This page provides an independent statistical analysis and educational interpretation of publicly reported results. ClinicalTrials.gov provides the official trial registry record. Numerical trial results on this page are restricted to the results posted in the ClinicalTrials.gov record; calculations added by Clinical Biostats are labelled as such.
1. Trial at a Glance
This page covers the registry results for GonoScreen: Efficacy of Screening STIs in MSM (NCT04269434), a randomized na study. The primary outcome is Incidence Rate of Ng Plus Ct Detected at Any Site Whilst Individuals Are Screened vs. Not Screened.
| Feature | Registry value |
|---|---|
| ClinicalTrials.gov identifier | NCT04269434 |
| Phase | NA |
| Condition | Neisseria Gonorrhoeae Infection, Chlamydia Trachomatis Infection |
| Allocation | Randomized |
| Design model | Parallel |
| Masking | None (open label) |
| Enrollment | 1014 |
| Arms in the primary outcome | 2 |
| Lead sponsor | Institute of Tropical Medicine, Belgium |
| Results posted | Yes |
2. Clinical Question
Population
Men who have sex with men taking HIV pre-exposure prophylaxis (PrEP) were the population described in the trial title. The registry identifies gonorrhea and chlamydia infections as the conditions under study. The primary outcome was analyzed in a per-protocol population. The registry record does not provide further eligibility details in the information reported for this analysis.
Intervention
The comparison was regular screening for gonorrhea and chlamydia at the urethra, pharynx and rectum. Samples were collected every 3 months. In the screening arm, results were sent to study physicians; participants with results requiring treatment were treated, and partner contact tracing was done.
Comparator
The comparator was no screening during the study. Samples from that arm were processed, but results were reported at the end of the study. The trial therefore compared a screening strategy with delayed reporting, rather than comparing two different screening schedules.
Primary question
Over 12 months, how did the rate of gonorrhea or chlamydia diagnoses per scheduled study visit with an available result differ between the screening and no-screening arms? The primary outcome counted diagnoses at any site. Its denominator excluded unscheduled visits, so it describes diagnoses per qualifying scheduled visit rather than a simple count of participants who ever tested positive.
3. Trial Design
GonoScreen was a randomized, multicentre controlled trial with a parallel-group design. The registry lists enrollment of 1014 and a completed status. The recorded start date is 2020-09-21, and the primary completion date is 2022-08-31. Masking is listed as none, so the trial was open label according to the registry information.
The trial compared screening with no screening. In the screening arm, sampling for gonorrhea and chlamydia was scheduled every 3 months at the urethra, pharynx and rectum. Laboratory results were sent to study physicians, with treatment and partner contact tracing described for participants in that arm. In the no-screening arm, the laboratory processed the samples but withheld reporting until the end of the study. Thus, the comparison included differences in the timing of result reporting and the clinical actions described for the arms.
The primary outcome was assessed over 12 months in a per-protocol analysis. The registry describes the outcome as the number of gonorrhea and chlamydia diagnoses divided by scheduled study visits with available results; unscheduled visits were excluded from the denominator. The reported arm estimates are 0.155 for screening and 0.205 for no screening, with 95% confidence intervals of 0.128 to 0.186 and 0.171 to 0.246, respectively.
The registry lists no masking and does not report additional details about the randomization procedure, allocation concealment, or the intended allocation ratio. Those design details 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.
Overall Study
| Stage | Screening | No Screening |
|---|---|---|
| STARTED | 506 | 508 |
| COMPLETED | 439 | 440 |
| NOT COMPLETED | 67 | 68 |
Analysis population for the primary outcome, as defined in the registry: Per protocol 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.
| Characteristic | Unit | Screening | No Screening | Total |
|---|---|---|---|---|
| Participants analysed | 506 | 508 | 1014 | |
| Age, Continuous | years | 39 (33 to 47) | 39 (32.5 to 48) | 39 (33 to 47) |
| Sex/Gender, Customized: Sex – Male | Participants | 506 | 505 | 1011 |
| Sex/Gender, Customized: Sex – Transwoman | Participants | 0 | 3 | 3 |
| Race and Ethnicity Not Collected | Participants | — | — | 0 |
| Number of sex partners (last 3 months) | Number of sex partners | 4 (2 to 8) | 4 (2 to 8) | 4 (2 to 8) |
| Number of sex partners (mean) | Number of sex partners | 6.28 (0 to 100) | 7.26 (0 to 150) | 6.77 (0 to 150) |
| Number of unprotected sex partners in the last 3 months | Number of unprotected sex partners | 2 (1 to 5) | 2 (1 to 5) | 2 (1 to 5) |
| Number of unprotected sex partners (mean) | Number of unprotected sex partners | 3.80 (0 to 40) | 4.28 (0 to 100) | 4.04 (0 to 100) |
| STI symptoms: Missing | Participants | 0 | 1 | 1 |
| STI symptoms: No | Participants | 504 | 503 | 1007 |
| STI symptoms: Yes | Participants | 2 | 4 | 6 |
| Antibiotics in the last 6 months: Any | Participants | 192 | 173 | 365 |
| Antibiotics in the last 6 months: Cephalosporins | Participants | 67 | 77 | 144 |
| Antibiotics in the last 6 months: Macroloides | Participants | 81 | 94 | 175 |
| Antibiotics in the last 6 months: Penicillins | Participants | 63 | 47 | 110 |
| Antibiotics in the last 6 months: Quinolones | Participants | 11 | 5 | 16 |
| Antibiotics in the last 6 months: Tetracyclines | Participants | 57 | 54 | 111 |
| Antibiotics in the last 6 months: Other | Participants | 3 | 3 | 6 |
6. Primary Endpoint
The registry lists the primary outcome as Incidence Rate of Ng Plus Ct Detected at Any Site Whilst Individuals Are Screened vs. Not Screened, measured over 12 months.
| Element | Registry entry |
|---|---|
| Result type | Number |
| Measure of spread | 95% Confidence Interval |
| Unit | Ng or CT cases per study visit |
| Endpoint classification | not reduced to a single comparison |
The primary endpoint was the rate of gonorrhea or chlamydia diagnoses detected at any site during 12 months, expressed as cases per study visit. Its numerator was the number of diagnoses in the relevant arm. Its denominator was the number of scheduled study visits with an available result for gonorrhea or chlamydia; unscheduled visits were excluded. The endpoint therefore measures diagnoses in relation to qualifying visits, not the number of participants who experienced infection.
A rate-per-visit outcome is useful for comparing diagnostic yield when the number of observed visits may differ or when results are not available for every scheduled visit. It also makes clear what the unit of comparison is: the posted estimates are not percentages of participants. The screening arm estimate was 0.155 cases per study visit, with a 95% confidence interval from 0.128 to 0.186. The no-screening estimate was 0.205, with a 95% confidence interval from 0.171 to 0.246.
The endpoint combines gonorrhea and chlamydia diagnoses and includes detections at any site. It does not, by itself, distinguish the two infections, identify which anatomical site accounted for a diagnosis, or indicate how many people had repeated diagnoses. The registry record does not report those breakdowns for the primary endpoint.
7. Why No Re-analysis Is Shown
8. Statistical Interpretation
The posted primary estimates are 0.155 diagnoses per study visit in the screening arm and 0.205 in the no-screening arm. The registry’s reported non-inferiority analysis gives an incidence rate ratio of 1.318 for no screening relative to screening, with a 95% confidence interval from 1.068 to 1.627. This is a relative comparison of rates, while the arm estimates describe the rates themselves. The interval is entirely above 1, indicating that the reported rate in the no-screening arm was higher in this comparison. The registry does not provide a p-value for this primary analysis.
These estimates should be read in light of the endpoint definition. The denominator includes scheduled visits with available results and excludes unscheduled visits. Accordingly, the rate is not a risk per participant, and the result does not directly state how many individuals experienced an infection. The analysis is also identified as per protocol, so its population is not necessarily identical to all randomized participants. The registry record does not report the per-protocol inclusion rules or the number of participants excluded from that analysis.
The non-inferiority label identifies the type of analysis reported, but the registry information does not state the non-inferiority margin or give the protocol’s decision rule. Without those details, the reported label alone is insufficient to evaluate the design criterion for non-inferiority. The confidence interval communicates uncertainty around the estimated rate ratio, but it does not supply the missing margin or explain the sponsor’s intended threshold.
The posted results support a descriptive comparison of the two observed rates and the reported rate ratio. They do not establish why the rates differed, whether differences in diagnoses reflect differences in infection occurrence or detection, or how results might generalize beyond the trial population and procedures. Nor do these estimates alone quantify effects on antimicrobial exposure, partner outcomes, or other consequences. Those questions require outcomes and analyses beyond the primary rate comparison.
Precision is conveyed by the confidence intervals rather than by the point estimates alone. The primary arm intervals overlap, but visual overlap is not a substitute for the reported comparison or for a prespecified non-inferiority assessment. The reported rate ratio and its interval are the relevant comparative statistics in the registry record. Their interpretation remains limited by absent details about the margin, missing-result handling, and analysis plan.
9. Statistical Methods Explained
What does a rate per study visit represent?
In general, a rate relates an event count to an amount of observation. For this trial, the stated unit is gonorrhea or chlamydia cases per study visit. The numerator is the number of diagnoses, and the denominator is scheduled visits with available results. This definition differs from a proportion of participants: a person could contribute more than one visit, and the endpoint is not reported as the number of people with at least one diagnosis. The registry explicitly excludes unscheduled visits from the denominator.
Using visits as the denominator makes the observed diagnostic rate interpretable in relation to scheduled opportunities for testing with an available result. It does not remove all possible differences in observation. The analysis can be affected by which scheduled results are available, and the record does not describe how unavailable results were handled or whether visit availability differed between arms. Those details are not reported in the registry record.
How should the confidence intervals be read?
In general, a confidence interval expresses the precision of an estimate under the statistical method used. A narrower interval indicates greater precision than a wider interval on the same scale; it does not mean that the true value has a stated probability of lying within the particular interval. The primary rates are accompanied by 95% confidence intervals: 0.128 to 0.186 for screening and 0.171 to 0.246 for no screening. The reported rate ratio also has a 95% confidence interval, from 1.068 to 1.627.
The arm-specific intervals describe uncertainty around each rate. The interval for the rate ratio describes uncertainty around the relative comparison and is more directly relevant to whether the rate differs between arms. A confidence interval does not address possible bias from missing results, protocol deviations, or limitations in the endpoint definition.
What does the rate ratio compare?
A rate ratio divides one rate by another. In general, a value of 1 represents equal rates, values above 1 indicate a higher rate in the numerator group, and values below 1 indicate a lower rate in that group. The registry’s primary analysis identifies the comparison group as no screening and the control as screening. Its reported ratio of 1.318 is therefore read as the no-screening rate relative to the screening rate. The confidence interval is reported as 1.068 to 1.627.
The registry labels this analysis non-inferiority. Non-inferiority analyses generally assess whether the outcome for an intervention falls within a prespecified margin relative to a comparator. That margin and the specific decision criterion are not reported in the registry record. The label and ratio can be described, but the full non-inferiority claim cannot be independently assessed from the reported details alone.
What is the role of the recalculated secondary comparison?
For symptomatic gonorrhea and chlamydia, a simple unadjusted recalculation from the posted arm rates gives a rate ratio of 1.353 for no screening relative to screening, with a confidence interval from 0.797 to 2.296. This calculation is based on posted summary statistics and is not a replacement for the sponsor’s analysis. The registry’s own reported analysis gives an incidence rate ratio of 1.373, with a 95% confidence interval from 0.963 to 1.956. The registry record does not provide a p-value for that analysis.
In general, a recalculation from summary statistics can be useful as a transparent check of the scale and direction of a comparison. It may not reproduce the sponsor’s method if the original analysis used details unavailable in the summary, such as a particular model or adjustment. The registry does not identify the method used for these reported analyses. The recalculated comparison should therefore be treated as a simple unadjusted illustration, not as a new primary analysis.
What do the planning illustrations mean?
The planning illustrations are based on the observed symptomatic infection effect, not on a sample-size justification for the trial. The stated assumptions are two-sided significance level 0.05, equal allocation between arms, and a normal approximation; the effect to detect is a stated fraction of the observed effect. These are hypothetical planning calculations. They are not observed power and do not establish what the trial was designed to detect. The original design assumptions and sample-size justification are not reported in the registry record.
General planning calculations depend on assumptions about the effect of interest, variability or event rates, analysis method, and allocation. Using an effect estimated in a study to illustrate planning for a future study can be informative, but the observed estimate is uncertain. The planning figures should therefore be read as scenarios rather than guarantees about the sample size needed in another setting.
Why can over-dispersion matter for count rates?
In general, a simple count model may assume that events vary according to a specified distribution. If observed counts vary more than that model allows, uncertainty can be understated unless the analysis accounts for over-dispersion. The registry does not report enough detail about the model used for the primary or secondary outcomes to establish whether over-dispersion was assessed or addressed. This is a general statistical consideration, not a claim about the trial’s actual data or analysis.
10. Analyses Reported by the Sponsor
| Method | Estimate | 95% CI | P-value |
|---|---|---|---|
| not stated | Incidence rate ratio 1.318 | 1.068 to 1.627 | not reported |
The registry reports a sponsor analysis for the primary endpoint as an incidence rate ratio of 1.318 for no screening relative to screening, with a 95% confidence interval from 1.068 to 1.627. The analysis is labelled non-inferiority. The record does not report a primary recalculation alongside this sponsor result, so there is no separate recalculated primary estimate to compare directly with it.
For symptomatic gonorrhea and chlamydia, the registry reports an incidence rate ratio of 1.373, with a 95% confidence interval from 0.963 to 1.956. A simple unadjusted recalculation from the posted arm rates gives a rate ratio of 1.353, with an interval from 0.797 to 2.296. These are close point estimates but not identical, and the intervals differ. The registry does not state the method used for the sponsor analysis, so the reason for the difference cannot be determined from the record.
A summary-statistic recalculation is a check, not a substitute for an analysis based on the full trial data and prespecified methods. The registry does not identify the model or adjustment details for the sponsor’s analyses. The non-inferiority margin is also not reported, which limits assessment of that analysis. The registry record marks no discrepancy, but the available information does not explain how the reported sponsor analysis was conducted.
11. Multiplicity, Missing Data and Analysis Populations
The primary analysis is described as a per-protocol analysis, but the registry record does not report the criteria used to define that population, the number excluded from it, or the handling of participants who did not follow the assigned strategy. The enrollment entry is 1014, while the arm-specific primary analysis entries are 357.0 for screening and 344.0 for no screening. The record does not explain the relationship between the enrollment entry and the analysis populations, so those figures should not be treated as interchangeable.
The primary endpoint denominator includes scheduled visits with available results and excludes unscheduled visits. This makes result availability part of the endpoint’s construction. The registry does not report how missing scheduled results were handled, whether any imputation was used, or whether analyses assessed sensitivity to missing data. It also does not provide a comparison of result availability between arms. If availability differed by arm or was related to infection status, the observed rate could be affected; this is a general concern, not a claim that such a difference occurred.
The registry labels the primary analysis non-inferiority, but a non-inferiority margin, decision rule, and detailed statistical analysis plan are not reported in the registry record. Interim analyses and stopping rules are also not reported. Their absence from the record does not establish that none were used; it means their use cannot be evaluated from the listed information.
The record lists 4 outcome measures and 6 analyses, including secondary outcomes. It does not report whether multiplicity was controlled, which analyses were confirmatory, or whether the outcome hierarchy was prespecified. Without those details, multiple secondary results should be interpreted cautiously because repeated testing can increase the chance of a seemingly persuasive result arising by chance. The missing information also prevents a full assessment of how selectively reported analyses may relate to the trial’s planned comparisons.
The posted analysis is therefore best read as a reported per-protocol comparison with a defined visit-based denominator, alongside incomplete information about missingness, protocol adherence, and multiplicity. These limitations affect confidence in the interpretation without changing the numerical estimates that the registry reports.
12. What the Result Does, and Does Not, Mean
The posted result describes diagnoses per scheduled study visit with an available result, not the proportion of participants infected. The primary comparison includes gonorrhea and chlamydia detected at any site. It does not indicate how many participants had at least one infection, how often a participant had repeated diagnoses, or which infection or site contributed to the rate. Those distinctions are not reported for the primary outcome in the registry record.
The reported primary rate ratio is a relative comparison between no screening and screening. Its 95% confidence interval is above 1, and the arm-specific point estimate is higher for no screening. This is a description of the observed registry result. It does not establish the mechanism behind the difference. In particular, the trial compared not only screening with no screening but also different timing of laboratory result reporting and the actions described for the screening arm. The outcome may reflect detection processes as well as underlying infection occurrence.
The non-inferiority label should not be treated as a complete statement of the study’s conclusion. The registry record does not report the margin needed to judge the result against a prespecified non-inferiority criterion. A confidence interval can quantify uncertainty around an estimate, but without the criterion it cannot answer whether the intended threshold was met.
The result also does not establish effects on antimicrobial exposure, syphilis, or other outcomes. Those are listed as secondary outcomes, but the record does not provide full results for each. The symptomatic infection outcome is narrower than the primary outcome and has a different reported comparison. Neither outcome alone answers every question about the trial’s screening strategy.
Finally, trial findings apply most directly to the study population, procedures, outcome definitions, and analysis actually used. The registry describes men who have sex with men taking PrEP, multicentre randomization, screening at 3 sites every 3 months, and a per-protocol primary analysis. Broader generalization requires information about participant eligibility, adherence, result availability, and settings that is not fully reported. The posted estimates are useful for understanding the recorded comparison, but they are not a complete account of every clinical or public-health consequence.
13. Why This Trial Matters Statistically
This trial is statistically informative because its primary outcome is explicitly defined as a rate per scheduled visit with an available result. That unit prevents a common interpretive error: treating a visit-based rate as if it were a percentage of people. It also draws attention to the role of result availability in the denominator, which is important when interpreting comparisons of screening strategies.
The study also illustrates the distinction between an arm-specific rate and a comparative measure. The arm estimates describe the diagnostic rate in each group, while the rate ratio expresses their relative comparison. Confidence intervals convey the precision of those estimates. Reading these measures together is more informative than focusing only on a point estimate, although the registry’s missing design details constrain assessment of the non-inferiority label.
The trial’s open-label design and differing procedures for reporting results are relevant to the interpretation of a screening study. The screening arm had results sent to study physicians, with treatment and partner contact tracing described; the no-screening arm’s results were reported at the end of the study. Thus, the outcome reflects a comparison of strategies with distinct result-handling procedures, not just a laboratory comparison in isolation.
Finally, the record highlights why statistical conclusions depend on the analysis population, missing-data handling, and multiplicity plan as well as on estimated rates. The per-protocol designation is reported, but details needed to evaluate it are not. Secondary outcomes are listed, but full results and multiplicity information are not. These omissions limit how far readers can take the posted comparisons while leaving the reported estimates interpretable on their stated scale.
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.
Several trial-specific limitations follow from the registry information. The primary result is based on a per-protocol analysis, but the record does not define protocol adherence, explain exclusions, or provide the number of participants in that analysis. The enrollment entry and arm-specific analysis entries therefore describe different reported quantities, and their relationship is not explained.
The primary denominator is restricted to scheduled visits with available results. The record does not report the amount or pattern of missing results, how they were handled, or whether availability differed by arm. Because the outcome is a rate per visit, these omissions matter to interpretation of the observed comparison. Unscheduled visits are explicitly excluded, so the endpoint also does not represent all testing encounters.
The registry describes a non-inferiority analysis but does not report its margin or complete decision rule. The statistical method for the reported primary result is not identified. Details of randomization, allocation concealment, planned analyses, and any interim monitoring are not reported in the registry record. These gaps prevent independent appraisal of important design and analysis choices.
The primary outcome combines gonorrhea and chlamydia detections across sites. The record does not provide the primary result broken down by infection or anatomical site, nor does it report participant-level counts of people with infections. The open-label procedures also differ in result reporting and clinical actions between arms, as described in the record. These features should be kept in mind when interpreting a diagnostic rate as an outcome of screening.
Finally, the registry lists secondary outcomes for antimicrobial exposure and syphilis without reporting the relevant results in the available outcome details. The record therefore does not support a complete assessment of all listed outcomes, or of multiplicity across analyses.
15. Related Tutorials
16. Related Statistical Calculators
17. Sources
- ClinicalTrials.gov: GonoScreen: Efficacy of Screening STIs in MSM — NCT04269434.
- Registry record last updated 2024-10-16; status last verified 2024-10.
- Results first posted 2024-10-16.
- 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.
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18. Record Summary
GonoScreen was a completed, randomized, multicentre, parallel-group trial with no masking. It enrolled 1014 and compared screening with no screening among men who have sex with men taking PrEP, as described in the trial title. Screening involved urethral, pharyngeal and rectal sampling every 3 months; no-screening results were reported at the end of the study.
The primary outcome was gonorrhea or chlamydia diagnoses per scheduled study visit with an available result over 12 months, analyzed per protocol. The reported rates were 0.155 for screening and 0.205 for no screening. The registry’s non-inferiority analysis reported an incidence rate ratio of 1.318, with a 95% confidence interval from 1.068 to 1.627.
Interpretation is limited by missing registry entries about the non-inferiority margin, statistical methods, missing results, analysis populations, and multiplicity. The record also lists secondary outcomes, but does not report full results for all of them.