Pilot Study of Using Copeptin to Predict Response to Tolvaptan: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Pilot Study of Using Copeptin to Predict Response to Tolvaptan (NCT01346072), a na phase 4 study. The primary outcome is Urine Output.

2. Clinical Question

The clinical question is about association and prediction within a treated group. It is not a test of tolvaptan against placebo or another therapy. Because everyone in the treatment phase received tolvaptan, any difference between copeptin strata would describe variation in response among treated patients; it would not establish what urine output would have been without treatment.

The study summary describes selection aimed at the upper and lower quartiles of copeptin at screening, while the analysis strata were based on a prospectively determined threshold applied to a blinded baseline copeptin value obtained before tolvaptan. These are related but distinct descriptions of selection and analysis. The registry record does not provide the measured copeptin values or the primary urine-output results, so the size and precision of any association cannot be assessed from this record.

3. Trial Design

This completed phase 4 pilot study was a prospective, single-group, open-label intervention study sponsored by the University of North Carolina, Chapel Hill. The registry lists enrollment of 21 and an allocation value of NA. The study was not randomized and did not include a separate control-treatment arm. Its design instead compared two copeptin-defined subgroups within the group receiving tolvaptan.

The summary describes stable outpatients with congestive heart failure and reduced left ventricular ejection fraction, with an ejection fraction at most 45. Recruitment targeted upper and lower quartile copeptin levels at screening. In the treatment phase, patients received a single oral dose of 30 mg of tolvaptan. Body weight, fluid intake, and urine output were monitored in a research unit for 24 hours.

For analysis, the single intervention arm was stratified according to a prospectively determined copeptin cut point: 10 pmol/L or above versus below 10 pmol/L. The copeptin value used for this classification was described as a blinded baseline value obtained during the hospital phase before tolvaptan administration. Masking is listed as none, so the trial was open label; the description of a blinded copeptin result does not establish that other trial procedures or outcome assessments were blinded.

The registry lists urine output as the primary outcome, expressed as a mean with standard deviation over 24 hours. The outcome population description says that all patients in the low-copeptin group were included. It also describes a patient in the high-copeptin group with reduced renal function at baseline relative to screening and transient hypovolemia during hospitalization, who was excluded before review of blinded data. The remaining text of that population description is incomplete, so the full analysis-population rule is not reported in the registry record.

The registry reports a completed study, with a start date of 2011-04 and primary completion in 2014-02. It does not report a randomized treatment contrast, and the copeptin groups should not be read as equivalent to randomized arms. The primary outcome values and their uncertainty are not reported in the registry record.

4. Participant Flow and Accounting

The registry reports how participants moved through each stage of the study. All counts below are exactly as posted; missing counts are not inferred from other stages.

Before assignment: Blinded copeptin results were reviewed in 43 patients for participation in the hospital phase. 11 patients were selected by targeting the upper quartile of baseline copetin and 10 patients by targeting the lower quartile of screening copeptin levels. A total of 21 patients were enrolled in hospital phase.

Overall Study

Analysis population for the primary outcome, as defined in the registry: All patients in the Low Copeptin group were included in the analysis. One patient in the High Copeptin group had reduced renal function at baseline versus screening and transient hypovolemia during hospitalization indicative of volume depletion. This patient was excluded prior to review of blinded copeptin results and study data analysis.

5. Baseline Characteristics

The table reproduces the baseline characteristics posted in the registry, which describe the participants analysed at baseline and help show whether the randomized groups were comparable.

Population: All patients in Low Copeptin group were included in analysis. One patient in High Copeptin group had reduced renal function at baseline versus screening and transient hypovolemia during hospitalization indicative of volume depletion. This patient was excluded prior to review of blinded copeptin results and study data analysis for a final N of 20.

6. Primary Endpoint

The registry lists the primary outcome as Urine Output, measured over 24 hours.

The primary endpoint is total urine output during the 24 hours following administration of tolvaptan, measured in milliliters. It is a continuous outcome summarized as a mean and standard deviation. Urine output was monitored alongside body weight and fluid intake during the research-unit observation period, but those additional monitored measures are not identified as the primary endpoint in the registry record.

Urine output is used in this study as an observable measure of the aquaretic response, meaning the change in water excretion associated with blocking vasopressin action. The study question is whether baseline copeptin level is associated with variation in this response. Comparing the mean urine output of the copeptin-defined groups could describe whether their observed responses differed, while the standard deviation conveys variation among the observations within a group.

That interpretation depends on the actual results, which are not reported in the registry record. The record contains no group means, standard deviations, estimated contrast, confidence interval, or test result for the primary endpoint. Therefore, it does not allow assessment of the direction, magnitude, or precision of the observed urine-output difference. Nor does the endpoint alone establish a causal effect of copeptin or show how urine output would compare under a different treatment or no treatment.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

The study’s primary endpoint is continuous: total urine output in milliliters during the 24 hours after tolvaptan administration. The registry specifies a mean and standard deviation. A mean describes the average observed outcome in a group. A standard deviation describes how spread out individual observations are around that group’s mean. These summaries are useful for describing the response distribution, but they do not by themselves quantify uncertainty in the difference between groups. The registry record does not report the endpoint summaries or a contrast estimate.

What does a copeptin-stratified comparison estimate?

The analysis compares patients classified by baseline copeptin: 10 pmol/L or above versus below 10 pmol/L. Since both groups received the same single-dose intervention, the comparison concerns observed response across biomarker strata. It is not a randomized estimate of tolvaptan versus an untreated or alternative-treatment group. A group contrast can describe association, but patients in the strata may differ in other ways. Without randomization to strata and without reported adjustment details, causal interpretation is not justified by the design description alone.

The threshold creates a categorical comparison from a biomarker that may otherwise vary continuously. Categorization can make results easier to communicate, but it also means the comparison does not describe the full relationship between copeptin and urine output across all possible values. The registry does not report a continuous dose-response analysis or the observed copeptin distribution. The stated cut point is prospective, and the value used for classification is described as blinded and measured at baseline before tolvaptan. These details help define the intended grouping, but they do not replace the missing endpoint results.

Why does the standard deviation matter?

Variation in urine output is relevant because patients can have different responses even within the same copeptin stratum. A relatively large standard deviation indicates greater spread of observed values around the mean; a relatively small one indicates less spread. If individual responses vary substantially, a difference between group means may be estimated less precisely. The standard deviation is not a confidence interval and does not, on its own, show how precisely the mean or between-group contrast has been estimated.

A confidence interval, when reported, would give a range of values compatible with the data and the specified analysis under its assumptions. Its width communicates precision: a narrower interval indicates a more precise estimate than a wider one. The registry record does not report a confidence interval for the primary outcome or an estimated group difference, so the precision of the comparison cannot be assessed. General statistical interpretation also depends on the analysis model and assumptions, which are not fully described in the available record.

What does the design imply for inference?

The open-label, single-group design has no randomized control arm. That means it is suited to describing the response observed after treatment and exploring whether it varies by baseline copeptin. It does not isolate treatment effects from changes that might occur over time or from other influences. Within-arm stratification does not confer the protection against confounding that random assignment can provide. A numerical association would therefore need to be described as an association in this cohort, not as proof that copeptin itself causes a different response.

The record lists enrollment of 21, while the safety listings identify 11 patients at risk in the high-copeptin group and 10 in the low-copeptin group. These are the group-specific counts reported for the adverse-event entries. They should not be assumed to define the endpoint analysis population: the primary endpoint description notes an exclusion in the high group, and its full population text is incomplete. The number of endpoint observations analyzed is not reported in the registry record.

How should planning illustrations be understood?

A sample-size illustration based on an observed effect is a planning exercise, not an outcome of the trial and not observed power. The stated assumptions describe a two-sided significance level of 0.05, equal allocation between two arms, a normal approximation, and a target effect expressed as a fraction of the observed effect. For time-to-event outcomes, the general planning approach described uses a total event count; for other outcomes, it gives participants per arm. The primary endpoint in this study is continuous rather than time-to-event.

Such an illustration depends on the observed effect estimate and the assumptions used for planning. It does not establish that the study had a particular probability of detecting an effect after the data were observed. In this record, the primary endpoint effect itself is not reported, so no numerical planning illustration can be interpreted as evidence about the trial’s observed result. The registry also does not report a sample-size justification for the original study, so the planning assumptions should not be mistaken for the original design rationale.

What analysis details are unavailable?

The available information identifies the outcome type, summary measure, time frame, copeptin grouping, and a specific exclusion described in the population text. It does not report a full statistical analysis plan, whether any covariates were included, the precise statistical test or model, handling of missing endpoint observations, or an endpoint estimate with uncertainty. Those details are not reported in the registry record. Without them, the analysis cannot be independently reconstructed or fully evaluated from the record alone.

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("Tolvaptan High Copeptin", "Tolvaptan Low Copeptin")
n   <- c(10, 10)
m   <- c(6498, 5784)
sp  <- c(2328, 1835)
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  Tolvaptan High Copeptin  10  6498.000  2328.000  4832.649  8163.351
CARM  Tolvaptan Low Copeptin  10  5784.000  1835.000  4471.320  7096.680

11. Multiplicity, Missing Data and Analysis Populations

The registry lists two outcome measures and two analyses, but the available record does not provide enough detail to determine how the analyses were prioritized beyond identifying urine output as the primary outcome. It does not report whether statistical testing was adjusted for multiple outcomes or multiple comparisons, whether a hierarchy was prespecified, or whether any exploratory analyses were conducted. The multiplicity approach is not reported in the registry record.

Multiplicity matters when several outcomes or comparisons are tested because the chance of obtaining at least one apparently notable result can increase as the number of tests grows. This is a general statistical concern, not evidence that the study’s findings are false. Without the analysis plan and test results, it is not possible to assess how multiplicity was handled or whether it affected interpretation.

The primary outcome population description says that all patients in the low-copeptin group were included in the analysis. It also describes a high-copeptin patient with reduced renal function at baseline compared with screening and transient hypovolemia during hospitalization who was excluded before review of blinded data. The text ends before the full population description is complete. The complete analysis population and the rationale for any further exclusions are not reported in the registry record.

The registry does not report how missing urine-output data were handled, whether any endpoint values were unavailable, or whether sensitivity analyses assessed the effect of exclusions or missingness. It also does not provide a full participant flow from screening through endpoint analysis. These omissions prevent assessment of whether the analyzed patients differed systematically from those not included in the primary analysis. The reported safety counts do not resolve this issue, because the safety risk sets are not necessarily the same as the primary endpoint analysis population.

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 5% reporting threshold.

The adverse-event listing reports the numbers at risk and the number affected in each copeptin group. For the high-copeptin group, 11 were at risk, with 0 serious adverse events, 0 other adverse events, and 0 deaths listed. For the low-copeptin group, 10 were at risk, with 0 serious adverse events, 0 other adverse events, and 0 deaths listed. No top serious-event terms are listed.

These entries should be read as the event counts recorded in the registry, not as proof that the intervention cannot be associated with adverse events. A zero count in a small pilot study leaves substantial uncertainty about the frequency of uncommon events. The registry record does not provide additional safety detail such as event timing, severity categories beyond the serious-versus-other listing, or the method used to collect adverse events.

The copeptin groups are not randomized treatment arms. Their safety counts do not support a controlled comparison of treatments, and they should not be interpreted as showing that one copeptin stratum is safer than another. The listed numbers at risk also should not automatically be treated as the primary endpoint analysis denominators, because the urine-output population description notes an exclusion in the high group and is incomplete.

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

This study’s statistical purpose was to examine whether baseline copeptin level was associated with the 24-hour urine-output response observed after a single dose of tolvaptan. The design used a single treated group divided into high- and low-copeptin strata. It was a pilot investigation, not a randomized comparison of tolvaptan with another treatment or with no treatment.

The registry record does not report the primary urine-output results. No group means, standard deviations, difference estimate, confidence interval, or p-value are given. Consequently, the posted record does not establish the direction or size of an association, or how precisely it may have been estimated. Any numerical conclusion about the primary endpoint would go beyond what is reported.

If a difference between copeptin groups were available, it would describe an observed contrast among patients who received tolvaptan. Because copeptin strata were not randomized and there was no control-treatment arm, the contrast would not by itself show that copeptin caused the response difference, nor would it estimate the effect of tolvaptan relative to an alternative. Other patient characteristics and chance could contribute to an observed contrast.

The study’s selection approach also affects generalizability. Screening targeted upper and lower quartile copeptin levels, and the analysis used a threshold of 10 pmol/L. The registry does not report enough information about the enrolled copeptin distribution or screening flow to establish how representative the cohort was of patients with heart failure more broadly. Results from this design would therefore need interpretation in the context of its selected population and setting.

The safety listing records zero serious adverse events, zero other adverse events, and zero deaths in both copeptin groups. These are reassuringly sparse recorded counts in the registry, but they do not demonstrate absence of risk or characterize uncommon events. In summary, the record identifies a clinically focused pilot question and describes how the groups and endpoint were defined. It does not report the outcome estimates needed to determine what association was observed or how precise it was.

14. Why This Trial Matters Statistically

The study is statistically interesting because it links a baseline biomarker to a measured response after an intervention. Rather than asking only whether a treatment changes an outcome on average, it explores whether response varies across patients according to copeptin. This is a prediction-oriented question: a biomarker may help identify variation in response, but that possibility must be supported by reported estimates and evaluated in appropriate populations.

The single-arm structure makes the distinction between association and treatment effect especially important. A comparison between copeptin strata can describe response heterogeneity among patients receiving tolvaptan. It cannot establish that copeptin modifies the effect of tolvaptan relative to another treatment, because no randomized treatment contrast is included. That distinction is central when interpreting biomarker-stratified analyses.

The endpoint is a quantitative measure collected over a defined 24-hour period, and the record specifies mean and standard deviation summaries. This provides a clear framework for describing response magnitude and variability. Yet those summaries and the contrast between groups are absent from the registry record. The example therefore also illustrates why a well-defined endpoint and a clear group definition are not enough for statistical interpretation without results and measures of uncertainty.

The population description raises another useful issue: analysis exclusions need to be transparent and complete. The record describes a clinically relevant exclusion in the high-copeptin group, but its full analysis-population text is incomplete. Without a complete account of the endpoint population and missing-data handling, readers cannot fully evaluate the consequences of that exclusion.

Finally, the zero event counts illustrate the limits of safety interpretation in a pilot study. Such counts are observations from the reported risk sets; they do not establish that adverse events are impossible. Taken together, the design, biomarker grouping, continuous endpoint, and incomplete results make the record a useful case for separating a study’s question from what can actually be concluded from its reported statistics.

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 most consequential limitation for interpretation is that the registry record does not report the primary outcome results. Without urine-output summaries or an estimated contrast and its uncertainty, the association under study cannot be quantified. The record therefore supports description of the question and design, but not a conclusion about the observed primary endpoint.

The study has no randomized control arm. Both copeptin groups received tolvaptan, so the design cannot estimate a treatment effect relative to placebo or another intervention. Since copeptin strata were not randomized, observed differences between them could reflect other patient characteristics as well as copeptin-related variation. The available record does not report adjustment details that would clarify whether measured covariates were considered.

The screening strategy targeted upper and lower quartile copeptin levels, while the endpoint analysis divided patients at a threshold of 10 pmol/L. This selected sampling approach may limit how readily findings generalize beyond the enrolled cohort. The record does not report the distribution of copeptin values or sufficient screening information to assess representativeness.

The primary endpoint population description is incomplete. It notes an exclusion in the high-copeptin group related to reduced renal function at baseline relative to screening and transient hypovolemia during hospitalization, but the complete analysis population is not reported in the registry record. The record also does not describe missing endpoint data, a complete participant flow, or sensitivity analyses for exclusions.

Additional limitations include open-label treatment and the absence of reported endpoint uncertainty or full analysis details. The record does not report the exact test or model, covariate adjustment, multiplicity procedures, or a complete statistical plan. The safety listings contain zero events in the reported groups, but the limited pilot setting does not allow those counts to rule out uncommon harms. Each of these limitations constrains interpretation independently of the biological rationale for the study.

16. Related Tutorials

17. Related Statistical Calculators

18. Sources

• ClinicalTrials.gov: Pilot Study of Using Copeptin to Predict Response to Tolvaptan — NCT01346072.
• Registry record last updated 2017-05-17; status last verified 2017-04.
• Results first posted 2017-05-17.
• 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

The registry describes a completed phase 4, prospective, open-label, single-group pilot study of tolvaptan in stable outpatients with heart failure and reduced left ventricular ejection fraction. Enrollment is listed as 21. Patients received a single oral dose of 30 mg, with body weight, fluid intake, and urine output monitored for 24 hours.

The primary endpoint was total urine output during the 24 hours after administration, summarized as a mean with standard deviation. The analysis classified baseline copeptin as 10 pmol/L or above versus below 10 pmol/L. The design was not randomized and did not include a control-treatment group.

The registry record does not report the primary endpoint results or their statistical uncertainty. It describes an exclusion in the high-copeptin group, but the full endpoint-population description is incomplete. The adverse-event listing reports zero serious events, zero other events, and zero deaths in each group. The record defines a pilot biomarker-response question, while leaving the magnitude and precision of the primary result unreported.