record does not explain how symptom complaints were collected, how an endoscopy indication was determined, or how consistently these processes were applied across arms. The record also excludes patients lost to follow-up from primary outcome analysis without reporting their number or outcome status. Those missing details limit assessment of bias and generalizability.

Finally, the registry does not report a full participant flow, detailed analysis populations, a statistical analysis plan, sample-size justification, or an adjusted analysis. Enrollment is listed as 40, while the primary outcome denominators are reported separately by arm; the relationship between those figures is not explained. These reporting gaps do not prove that particular procedures were absent, but they prevent a reader from evaluating them from the registry record.

17. Related Tutorials

18. Related Statistical Calculators

19. Sources

• ClinicalTrials.gov: Ulcer Prevention Study in Post Gastric Bypass Patients — NCT00557349.
• Registry record last updated 2017-01-19; status last verified 2016-09.
• Results first posted 2017-01-19.
• 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.

20. Record Summary

NCT00557349 is a completed phase 4 randomized, double-blind, parallel-group trial sponsored by the University of Missouri-Columbia. The listed enrollment is 40, with a start of 2006-11 and primary completion in 2007-11. It compared omeprazole 40 mg daily with famotidine 40 mg daily after gastric bypass surgery.

The primary outcome counted participants with pain, vomiting, dyspepsia, and/or dysphagia during the first 14 weeks after surgery. The reported counts were 17 of 17 for omeprazole and 13 of 13 for famotidine, with a risk difference of 0.0 and an interval from 0.0 to 0.0. The secondary outcome concerned upper endoscopy indicated due to complaints. Its comparison is reported with an unadjusted p-value of <0.001.

The registry does not report confirmed ulcer results, endoscopic findings, a detailed statistical analysis plan, or the extent of loss to follow-up. Those omissions limit conclusions about ulcer prevention and the precision or generalizability of the posted comparisons.