How Vitamin D Analogues Affect Endothelial Cells in Patients on Dialysis: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for How Vitamin D Analogues Affect Endothelial Cells in Patients on Dialysis (NCT00528788), a na phase 4 study. The primary outcome is Change in Endothelial Cell Function.

2. Clinical Question

3. Trial Design

This is a completed Phase 4, prospective, open-label, single-group study of doxercalciferol. The registry lists Duke University as the sponsor. It records a start date of 2007-09 and a primary completion date of 2012-08. The listed enrollment is 24. The outcome description, however, refers to 20 dialysis patients; the registry record does not explain this difference or clarify which number applies to the primary outcome analysis.

All participants in the described group were to receive doxercalciferol. The arm description gives a dose of 2 mcg or 4 mcg 3 times per week for 30 days, with blood work and vascular laboratory studies performed before and after treatment. The record does not say how participants were assigned to the dose levels, whether dose assignment was guided by clinical considerations, or whether dose-specific outcomes were planned. Allocation is listed as NA, and masking is listed as none.

The design uses each participant's pre-treatment measurement as the reference for the post-treatment measurement; it does not include a concurrent untreated or active-comparator group. Such a design can describe change over time, but it cannot by itself separate a treatment-associated change from changes related to time, measurement conditions, or other influences. The registry record does not report a protocol, a sample-size justification, interim analyses, or a detailed statistical analysis plan. The record marks the study completed, but completion does not establish that outcome results were posted.

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: Participants underwent review of history and meds. Hemodialysis (HD) blood was drawn. seen in vascular laboratory for testing of endothelial cell function and arterial stiffness. Doxercalciferol was initiated at dose of 2 mcg IV 3x/wk or 4 mcg IV 3x/wk. After 30 days of medication, repeat blood drawn and repeat testing in vascular laboratory.

Overall Study

Analysis population for the primary outcome, as defined in the registry: prospective open-label 30-day observational study testing the effects of doxercalciferol on mediated vasodilation (FMD) among 20 dialysis pts with secondary hyperparathyroidism

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 Endothelial Cell Function, measured over 1 month.

The primary endpoint is titled “Change in Endothelial Cell Function.” The registry specifies percent change in FMD and a timeframe of 1 month. FMD, or flow-mediated vasodilatation, is assessed in a vascular laboratory. In general, this type of measure evaluates a blood vessel's dilation response to a change in blood flow and is used as an indicator of endothelial function. The registry describes measurements before doxercalciferol and after 30 days of treatment.

The endpoint is continuous, and the planned summary is a mean with standard deviation. A mean describes the average measured value or change in a group; a standard deviation describes how dispersed individual measurements are around that average. Neither statistic is reported for the primary outcome in the registry record, so the observed mean change and its variability cannot be assessed from that record.

FMD is a vascular laboratory measure, not a direct count of cardiovascular events or a measure of mortality. The study summary presents vascular function as a possible mechanism to investigate in relation to cardiovascular risk, but the registry record does not report a clinical-event endpoint. It also does not report technical details of the FMD procedure or how the percent change was calculated.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

The primary measure is continuous: percent change in FMD. A continuous outcome can take a range of values rather than belonging to a limited set of categories. The registry specifies that the outcome is summarized using a mean and standard deviation. The mean conveys the average in the analyzed group, while the standard deviation describes the spread of individual measurements around that average. Neither summary for the primary endpoint appears in the registry record.

What does a before-and-after comparison estimate?

In a single-group pre-post design, the natural unit of comparison is the change within a participant between the pre-treatment and post-treatment measurement. A statistical analysis would ordinarily need to preserve that pairing: measurements from the same person are related, rather than independent observations. In general, ignoring that relationship can misstate uncertainty. The registry record does not report the analysis method, the individual or group-level change estimates, or whether the analysis used paired data.

The title and unit indicate an endpoint of change expressed as percent change in FMD. The registry does not give a formula for deriving that percentage, explain how measurements were processed, or specify how multiple measurements, if any, were handled. Those details matter because a reported estimate depends on how the endpoint is defined and measured. They are not reported in the registry record.

How should uncertainty be assessed?

In general, an estimate describes the observed pattern in a sample, while a confidence interval conveys the range of values compatible with the data under the analysis assumptions. A narrower interval indicates greater statistical precision than a wider interval. A test result can address compatibility with a specified null hypothesis, but a p-value alone does not indicate the size or clinical importance of an effect. The registry record reports no confidence interval or test result for the primary outcome, so there is no basis for judging its precision or statistical evidence.

Statistical significance and clinical importance are distinct. A small estimated change can be statistically distinguishable from no change in a sufficiently precise study, yet still have limited practical importance; a potentially meaningful estimate can also be too imprecise to resolve. These are general principles, not findings about this study. The record gives no clinically meaningful threshold for FMD and no analysis estimate with which to assess either issue.

What do the planning illustrations represent?

The sample-size assumptions describe a planning illustration using a two-sided significance level of 0.05, equal allocation between two arms, and a normal approximation. The target effect is expressed as a stated fraction of the observed effect. The record says that non-time-to-event illustrations are expressed as participants per arm for 80% power or other stated power targets, while time-to-event illustrations use total events. These are hypothetical planning calculations under assumptions, not the power observed in the completed study.

Those assumptions do not match the study's listed single-group pre-post design: the trial record does not describe two randomized arms or equal allocation. The stated illustration is therefore not a description of how participants were assigned in this study. In addition, the registry record does not post the outcome analysis needed to inspect an observed effect. The planning calculations should not be treated as a substitute for an outcome estimate or as evidence that the trial did or did not detect a change.

Why does over-dispersion matter in general?

Over-dispersion is a concern in some models for count outcomes when the observed variability exceeds what the model assumes. It can make standard errors too small if not accounted for. The primary endpoint in this study is continuous, not a reported count outcome, and the registry gives no model or outcome analysis to which an over-dispersion adjustment could be applied. It is therefore a general statistical concept rather than a reported feature of this analysis.

More broadly, understanding an analysis requires knowing the exact outcome definition, analysis population, handling of missing measurements, model, and uncertainty method. These details are not reported in the registry record. Without them and without the estimated outcome values, readers can describe the planned measurement and design but cannot reproduce or independently evaluate a treatment-associated estimate.

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("Pre-post Comparison")
n   <- c(20)
m   <- c(0.23)
sp  <- c(1.4)
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  Pre-post Comparison  20  0.230  1.400  -0.425  0.885

11. Multiplicity, Missing Data and Analysis Populations

The registry record lists one outcome measure, the primary endpoint of change in endothelial cell function. It lists zero analyses. No statistical analysis plan is reported, and the record does not explain whether additional outcomes or comparisons were considered outside the posted outcome entry.

Multiplicity arises when multiple hypotheses or outcomes are tested: the more opportunities to identify an apparently notable result, the more important it is to understand whether and how the analysis accounted for those tests. This is a general concern, not evidence that multiplicity affected this study. With no reported analysis results or analysis plan, it is not possible to determine whether any adjustment was planned or needed.

The record gives an enrollment of 24 and describes the primary outcome population as 20 dialysis patients. It does not report a participant flow, the number with pre-treatment and post-treatment FMD measurements, reasons for missing measurements, or an analysis population definition. The difference between the enrollment and outcome-description figures cannot by itself establish how many measurements were missing or why.

Missing-data methods, exclusions, and sensitivity analyses are not reported in the registry record. Nor does it say whether the analysis included all enrolled participants, all participants who started treatment, or only those with both measurements. These omissions limit assessment of potential selection effects and of how representative any analyzed group might be. Since no primary outcome analysis is posted, the possible effect of missing data on the endpoint estimate cannot be assessed.

12. 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 0.1% reporting threshold.

Most frequent serious adverse event terms

Most frequent non-serious adverse event terms

Serious events with the highest arm-level rate: atypical chest pain (5.0%), diabetic gastroparesis (5.0%).

The adverse-event entry lists 20 participants at risk for serious adverse events and 1 affected participant. It separately lists 20 at risk for other adverse events and 1 affected participant. The record gives atypical chest pain and diabetic gastroparesis as serious adverse-event terms, each with a reported frequency of 5.0%. These are registry safety figures; they do not establish that doxercalciferol caused an event.

The record does not specify whether the listed terms correspond to the serious-event count, the other-event count, or both. The terms should not be added together to create a total, since the registry does not state whether they refer to different participants or event categories. It also does not provide event narratives, timing, severity details beyond the serious-event classification, or an assessment of relation to treatment.

Deaths are listed as null, which means death information is not reported in the registry record. The safety entry does not provide enough detail to assess comparative risk: this was a single-group study without a separate control group, and no comparative safety analysis is reported.

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

The record establishes that investigators planned a prospective, open-label assessment of FMD before and after doxercalciferol in dialysis patients with secondary hyperparathyroidism. It specifies the treatment period and the primary endpoint, and it marks the study completed. Those details describe the study's purpose and structure; they do not answer whether FMD changed.

The primary outcome result is not reported in the registry record. As a result, the posted information does not support a statement about the direction or magnitude of the observed change, its precision, or whether an analysis found evidence against no change. It also cannot establish whether any change was clinically important. A completed study status is not a substitute for a numerical result.

Even if a pre-post change were available, a single-group design would describe a pattern over time rather than isolate the effect of treatment from other possible influences. A concurrent comparison group can help address some alternative explanations; this record describes no such group. No causal conclusion can be drawn from the design description alone.

The study's stated rationale concerns vascular function as a possible mechanism related to cardiovascular risk. FMD is a vascular laboratory endpoint, not a direct measure of cardiovascular events or mortality. The registry record reports no cardiovascular-event or mortality result that would allow those outcomes to be assessed. It also gives no dose-specific analysis, so it cannot establish whether the two listed dose levels differed in their outcome patterns.

Thus, the record is informative about the question asked and the measurement planned, but it does not report the result needed to judge the observed endpoint. This distinction is important: absence of a posted result is not evidence of no change, nor is it evidence of a beneficial or harmful change.

14. Why This Trial Matters Statistically

This trial is statistically instructive because its research question and design are clear enough to identify what an analysis would need to estimate, while its registry entry does not report that estimate. The primary outcome is a continuous FMD measure assessed before and after treatment in a single group. That makes the relation between each participant's repeated measurements central to appropriate uncertainty estimation.

The record also illustrates why a stated enrollment and an outcome population description should not be treated as interchangeable without an explanation. It lists 24 enrolled participants and describes the primary outcome population as 20 dialysis patients, but does not reconcile the figures. That gap limits assessment of the analysis population and possible missing measurements.

A further statistical lesson is the difference between a planning exercise and an observed result. The sample-size assumptions describe hypothetical calculations based on an effect and an assumed two-arm design. The study itself is described as single group, and the registry entry does not report the primary outcome analysis. Planning illustrations cannot supply the missing estimate, its uncertainty, or evidence about the study's findings.

Finally, the study highlights the limits of interpreting a surrogate or laboratory endpoint in isolation. FMD addresses vascular function; it is not itself a cardiovascular-event or mortality outcome. The registry's rationale links vascular function to a possible mechanism, but no clinical-event result is reported. Statistical interpretation should stay aligned with the endpoint actually defined and the data actually posted.

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 principal limitation for interpreting the registry entry is that it reports no primary outcome analysis. No mean change, standard deviation, confidence interval, or test result is available in the record. Thus, neither the size nor the precision of the endpoint result can be evaluated.

The single-group pre-post design has no concurrent control group. Changes over time may reflect influences other than treatment, and the record does not describe methods to address such influences. The open-label design also means masking was not used, as recorded; the registry does not describe safeguards or procedures for limiting measurement or assessment bias.

The record lists enrollment of 24 but describes 20 dialysis patients in the primary outcome population. It does not explain the difference, provide participant disposition, or report missing-data handling. Dose assignment and dose-specific analyses are also not described. The record does not provide technical details for the FMD assessment or the formula for percent change, a protocol, or a detailed analysis plan.

Finally, the endpoint is a vascular laboratory measure, while the registry rationale refers to broader cardiovascular risk. No cardiovascular-event or mortality analysis is reported. The serious adverse-event entry is limited and does not provide causal attribution or comparative safety estimates. These limitations mean that the registry description cannot support conclusions about the treatment's effect or broader clinical outcomes.

16. Related Tutorials

17. Related Statistical Calculators

18. Sources

• ClinicalTrials.gov: How Vitamin D Analogues Affect Endothelial Cells in Patients on Dialysis — NCT00528788.
• Registry record last updated 2016-01-22; status last verified 2012-08.
• Results first posted 2014-04-14.
• 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

This completed Phase 4 study was a prospective, open-label, single-group assessment of doxercalciferol in dialysis patients with secondary hyperparathyroidism. The registry lists 24 enrolled participants and describes the primary outcome population as 20 patients. Participants were to receive 2 mcg or 4 mcg 3 times per week for 30 days.

The primary endpoint was change in endothelial cell function, measured by FMD and expressed as percent change over a 1-month timeframe. The registry record lists one outcome measure and zero analyses; it does not report the primary outcome result, its variability, or an inferential analysis. The safety entry lists serious and other adverse-event counts and two serious-event terms, but does not establish treatment causation. Overall, the record describes the study design and question without posting the outcome analysis needed to assess the observed effect.