Apixaban Pharmacokinetics in Bariatric Patients (APB): Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Apixaban Pharmacokinetics in Bariatric Patients (APB) (NCT02406885), a non-randomized phase 4 study. The primary outcome is Change in Pharmacokinetics as Assessed by Cmax (Max Concentration).

2. Clinical Question

The central question is about drug exposure over time in a population undergoing bariatric surgery, rather than whether surgery or apixaban is superior to an alternative. Cmax is one pharmacokinetic measure: it describes the maximum concentration observed for a drug during a measurement interval. The primary outcome is the change in Cmax between baseline and 12 months, recorded in ng/ml.

The registry describes two surgical cohorts, vertical sleeve gastrectomy (VSG) and Roux-en-Y gastric bypass (RYGB), and identifies comparisons between them among the study objectives. Since allocation was non-randomized, these cohorts should not be read as randomized treatment groups. Differences between them could reflect differences in the people who underwent each procedure, in addition to any relation to the procedures themselves.

The registry's broader aims include assessing pharmacokinetic and pharmacodynamic changes and comparing the two procedures. A secondary outcome is change in Factor Xa levels, expressed as percent activity. The registry record does not report numerical results for Cmax or Factor Xa, so the direction and precision of any observed change cannot be described from this record.

3. Trial Design

APB, titled “Apixaban Pharmacokinetics in Bariatric Patients,” is listed as a Phase 4 study sponsored by Johns Hopkins University. The registry status is completed. The record gives a study start date of 2017-07-19 and a primary completion date of 2020-02-01, with enrollment of 33.

The registry categorizes allocation as non-randomized, masking as none, and the model as single group. Its arm descriptions nevertheless identify separate cohorts for VSG and RYGB. These details indicate that the study is not a masked randomized comparison; the record does not describe assignment to surgery or a parallel control group receiving a different drug or no drug.

The primary outcome is change in Cmax, in ng/ml, between baseline and 12 months. The stated primary aim is to assess durability or change in apixaban pharmacokinetics following one of the bariatric procedures. A secondary outcome listed in the record is change in pharmacodynamics assessed through Factor Xa levels, reported as percent activity. Other stated objectives include contrasting pharmacokinetic and pharmacodynamic measures between RYGB and VSG and assessing measures after significant postoperative weight loss. The complete details of those objectives and their timing are not reported in the registry record.

The record lists four outcome measures and zero analyses. It does not report the numerical primary endpoint results, estimates of variability, confidence intervals, p-values, or a detailed statistical analysis plan. Although the study status is completed, completion status alone does not establish that all outcomes are reported in the registry. The record does not provide enough information to determine how many participants contributed to each outcome or how missing measurements were handled.

Because the design is non-randomized, the surgical cohorts are descriptive groups rather than exchangeable groups guaranteed by random allocation. Baseline-to-follow-up measurement can describe change over time, but it does not by itself establish that surgery caused a change in apixaban exposure. The registry record does not report methods intended to address potential differences between the cohorts or other sources of confounding.

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

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 Change in Pharmacokinetics as Assessed by Cmax (Max Concentration), measured over Baseline and 12 months.

The primary endpoint is change in apixaban Cmax between baseline and 12 months. Cmax means maximum concentration, and the registry gives ng/ml as its unit. The endpoint is a continuous measure, with the registry identifying the mean and standard deviation as the summary form.

As a general pharmacokinetic concept, concentration measurements describe drug exposure in the body over time. Maximum concentration captures the peak level during a measurement interval; it does not by itself describe the full concentration-time profile. Measures such as total exposure or the time to peak concentration answer different questions, and the registry record does not identify them as the primary endpoint.

Comparing Cmax at baseline and 12 months is intended to assess whether the measured peak concentration changes over the period surrounding bariatric surgery. The registry's description frames this as a question about the durability or change in apixaban pharmacokinetics. A change in this measure is not, by itself, a direct measure of a clinical event or a complete account of pharmacodynamic response.

The record identifies the endpoint, unit, summary type, and timeframe, but does not report the observed means, standard deviations, change estimate, or uncertainty interval. It also does not report the exact measurement schedule or the statistical method used to compare baseline with 12 months. Accordingly, the endpoint's definition can be described, but the magnitude and precision of the observed change cannot be evaluated from the registry record.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

The registry describes a continuous primary outcome summarized using a mean and standard deviation. It names the endpoint and timeframe but does not report the analysis used to estimate change or the results. The methods below explain general statistical ideas relevant to this type of endpoint; they should not be read as a description of an unreported analysis plan.

What does a mean change represent?

For a continuous measure such as Cmax, a mean summarizes the average value in a group. A change score compares measurements at two times for the outcome definition in this study: baseline and 12 months. In general, a mean change describes the average direction and magnitude of change across the observations included in the calculation. It does not show whether every participant changed in the same direction, and it can conceal a wide range of individual responses.

The registry identifies change between the two time points but does not state whether the reported change would be calculated separately for each participant and then averaged, or estimated through another model. It does not describe whether values were transformed, how repeated observations were handled, or which participants were included. Those are important details for understanding the estimand—the precise quantity the analysis aims to estimate—and they are not reported in the registry record.

What does the standard deviation tell a reader?

As a general concept, the standard deviation describes spread among individual measurements around a group mean. It is not the same as the standard error, which describes uncertainty in an estimated mean, and it is not a confidence interval. A small or large standard deviation alone does not establish whether an estimated change is precise; precision also depends on the amount and pattern of available data and the analysis method.

No Cmax standard deviations or standard errors are reported in the registry record. Nor are confidence intervals provided. A confidence interval is, in general, a range of values produced by a specified method that conveys uncertainty around an estimate. Its width depends on the data and method; without an estimate and interval, the precision of the primary result cannot be judged.

Why distinguish within-cohort change from a procedure comparison?

A before-and-after description addresses how a measure differs across time in the observed participants. A comparison of change between VSG and RYGB asks whether the time patterns differ across cohorts. The latter is not equivalent to observing change in each cohort separately: two groups can each change while the difference between their changes is small, or they can have different changes. The registry describes both procedure cohorts and a comparative objective, but it reports no estimates for either interpretation.

Because allocation was non-randomized, any between-cohort contrast requires caution. In general, measured and unmeasured characteristics may differ between non-randomized groups. Statistical adjustment can address some measured differences if appropriate information and a suitable model are available, but it cannot automatically remove unmeasured confounding. The registry record does not report the covariates collected for this purpose or whether adjustment was performed.

What role do p-values and confidence intervals play?

In general, a p-value measures how incompatible the observed data are with a specified null model, under the assumptions of the statistical test. It does not measure the probability that a treatment works, nor does it convey the size or practical importance of an effect. A confidence interval adds information about the range of effect sizes reasonably compatible with the data under its method. Both require a defined analysis and an estimated result.

The registry record contains no primary-outcome p-value, confidence interval, or numerical effect estimate. The absence of these items means statistical significance and precision cannot be assessed from this record. It is not valid to infer that the result was null, statistically significant, or clinically important from the study status or endpoint description.

What is over-dispersion, and is it relevant?

As a general statistical concept, over-dispersion occurs when observed variability is greater than a model assumes. It is especially relevant to count or binary outcomes analyzed with models that impose a particular variance structure; failing to account for over-dispersion can make uncertainty estimates too narrow. The primary endpoint in this record is continuous, and no fitted model or residual diagnostics are reported. There is no basis to claim that over-dispersion was observed or that a particular correction was used for Cmax.

For continuous outcomes, analysts may instead need to consider skewness, outliers, unequal variability, repeated measurements, and influential observations, depending on the data and model. These are general considerations, not reported features of this study. The registry record does not provide distributions, diagnostic results, or details about transformations or robust methods.

How should planning illustrations be understood?

The record describes a planning illustration based on the observed effect, using a two-sided significance level of 0.05, equal allocation between the two arms, and a normal approximation. It states that the effect to detect is a specified fraction of the observed effect. For time-to-event outcomes, the illustration is based on total events; for other outcomes, it is expressed as participants per arm. These are planning assumptions, not evidence that the study was originally designed with those assumptions.

Any sample-size illustration is a conditional calculation: it depends on the chosen effect size, variability assumptions, allocation, significance level, and model. Such an illustration is not observed power and does not establish the chance that a completed study detected an effect. No numerical illustration results are reported in the registry record. The record also does not give a sample-size justification for the original trial design.

10. Confidence Intervals for Each Arm

No two-arm comparison is shown for this trial, but each arm's mean can be given with a t-based 95% confidence interval computed from the posted mean, spread and number of participants.

R code and output

arm <- c("APB Study: Apixaban Pharmacokinetics in VSG", "APB Study: Apixaban Pharmacokinetics in RYGB")
n   <- c(14, 14)
m   <- c(5.3, -6.7)
sp  <- c(19.98, 25.35)
sd  <- sp      # posted standard deviations
for (i in seq_along(n)) {
  se <- sd[i] / sqrt(n[i]); h <- qt(0.975, n[i] - 1) * se
  cat(sprintf("CARM|%s|%d|%.3f|%.3f|%.3f|%.3f\n", arm[i], n[i], m[i], sd[i], m[i] - h, m[i] + h))
}

CARM  APB Study: Apixaban Pharmacokinetics in VSG  14  5.300  19.980  -6.236  16.836
CARM  APB Study: Apixaban Pharmacokinetics in RYGB  14  -6.700  25.350  -21.337  7.937

11. Multiplicity, Missing Data and Analysis Populations

The registry lists four outcome measures and zero analyses. It names Cmax change as the primary outcome and Factor Xa change as a secondary outcome, while the complete details for the other listed measures are not reported in the registry record. It does not state how many hypotheses were tested, which outcome comparisons were prespecified for formal testing, or whether any multiplicity adjustment was used.

As a general statistical issue, evaluating multiple outcomes, time points, or subgroup comparisons can increase the chance of at least one apparently unusual result when each is assessed separately. Methods such as prespecifying a primary comparison or adjusting for multiple tests can help control that concern, depending on the question and analysis plan. No specific multiplicity strategy is reported for this study, so it cannot be assumed that the listed endpoints were tested with or without adjustment.

The record does not report missing-data counts for the primary endpoint, how incomplete follow-up was handled, or whether participants with incomplete measurements were included in an analysis. It does not state whether an imputation method, complete-case analysis, or sensitivity analysis was used. These details matter because the amount and pattern of missing measurements can affect both the estimate and its uncertainty.

Analysis populations are also not described. The record does not say whether the primary analysis included all enrolled participants, only those with paired baseline and 12-month measurements, or another defined population. The enrollment figure alone cannot establish how many participants contributed to each outcome. With no analysis population or missing-data method reported, the precision and representativeness of any unposted result cannot be assessed.

12. Safety Results

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

Deaths

Other (non-serious) adverse events

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

Most frequent serious adverse event terms

Most frequent non-serious adverse event terms

Serious events with the highest arm-level rate: sudden cardiac death (6.2%), bowel obstruction (6.2%), pulmonary emboli (5.9%).

The registry lists serious adverse event figures separately for the RYGB and VSG cohorts. For RYGB, 17 were at risk for serious events, with 1 affected; for VSG, 16 were at risk, with 2 affected. The deaths field records 0 for RYGB and 1 for VSG. These figures are registry counts, not estimates of comparative risk, and they do not establish that a procedure or apixaban caused an event.

For other adverse events, the record lists 17 at risk and 1 affected in RYGB, and 16 at risk and 1 affected in VSG. The registry's listed top serious-event terms are Sudden Cardiac Death at 6.2%, Bowel Obstruction at 6.2%, and Pulmonary Emboli at 5.9%. The record does not make clear in these figures how the terms map to the group-specific counts, and no event-level causal assessment is reported.

As a general principle, adverse-event counts need context about definitions, observation periods, ascertainment, and denominators. The registry record does not report those details fully, nor does it provide a randomized comparison. The listed counts therefore describe the safety information recorded, but do not support a causal or comparative safety conclusion.

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

The APB registry record identifies a focused pharmacokinetic question: whether apixaban Cmax changes between baseline and 12 months in people undergoing bariatric surgery, with VSG and RYGB cohorts described. It also identifies a secondary pharmacodynamic measure, Factor Xa levels. These features clarify what the study set out to examine, but the registry record does not report the numerical primary or secondary results.

Consequently, the posted information does not establish the direction or size of Cmax change, its statistical precision, or whether the two surgical cohorts had different patterns. No conclusion about the size or uncertainty of a change can be recovered from the study title, completed status, outcome definition, or enrollment figure. The actual result is not reported in the registry record.

Even if a before-and-after change were reported, it would describe change over time in the measured participants, not automatically establish that surgery caused the change. The study is non-randomized and the registry does not describe a concurrent control group. A difference between the surgical cohorts would likewise not be a randomized comparison and could reflect group characteristics or other factors.

Pharmacokinetic and pharmacodynamic measurements are not the same as clinical outcomes. Cmax describes a peak concentration, while Factor Xa activity is a pharmacodynamic measure. The record does not provide clinical outcome results linking these measures to a treatment benefit or harm. Nor does it report enough methods detail to assess missing measurements, analysis populations, or adjustment for confounding.

The safety figures are descriptive registry entries and do not identify the cause of the listed events. They should not be interpreted as proof that apixaban or either procedure caused an event, or as a reliable comparative estimate between procedures. Overall, the registry describes the design and questions of a completed study, but leaves the main efficacy-related statistical interpretation unresolved because its endpoint results and analysis details are not reported.

14. Why This Trial Matters Statistically

Pharmacokinetic studies ask how drug exposure behaves in a particular population and context. Bariatric surgery can make the question especially relevant to researchers because the study's stated objective is to examine whether apixaban exposure changes before and after surgery. The APB record focuses on this question using Cmax and includes two different bariatric procedures, along with a pharmacodynamic outcome based on Factor Xa activity.

Statistically, the study illustrates why outcome definition and design both matter. A defined continuous endpoint and timeframe make the target question understandable: Cmax change from baseline to 12 months. But a before-and-after outcome and a non-randomized comparison between surgical cohorts have different interpretive limits from a randomized treatment comparison. Change over time and differences between procedures should not be conflated.

The study also highlights the value of complete results reporting. An endpoint name is not enough to judge an effect. Readers need an estimate, uncertainty information, clear analysis populations, and missing-data methods to assess what the data show and how precisely. The registry record does not report these details for the primary outcome, so the question can be understood while the result remains unevaluable from the registry.

For biostatisticians and clinical researchers, this distinction is important: a completed study may have an informative design and still have an incomplete public statistical record. The registered objectives help identify what was intended to be measured; transparent estimates and methods are needed to determine what the observed data support. In this case, that separation prevents design details and safety counts from being mistaken for a reported primary finding.

15. 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 record has important limitations for interpreting the study. It describes the study as completed but does not report the primary Cmax change estimate, its standard deviation, confidence interval, or p-value. As a result, the magnitude, direction, and precision of the primary outcome cannot be assessed.

Although the record identifies baseline and 12 months as the primary outcome timeframe, it does not report the exact measurement schedule, the analysis model, or how repeated measurements were handled. The analysis population, number contributing data to each outcome, and handling of missing observations are also not reported. Enrollment does not establish the number analyzed for any specific endpoint.

The study is non-randomized, and the record identifies RYGB and VSG cohorts. It does not report methods for adjusting for differences between those cohorts, so any comparison would require particular caution. The registry categorizes the model as single group while also describing two procedure cohorts; the record does not further clarify that design classification.

Four outcome measures are listed, but only the primary Cmax outcome and one secondary Factor Xa outcome are described in the available record. Details of other measures, multiplicity handling, prespecified comparisons, and a statistical analysis plan are not reported. The safety entries also lack full detail on ascertainment and event attribution. These limitations mean the registry record can describe the intended question and selected safety counts, but cannot support a complete assessment of the study's statistical findings.

16. Related Tutorials

17. Related Statistical Calculators

18. Sources

• ClinicalTrials.gov: Apixaban Pharmacokinetics in Bariatric Patients (APB) — NCT02406885.
• Registry record last updated 2020-12-22; status last verified 2020-12.
• Results first posted 2020-12-22.
• 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.

19. Record Summary

APB is a completed Phase 4, non-randomized study of apixaban pharmacokinetics in bariatric patients, sponsored by Johns Hopkins University. The registry lists enrollment of 33 and identifies VSG and RYGB cohorts. The study start date is 2017-07-19, and the primary completion date is 2020-02-01.

The primary outcome is change in Cmax, measured in ng/ml, between baseline and 12 months. A secondary outcome is change in Factor Xa levels, expressed as percent activity. The record lists four outcome measures and zero analyses, but does not report numerical results for the primary endpoint or the statistical methods used to analyze it.

Serious and other adverse-event counts are recorded by cohort, alongside several listed serious-event terms. The registry does not provide sufficient information to interpret those entries causally or as a randomized safety comparison. Overall, the record defines the pharmacokinetic question and study structure, while leaving the primary result and its precision unreported.