Suppressive Therapy With Oral Antibiotics for Prevention of Postoperative Urinary Tract Infection (UTI): Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Suppressive Therapy With Oral Antibiotics for Prevention of Postoperative Urinary Tract Infection (UTI) (NCT01450800), a randomized phase 4 study. The primary outcome is Urinary Tract Infections.

2. Clinical Question

The registry describes the study as testing whether prophylactic antibiotic treatment decreases the risk of postoperative urinary tract infection in patients who require catheterization. The primary outcome includes treatment for a clinically suspected or culture-proven infection within three weeks of surgery. Clinically suspected treatment could be empirical after urinary symptoms developed or prescribed based on urine test results. Culture-proven infection was defined as a urine culture with >100,000 colony-forming units of a single organism.

The comparison therefore concerns recorded treatment for suspected or culture-proven infection, not culture results alone. The registry does not describe the specific surgical procedures, catheter duration for individual participants, or detailed eligibility criteria. Further details about which patients were included beyond the stated population are not reported in the registry record.

3. Trial Design

The registry describes a phase 4, randomized, quadruple-masked, parallel-group trial sponsored by Duke University. The study status is completed. The recorded start was 2011-08, and primary completion was 2013-03. Enrollment was 163. The registry does not identify which parties were masked under the quadruple-masking label, nor does it provide additional details about the randomization process, allocation concealment, or the planned treatment of participants who stopped study medication.

The parallel design assigns participants to one of two intervention arms for comparison: nitrofurantoin or placebo. The nitrofurantoin regimen was 100mg by mouth daily, beginning on postoperative day 1 and continuing for up to 7 days during catheterization. The placebo regimen was one tablet by mouth daily over the same stated period. The registry does not report the precise appearance of the placebo, adherence to either regimen, or how treatment duration varied across participants.

The primary outcome is identified as urinary tract infections, measured as the number of participants receiving treatment for UTI within three weeks after surgery. Counts are reported for 18 of 81 participants in the nitrofurantoin arm and 10 of 78 in the placebo arm. The outcome is labelled as an intent-to-treat analysis. The registry does not report participant flow, reasons for any exclusions from the outcome denominators, or whether all randomized participants were included in that analysis.

Seven secondary outcomes are listed under the repeated general label “Other Risk Factors for UTI.” The registry record does not provide distinct descriptions for those entries in the posted summary. It reports nine outcome measures in total and no analysis entries. No additional details about the statistical analysis plan, interim analyses, or the original sample-size justification are 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: Randomization occurred within a few hours after surgery. Participants were excluded prior to randomization if they sustained intraoperative injury to the urinary tract requiring prolonged catheterization, or if they passed an immediate void trial on day of surgery (as these participants did not receive additional postoperative catheterization).

Overall Study

Reasons for not completing this period:

Analysis population for the primary outcome, as defined in the registry: Intent-to-treat 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.

Population: 163 participants were randomized, 82 to nitrofurantoin and 81 to placebo. Four randomized participants were excluded from the final analysis. One in the placebo group was excluded, as she had previously been included in the study during a prior surgery. Three participants in the nitrofurantoin group were excluded due to protocol deviations.

6. Primary Endpoint

The registry lists the primary outcome as Urinary Tract Infections, measured over three weeks post-operative.

The primary endpoint counts participants who received treatment for a urinary tract infection within the first three weeks after surgery. It includes treatment for either clinically suspected or culture-proven UTI. Clinically suspected treatment was defined to include empirical treatment after urinary symptoms developed, or treatment prescribed based on urine test results. Culture-proven UTI was defined as a urine culture with >100,000 colony-forming units of a single organism.

This endpoint captures treatment decisions for infections considered clinically suspected as well as those meeting the stated culture definition. It is therefore broader than an endpoint based only on positive cultures. The registry reports the endpoint in participants, with 18 events among 81 participants assigned to nitrofurantoin and 10 among 78 assigned to placebo. The outcome timeframe is three weeks post-operative.

General explanation: a binary participant-level endpoint records whether each participant experienced the defined event during the stated period. Counting participants rather than repeated episodes can make the outcome easier to interpret as the proportion experiencing at least one qualifying event, although the registry does not separately report episode counts. The endpoint is relevant to the stated study question because the trial concerns treatment for postoperative UTI during the early period after surgery. The registry does not report a separate primary endpoint based solely on culture-confirmed infection, or the timing and type of treatment for each event.

7. Results Posted in the Registry

• Nitrofurantoin: 22.2% (18/81) versus 12.8% (10/78) on control; risk difference 9.4 percentage points (95% CI -2.3 to 21.1), risk ratio 1.73 (0.85–3.52); unadjusted two-proportion z-test p = 0.120.

8. Statistical Interpretation

9. Statistical Methods Explained

The posted comparison is a binary-outcome comparison, where the event is treatment for UTI during the stated postoperative period. The registry gives the count of participants with the event and the denominator in each arm. Nitrofurantoin had 18 participants with the outcome among 81; placebo had 10 among 78. The registry reports a risk difference estimate of 0.094, an interval from -0.023 to 0.211, and an unadjusted p-value of 0.12. The specific statistical test or model used for the sponsor's analysis is not reported in the registry record.

What does a risk difference express?

General explanation: a risk difference subtracts the event probability in one group from the event probability in another. Its zero value corresponds to equal event probabilities under the comparison. The sign depends on the order of the groups. In this comparison, the estimate is described as nitrofurantoin relative to placebo, and the positive estimate aligns with the higher observed event frequency in the nitrofurantoin arm. The estimate is on an absolute scale, unlike a ratio measure. The registry does not give a clinically important threshold for interpreting its magnitude.

How should the confidence interval be understood?

General explanation: a confidence interval describes a range of values that are compatible with the data and the method used to calculate the interval. It reflects sampling uncertainty and relies on assumptions made by the analysis. Here, the interval for the risk difference runs from a negative value to a positive value and includes zero. That means the posted results do not precisely distinguish a difference in either direction from no difference. The interval is not a range containing every plausible value with certainty, and it does not describe individual participant outcomes.

What does the p-value add?

The reported unadjusted p-value is 0.12. General explanation: a p-value compares the observed data with what would be expected under a specified null hypothesis and statistical model. “Unadjusted” indicates that the displayed value is not adjusted for multiple comparisons in the posted result. It should not be read as the probability that the treatment has no effect. The registry does not report the exact test, the test's assumptions, or a multiplicity procedure for the primary and secondary analyses. The p-value and interval provide related but distinct descriptions of uncertainty.

Why do denominators and outcome definitions matter?

The primary endpoint is a participant count, not a count of all infection episodes. An event qualifies when treatment is received for a clinically suspected or culture-proven UTI within three weeks of surgery. The definition includes empirical treatment after urinary symptoms or treatment prescribed based on urine test results, as well as a culture meeting the stated threshold of >100,000 colony-forming units of a single organism. This definition may capture events that would not be counted by a culture-only endpoint. The registry does not report separate counts for suspected and culture-proven infections, or details that would allow the components to be examined independently.

The primary outcome is labelled intent-to-treat. General explanation: an intent-to-treat approach typically aims to analyze participants according to their randomized assignment, regardless of treatment adherence or changes after assignment. The label alone does not reveal how missing outcomes or participants without a recorded endpoint were handled. The record gives outcome denominators of 81 and 78 and enrollment of 163, but does not provide a flow accounting for those numbers. A reader therefore cannot determine from the registry record whether the outcome denominators reflect missing measurements, exclusions, or another reporting convention.

What is not established by the registry entry?

The record does not report the original statistical analysis plan, a model specification, adjustment variables, a missing-data method, interim analyses, or a sample-size justification for the trial itself. The planning illustration associated with the observed result is a separate calculation and does not establish what methods the investigators planned before the trial. No analysis entries are posted, and detailed results for the listed secondary outcomes are not included in the summary. These gaps mean that the displayed comparison can be interpreted as a reported arm-level estimate, but the full analysis pathway cannot be independently reconstructed from the registry entry.

General explanation: over-dispersion is a concern in some models when outcome variability exceeds what the model assumes. It matters because standard errors and intervals can be too narrow when variability is understated. This trial's primary outcome is a binary participant-level count, and the registry does not report a dispersion parameter or indicate that an over-dispersion model was used. It would therefore be inappropriate to infer that such a model was part of the analysis. More generally, statistical interpretation depends on matching the analysis method to the outcome and design, but the method details are not reported in the registry record.

The listed secondary outcome labels repeat the phrase “Other Risk Factors for UTI.” The record reports seven secondary outcomes and nine outcome measures overall, but the posted summary does not distinguish the secondary measures or provide their results. Without those details, multiplicity across the outcome family cannot be assessed. The unadjusted primary p-value should not be mistaken for a multiplicity-adjusted result.

10. Confidence Intervals and Precision

The reported risk difference is 0.094, with a confidence interval from -0.023 to 0.211. The interval crosses zero, so the results are consistent with a range that includes no difference as well as differences in either direction. Its breadth indicates that the point estimate is not highly precise: the data do not pin down the size or direction of the underlying difference narrowly. The registry does not state the confidence level used for this interval, so no confidence percentage is assigned.

The counts underlying the comparison are 18 of 81 participants in the nitrofurantoin arm and 10 of 78 in the placebo arm. A point estimate condenses those observations into one value, but the interval communicates that other values remain compatible with the data under the calculation method. General explanation: a narrower interval indicates greater precision, while a wider interval indicates more uncertainty about the estimated quantity. Precision is not the same as accuracy, and neither is a measure of clinical importance.

The reported unadjusted p-value is 0.12. This is consistent with the interval's inclusion of zero, but the p-value does not convey the range of effect sizes as directly as the interval does. The registry does not provide the statistical test or the confidence-interval method, which limits assessment of the assumptions used. It also does not report a clinically important difference against which to judge whether any particular value in the interval would matter.

The trial's enrollment was 163, while the outcome denominators are 81 and 78. The record does not explain the difference between enrollment and those denominators, so the potential impact of incomplete endpoint data cannot be assessed from this entry. More complete participant flow and separate event counts by clinical suspicion and culture confirmation would help readers judge the precision and interpretation of the primary endpoint.

11. 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 figures are participants per arm, not total events. They illustrate how many participants might be needed in each arm under the stated planning assumptions, using the observed effect as a reference. For the full observed effect, the figures are 256 participants per arm for 80% power and 342 participants per arm for 90% power. For planning at 75% of the observed effect, they are 431 participants per arm for 80% power and 576 participants per arm for 90% power. For 50% of the observed effect, they are 914 participants per arm for 80% power and 1,222 participants per arm for 90% power.

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. General explanation: power is the probability, under a specified effect and analysis assumptions, of detecting an effect using the chosen statistical criterion. The 80% and 90% columns therefore represent different target probabilities under those assumptions, not guarantees about the result of a future study.

Smaller effects require more participants to detect under the same planning framework, which is why the displayed participant counts rise as the assumed fraction of the observed effect becomes smaller. These figures are a planning illustration based on the observed effect, not the original design assumptions for this trial; those original assumptions are not reported in the registry record. This is not observed power. A sample-size curve should be read as a planning aid for relating the assumed effect to the required participant count, rather than as evidence about what the completed trial was capable of detecting after its results were known.

12. 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 = 0.128; treatment proportion p₁ = 0.222; average p̄ = 0.175.
2. First term: z_1−α/2 × √[2p̄(1−p̄)] = 1.96 × 0.5376 = 1.0537.
3. Second term: z_1−β × √[p₀(1−p₀) + p₁(1−p₁)] = 0.842 × 0.5335 = 0.4490.
4. n = (1.0537 + 0.4490)² / (0.0940)² = 255.5, rounded up to 256 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 <- 0.128205128205128                          # control proportion
p1 <- 0.222222222222222                          # 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(1450800)
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 = 256 (80% power), 342 (90% power)
 75% of observed effect: n per arm = 431 (80% power), 576 (90% power)
 50% of observed effect: n per arm = 914 (80% power), 1,222 (90% power)
power.prop.test: 255.5 
Simulated power at n = 256 per arm: 0.805 (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.

13. Multiplicity, Missing Data and Analysis Populations

The registry lists seven secondary outcomes, each under the repeated general label “Other Risk Factors for UTI,” and reports nine outcome measures in total. It does not provide distinct descriptions for those secondary outcomes in the posted summary. The registry also reports zero analysis entries. Consequently, the full set of analyses and their results cannot be assessed from this record.

Multiplicity arises when several outcomes or comparisons are tested. General explanation: as the number of statistical tests increases, the chance of at least one apparently unusual result can increase even when none of the underlying hypotheses is true. Methods such as prespecifying a primary outcome or adjusting for multiple testing can address particular multiplicity concerns, but the registry record does not state whether any such approach was planned or used. The primary comparison is reported with an unadjusted p-value of 0.12. It should not be described as adjusted for the secondary outcomes.

The primary outcome is labelled as an intent-to-treat analysis. That label identifies the stated analysis population approach, but it does not show how missing outcome values were treated or whether all randomized participants contributed data. The registry reports 163 enrolled participants and primary outcome denominators of 81 and 78. No participant-flow explanation, missing-data method, or reasons for any difference between these numbers are reported in the registry record. The adverse-event denominators are also reported separately, without an explanation of the difference from the primary outcome denominators.

Interim analyses, stopping rules, and the original statistical analysis plan are not reported in the registry record. Nor are details available about adjustment variables, prespecified subgroup analyses, or the handling of repeated outcomes. These omissions do not prove that such procedures were absent; they mean their use cannot be established from the registry entry. The displayed primary estimate can be read as the reported unadjusted arm comparison, while conclusions about the broader family of outcomes and the complete analysis strategy remain limited by the posted information.

14. 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 registry reports serious adverse events for 82 participants at risk in the nitrofurantoin group and 81 at risk in the placebo group. The number affected is recorded as 0 in both groups. Other adverse events are also reported as affecting 0 participants among 82 at risk in the nitrofurantoin group and 0 among 81 at risk in the placebo group. No serious adverse-event terms are listed. Deaths are not reported in the registry record.

These figures describe the safety information posted for the stated at-risk groups. They do not establish that no adverse events of any kind occurred outside the record's definitions or reporting process. The registry does not give event-specific detail, severity grades, timing, duration, or a fuller explanation of how adverse events were collected. It also does not explain why these at-risk denominators differ from the primary outcome denominators of 81 and 78.

General explanation: zero reported events in a group is not proof that the true risk is zero. The interpretation depends on the number of participants observed, the definitions used, the completeness of reporting, and the observation period. These figures should therefore be read as the counts recorded in the registry, not as a comprehensive assessment of all possible harms or as a basis for treatment decisions.

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

The registry reports a higher observed number of participants treated for UTI in the nitrofurantoin arm than in the placebo arm: 18 of 81 compared with 10 of 78 during the first three weeks after surgery. The estimated risk difference is 0.094 in the direction of more recorded events in the nitrofurantoin arm. Its confidence interval, from -0.023 to 0.211, includes zero, and the unadjusted p-value is 0.12. The estimate is therefore uncertain and does not precisely establish the size or direction of the underlying difference.

These findings do not establish that nitrofurantoin caused the observed difference, or that the treatment has or lacks an effect. A randomized design is reported, but the registry entry does not provide all details needed to evaluate trial conduct, missing outcomes, or the sponsor's full analysis. The primary outcome also combines treatment for clinically suspected and culture-proven UTI. Because separate counts are not reported, the posted result cannot show how many events met the culture definition or how many were based on clinical suspicion or urine test results.

The result applies to the recorded endpoint and its stated timeframe among the participants included in the posted outcome denominators. It does not answer questions about outcomes beyond three weeks, recurrence, other infection definitions, or other surgical settings. The registry does not provide detailed results for the listed secondary outcomes or a stated threshold for a clinically important difference. Nor does the posted safety summary establish that all possible harms were absent; it records zero affected participants for the reported serious and other adverse-event categories in the stated groups.

General explanation: statistical uncertainty and clinical importance are different questions. An estimate can be uncertain even when the point estimate is not zero, and a statistically unusual result would not by itself establish that a difference is important to patients. Judging importance requires context and a prespecified threshold, neither of which is reported. The registry result is best read as a comparison of the posted participant counts, with substantial uncertainty about the true difference and limitations in the available reporting detail.

16. Why This Trial Matters Statistically

This trial is statistically instructive because its primary outcome is a participant-level binary event, while the posted results also include the event counts and denominators needed to understand the arm comparison. The endpoint is defined through treatment received for clinically suspected or culture-proven UTI, rather than through culture confirmation alone. That definition makes the analysis clinically oriented but also combines events that may have been identified in different ways. Without separate counts for the components, readers cannot determine how much each contributed to the total.

The positive risk-difference estimate and the confidence interval crossing zero illustrate why a point estimate should be interpreted alongside its uncertainty. The observed direction is not the same as a precise conclusion about the underlying difference. The unadjusted p-value and the interval together indicate uncertainty, while neither identifies whether a particular effect would be clinically important. The registry does not report a threshold for that judgment.

The entry also highlights the distinction between a completed trial's observed results and a planning exercise based on those results. The sample-size illustration uses the observed effect and stated assumptions; it is not a record of the trial's original design rationale and is not observed power. Reading the planning counts as if they described the trial's prespecified target would overstate what the registry establishes.

Finally, the reported enrollment, outcome denominators, and adverse-event denominators do not have an accompanying participant-flow explanation. The analysis population is labelled intent-to-treat, but the handling of missing data is not reported. These details matter because transparent accounting helps readers evaluate who contributed to each analysis. The registry's limited descriptions of secondary outcomes and absence of posted analysis entries also make multiplicity and the full analysis strategy difficult to assess. Together, these points show why statistical interpretation depends not only on estimates and p-values, but also on endpoint definitions, precision, analysis populations, 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.

The registry entry does not provide detailed eligibility criteria, descriptions of the surgical procedures, or information about catheter duration for individual participants. It states that the population required postoperative catheterization after gynecologic surgery, but the limits of generalizability to particular procedures or patient groups cannot be determined from the posted summary. The masking label is quadruple, although the registry does not identify which roles or groups were masked.

The primary endpoint combines treatment for clinically suspected and culture-proven infection. Separate counts for those categories, the number of cultures obtained, and the number meeting the stated culture definition are not reported. The event counts therefore cannot be separated into clinical-treatment and culture-confirmed components. The registry also does not report infection episodes beyond the participant-level primary count or the timing of events within the three-week period.

Enrollment is 163, while the primary outcome denominators are 81 and 78. Adverse-event denominators are 82 and 81. The record does not explain these denominators or provide participant flow, so the completeness of the reported outcome data cannot be evaluated. Although the primary analysis is labelled intent-to-treat, the missing-data method and reasons for any absent outcome data are not reported in the registry record.

The statistical analysis plan, exact test or model, confidence-interval method, interim analyses, and original sample-size justification are not reported. The registry lists secondary outcomes under a repeated general label and gives no analysis entries or detailed secondary results. Consequently, the full analysis strategy, multiplicity handling, and evidence across outcomes cannot be reconstructed. The reported adverse-event counts also lack event-specific descriptions and details about collection or follow-up. These are limitations of what can be learned from the registry entry, rather than evidence that a particular procedure was or was not performed.

18. Related Tutorials

19. Related Statistical Calculators

20. Sources

• ClinicalTrials.gov: Suppressive Therapy With Oral Antibiotics for Prevention of Postoperative Urinary Tract Infection (UTI) — NCT01450800.
• Registry record last updated 2017-02-13; status last verified 2016-12.
• Results first posted 2014-05-01.
• 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

NCT01450800 is a completed phase 4, randomized, quadruple-masked, parallel-group study sponsored by Duke University. It enrolled 163 participants and compared daily nitrofurantoin with placebo during postoperative catheterization after gynecologic surgery. The recorded study start was 2011-08, and primary completion was 2013-03.

The primary endpoint counted participants treated for clinically suspected or culture-proven UTI within three weeks after surgery. The registry reports 18 of 81 participants in the nitrofurantoin arm and 10 of 78 in the placebo arm. The risk difference is 0.094, with a confidence interval from -0.023 to 0.211 and an unadjusted p-value of 0.12. The interval includes zero, and the posted estimate remains uncertain.

Seven secondary outcomes are listed, but detailed descriptions and results are not available in the posted summary. The record reports zero participants affected by serious and other adverse events in the stated at-risk groups; deaths are not reported in the registry record. The registry does not report the original design assumptions, missing-data procedures, or full analysis plan.