Effects of Sitagliptin on Endothelial Function in Type 2 Diabetes on Background Metformin Therapy: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Effects of Sitagliptin on Endothelial Function in Type 2 Diabetes on Background Metformin Therapy (NCT01859793), a randomized phase 4 study. The primary outcome is Brachial Artery Flow Mediated Dilation.

2. Clinical Question

The registry describes a randomized, double-blind crossover study in people with type 2 diabetes who were on metformin. The main statistical opportunity in a crossover design is to compare each participant’s response under the two treatments, rather than relying only on differences between separate treatment groups. That structure can help account for stable differences between participants, although it also makes the handling of period, sequence, and possible carryover effects important.

The endpoint is described as change in brachial artery flow mediated dilation before and after a single dose, with the post-dose measurement at 2 hours, and after 8 weeks of daily dosing. The registry record does not report the numerical changes or a treatment comparison. It therefore does not establish from the posted results whether the observed responses differed, or how precisely any difference was estimated.

3. Trial Design

This was a randomized, double-blind, crossover study, listed as phase 4. Enrollment was 38, and the registry status is completed. The record identifies the Medical College of Wisconsin as the sponsor. The study began in 2013-06 and its primary completion was 2016-01.

Participants were to receive a sitagliptin pill once daily for 8 weeks and a matching placebo pill during a separate 8 weeks. The treatment periods were separated by a 4 week period. The study description also specifies ultrasound assessments before and after a single dose of each treatment, as well as after the daily-treatment periods. Blood was collected at the ultrasound assessment times to measure inflammatory markers. The two named secondary outcomes are circulating ICAM-1 and circulating VCAM-1.

In a crossover trial, treatment is compared within participants because participants receive both treatments in different periods. Random allocation helps determine treatment sequence, while masking is intended to limit expectations from influencing care or measurement. Those design features do not by themselves show how the analysis handled treatment sequence, period effects, or the possibility that a treatment effect persisted into the next period.

The primary outcome population is described as all 30 subjects who completed both arms. The registry does not give a detailed account of participants who did not complete both arms, why they did not complete them, or how their data were handled. Although the trial is marked completed, the numerical primary endpoint results and detailed statistical analysis plan 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.

First Intervention (8 Weeks)

Reasons for not completing this period:

Washout Period (4 Weeks)

Reasons for not completing this period:

2nd Intervention Period

Analysis population for the primary outcome, as defined in the registry: All 30 subjects who completed both arms of the cross-over study

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 Brachial Artery Flow Mediated Dilation, measured over Change before and after a single dose (2 hours post) and 8 weeks after daily dosing.

The primary endpoint is brachial artery flow mediated dilation, reported as %FMD. It is a continuous measurement, with the registry describing the summary parameter as a mean and the dispersion as a standard deviation. The endpoint description calls it a measurement of endothelial function in humans.

In general, flow mediated dilation assesses the change in the diameter of an artery following a stimulus that increases blood flow. It is used as a physiological measure related to blood-vessel function. A continuous endpoint can retain information about the size of measured responses rather than reducing them to categories such as improvement or no improvement.

The registry specifies measurements before and after a single dose, with the post-dose assessment at 2 hours, and after 8 weeks of daily dosing. These represent different assessment times, so a treatment comparison needs to make clear which time point or change is being summarized. The registry record notes that the primary outcome has several categories or time points and is not reduced to one comparison. It does not report the mean values, standard deviations, treatment contrasts, confidence intervals, or p-values for these assessments. As a result, the endpoint’s definition is available, but the size and precision of the observed treatment response are not.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

What does a crossover design compare?

In general, a crossover design assigns each participant to receive each treatment in a sequence. Because the same participant contributes observations under both treatments, a paired analysis can compare the participant’s response under sitagliptin with that participant’s response under placebo. This can reduce the influence of stable participant characteristics on the comparison. It does not remove the need to account for changes over time or for effects that may carry from one treatment period into another.

The registry describes randomized allocation, double masking, and a crossover model, with a 4 week period separating the 8-week treatment periods. It does not report the randomized sequences, the analytic method used to model the crossover, or whether the analysis assessed period, sequence, or carryover effects. Those details are important in general because a crossover estimate can be affected if responses differ by period or if effects of an earlier treatment persist into a later period.

How should a continuous endpoint be summarized?

The primary endpoint is continuous, and the registry specifies a mean with standard deviation as the summary and dispersion. In general, the mean describes the center of measured values, while the standard deviation describes their spread around that mean. For a treatment comparison, readers would also need the estimated contrast between treatments and a measure of its uncertainty. A confidence interval is one way to express that uncertainty; it indicates a range of effect values compatible with the observed data under the analysis assumptions. The registry record does not report these numerical summaries for the primary endpoint.

A paired treatment contrast and a change from baseline are distinct quantities. The registry description includes changes before and after dosing, but the record does not report the exact contrast used for each assessment or the estimates. Without those details, it is not possible to determine whether a posted value represents change within a treatment period, a difference between treatments, or another summary.

Why do assessment times matter?

The study includes an acute assessment after a single dose and a chronic assessment after 8 weeks of daily dosing. In general, those assessments address different temporal questions. A single-dose measurement concerns an immediate response at the specified post-dose time, while a measurement after repeated dosing concerns response after an extended treatment period. The registry notes multiple categories or time points and does not reduce the primary endpoint to one comparison. It does not report a hierarchy of time points or identify which comparison should be treated as the principal inferential result.

What does dispersion tell the reader?

Standard deviation describes variability among measured observations; it is not the same as the uncertainty in a treatment-effect estimate. In general, an estimate’s precision also depends on the analysis and the amount of usable information. In crossover data, within-participant variability is particularly relevant to a paired comparison. The registry identifies standard deviation as the dispersion measure but does not report its endpoint values or a paired treatment estimate.

How should significance and precision be distinguished?

In general, a p-value assesses how compatible the data are with a specified null hypothesis under a statistical model. It does not show the size of a treatment effect, and it is not the probability that the null hypothesis is true. A confidence interval adds information about the range and precision of an estimate. Neither a p-value nor a confidence interval for the primary endpoint is reported in the registry record, so a significance or precision judgment cannot be made.

What is the role of the sample-size illustration?

The sample-size illustration is described as planning based on the observed effect, not as the trial’s own design. Its stated assumptions are a two-sided significance level of 0.05, equal allocation between two arms, and a normal approximation; the effect to detect is a stated fraction of the observed effect. In general, such calculations illustrate the participant count that might be needed under their assumptions. They are not observed power and do not establish that the completed trial was designed with those assumptions. The registry record does not report the trial’s sample-size justification or a detailed original statistical plan.

Why can over-dispersion matter?

In general, over-dispersion means that observed variability is greater than a statistical model expects. If it is ignored, uncertainty estimates can be too small and inference can appear more precise than warranted. The primary endpoint is a continuous measure, and the registry specifies a standard deviation; it does not report an over-dispersion assessment or indicate that such a model was used. This general concept should not be taken as a description of an analysis performed in this trial.

10. Multiplicity, Missing Data and Analysis Populations

The primary endpoint is recorded across several categories or time points, including assessments after a single dose and after 8 weeks of daily dosing. The registry explicitly does not reduce these observations to one comparison. The record also lists circulating ICAM-1 and circulating VCAM-1 as secondary outcomes. Considering several outcomes or time points can raise the chance of obtaining at least one apparently notable result when many tests are performed. In general, multiplicity procedures can address that issue by controlling error rates or by defining a hierarchy of comparisons.

The registry record does not report whether multiplicity was addressed, which comparisons were prespecified as primary, whether any testing hierarchy was used, or how the analyses were adjusted. It also does not report numerical results for the primary or secondary outcomes. Without the estimates and analysis details, the strength of evidence across the outcome set cannot be assessed.

The primary outcome population is described as all 30 subjects who completed both arms. That is not enough information to establish how missing data were handled. The registry record does not report the number of missing endpoint measurements, reasons for missingness, imputation methods, sensitivity analyses, or whether participants with incomplete treatment periods were included in any analysis. It also does not specify whether the analysis followed an intention-to-treat principle or another analysis population definition.

In general, missingness matters when the probability of an unobserved measurement is related to the unobserved value or treatment response. Methods can address missing data under stated assumptions, but those assumptions and the method used need to be reported for interpretation. The registry record does not provide that information. It also does not report interim analyses or a detailed statistical analysis plan.

11. 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.

Most frequent non-serious adverse event terms

The registry safety record reports serious adverse events for the matching placebo group as 0 affected among 36 at risk, and for the sitagliptin group as 0 affected among 32 at risk. For other adverse events, it reports 1 affected among 36 at risk in the matching placebo group and 0 affected among 32 at risk in the sitagliptin group.

These are recorded counts with group-specific denominators. They should not be combined into an overall total or treated as a direct estimate of comparative risk without more information about event definitions, observation periods, and analysis. The safety denominators are not the same as the primary outcome population description of all 30 subjects who completed both arms, and the registry record does not explain the relationship between those populations.

The serious adverse event terms are not listed. Death counts are not reported in the registry record. The registry’s counts therefore give a limited description of recorded events, not a complete account of event types, timing, severity, or comparative safety. Zero recorded serious events in these groups does not, by itself, establish that no serious event could occur or that the treatments have equivalent safety.

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

The record describes a randomized, double-blind crossover investigation of sitagliptin and matching placebo in people with type 2 diabetes receiving metformin. Its primary endpoint was brachial artery flow mediated dilation, measured after a single dose and after 8 weeks of daily dosing. The record gives the endpoint definition and identifies the population described for that outcome.

It does not provide the numerical primary endpoint results. Consequently, the treatment effect’s direction, magnitude, and precision cannot be determined from the registry record. The record does not support a conclusion about whether the measured endpoint differed between treatments, and the absence of results should not be interpreted as evidence that there was no difference.

Even if a result were available, a physiological measurement would need to be interpreted as that measure, not as proof of a change in clinical outcomes. General interpretation of any treatment estimate would also depend on the selected time point, the crossover analysis, missing observations, multiplicity, and uncertainty around the estimate. Those details are not reported sufficiently to resolve them.

The safety counts are limited observations: the registry records no serious adverse events in either group and reports other adverse events as 1 in the placebo group and 0 in the sitagliptin group, using their stated denominators. They do not establish overall safety or compare all possible harms. The record does not report death counts or the serious event terms.

Overall, the registry record allows readers to understand the study question and broad design, but it does not contain the primary outcome estimates needed to judge the observed endothelial-function response. The reported design and safety counts should be distinguished from evidence about treatment efficacy or complete safety.

13. Why This Trial Matters Statistically

This trial is statistically notable for using a crossover design to study a continuous physiological endpoint. In general, crossover studies can make within-participant comparisons, which may reduce the influence of stable differences between participants. At the same time, interpretation depends on the sequence and timing of treatments and on whether period or carryover effects are addressed. The registry identifies the crossover structure and the interval between treatment periods, but does not report the corresponding analytic details.

The study also includes assessments at different times: after a single dose and after 8 weeks of daily treatment. Separating acute and longer-term measurements matters because they represent different questions and should not be collapsed into one result without an explicit analysis plan. The registry notes multiple categories or time points but does not identify a single comparison as the primary reported contrast.

The record gives a defined primary outcome population of participants who completed both arms, while leaving missing-data handling and the analysis population details unreported. That gap is important in a crossover study, where incomplete treatment sequences can affect which within-person comparisons are available.

Finally, the registry record describes the outcome and design but does not report the primary results. Statistical interpretation requires estimates and uncertainty, not design features alone. This distinction is useful for readers assessing what a trial record can establish: a clear research question and a potentially informative design do not replace outcome reporting, multiplicity details, or transparent handling of incomplete data.

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.

A major limitation of interpretation is that the registry record does not report numerical results for the primary endpoint. It gives neither mean values nor standard deviations, treatment contrasts, confidence intervals, or p-values. Therefore, the observed endothelial-function response and its precision cannot be assessed.

The primary endpoint includes several assessment categories or time points, and the record does not reduce them to one comparison. Although it describes measurements after a single dose and after 8 weeks of daily dosing, it does not identify a prespecified hierarchy or the exact contrast used at each time. This makes it difficult to determine what result would be the principal test of the study question.

The endpoint population is described as all 30 subjects who completed both arms. The registry record does not explain the reasons for non-completion, how many measurements were missing, how incomplete crossover data were handled, or whether any imputation or sensitivity analysis was performed. It also does not report the method used to account for period, sequence, or carryover effects.

The record lists two inflammatory-marker secondary outcomes but does not report their numerical results or whether multiplicity was addressed. The safety information is also limited: event terms are not listed for serious events, death counts are not reported, and the relationship between safety denominators and the primary outcome population is not explained. These limitations constrain interpretation of both efficacy and safety from the registry record.

15. Related Tutorials

16. Related Statistical Calculators

17. Sources

• ClinicalTrials.gov: Effects of Sitagliptin on Endothelial Function in Type 2 Diabetes on Background Metformin Therapy — NCT01859793.
• Registry record last updated 2016-10-21; status last verified 2016-08.
• Results first posted 2016-10-21.
• 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 record describes a completed phase 4, randomized, double-blind crossover study sponsored by the Medical College of Wisconsin. Enrollment was 38. The study compared sitagliptin with matching placebo in people with type 2 diabetes receiving metformin, using separate 8-week treatment periods divided by a 4 week period.

The primary outcome was brachial artery flow mediated dilation, assessed after a single dose and after 8 weeks of daily dosing. The outcome population is described as all 30 subjects who completed both arms. Secondary outcomes were circulating ICAM-1 and VCAM-1. The record does not report the numerical primary or secondary outcome results or detailed analysis methods.

Safety counts report no serious adverse events in either group, and other adverse events of 1 in the matching placebo group and 0 in the sitagliptin group, with the stated group-specific denominators. Death counts are not reported in the registry record.