Regional Prophylactic Vancomycin With Restricted Tourniquet Time in Primary Total Knee Replacement: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Regional Prophylactic Vancomycin With Restricted Tourniquet Time in Primary Total Knee Replacement (NCT03506347), a randomized phase 2, phase 3 study. The primary outcome is Vancomycin Concentration in Fat.

2. Clinical Question

The practical comparison is between two ways of administering vancomycin before surgery, and the measured outcome is drug concentration in fat rather than a clinical infection outcome. The registry record describes the intravenous regimen as the current standard in its study summary, but it does not report a comparison of infection rates or other patient outcomes. The statistical question addressed by the posted primary result is whether the group means for the tissue-concentration measurement differ. The record reports a randomized, parallel-group study without masking. It does not report details such as the randomization procedure, how allocation was concealed, or how the tissue samples were collected and analyzed.

3. Trial Design

The registry identifies this as a completed, randomized, parallel-group trial with no masking. Its phase is listed as both PHASE2 and PHASE3. The sponsor is Mayo Clinic. Enrollment was 24. The two groups each included 12 participants: Vancomycin 15mg/kg IV and Vancomycin 500mg Intraosseous. The condition listed is arthropathy of the knee joint.

The intravenous group received vancomycin through a forearm vein, based on actual body weight, with a maximum dose of 2g. The infusion was to finish immediately before surgery. In the intraosseous group, the limb was exsanguinated and an above-knee tourniquet inflated to 300 mmHg before vancomycin was given through an intraosseous cannula. The description specifies 500mg in 150ml of saline solution. The registry's description of this arm is truncated, so additional procedural details are not reported in the registry record.

The primary outcome is mean vancomycin concentration in fat, measured in ug/g. Its timeframe is described as approximately 60 minutes post surgical incision, while the outcome description says the measurement was made at surgical closure. These descriptions are both retained in the registry record; further clarification of timing is not reported. The study summary emphasizes assessing tissue concentrations with tourniquet use minimized, but the precise protocol for restricting tourniquet time is not reported in the registry record.

The record lists a start date of 2018-09-24, primary completion on 2019-01-17, and status as completed. These dates and the enrollment figure describe the trial, but do not establish how participants were selected or whether the enrolled sample represents other surgical populations. The original sample-size rationale, target enrollment, and the trial's design assumptions are not reported in the registry record. Masking is listed as none; who knew each participant's assigned group and whether laboratory personnel were masked are not reported.

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 Vancomycin Concentration in Fat, measured over Approximately 60 minutes post surgical incision.

The primary endpoint is mean vancomycin concentration in fat, expressed in ug/g. It is a continuous measurement: each participant contributes a measured tissue concentration, and the registry reports a mean and standard deviation for each treatment group. The outcome description places measurement at surgical closure, while the listed timeframe says approximately 60 minutes post surgical incision. More detail on the sampling time and tissue-collection procedure is not reported in the registry record.

The study's stated rationale is to compare antibiotic concentration in the tissue near the surgical site after local intraosseous administration with concentration after systemic intravenous administration. Measuring tissue concentration can address whether the drug is present in the sampled tissue at the measurement time. It is not itself a direct measure of infection prevention, postoperative recovery, or other clinical outcomes. A target concentration or criterion for what counts as “adequate” or “effective” is not reported in the registry record, so the results cannot be classified against a stated threshold using this record alone.

Concentration is also a single-time measurement as described in the registry. The record does not provide a concentration-time profile, nor does it report how concentration values relate to later clinical events. The primary endpoint therefore addresses a specific pharmacologic measurement at a stated perioperative time, not the full range of possible benefits, harms, or clinical consequences of either administration strategy.

7. Results Posted in the Registry

• Vancomycin 500mg Intraosseous: mean 40.50 versus 6.00 on control; difference 34.50 (95% CI 29.90 to 39.10), standardized effect d = 6.01; z-test p = <0.001.

8. Statistical Interpretation

9. Statistical Methods Explained

The primary measurement is continuous, and the registry reports a mean and standard deviation in each group. The registry's own analysis is described as ANOVA, with a superiority analysis and p = 0.001. The additional mean difference and confidence interval are unadjusted recalculations from the posted group summaries. These are distinct descriptions of the analysis: the registry reports its ANOVA result, while the recalculation uses the arm-level means and standard deviations.

What does a mean difference represent?

For a continuous outcome, a mean summarizes the central value of the observations in a group. The mean difference expresses the separation between two group means in the outcome's original units. Here, the reported comparison is intraosseous minus intravenous, so a positive difference corresponds to a higher observed mean concentration in the intraosseous group. The reported difference is 34.5 ug/g. This does not mean that every participant's concentration was higher by that amount; it is a comparison of group averages.

The standard deviation describes spread among observations around a group's mean. The posted standard deviations are 0.85 ug/g for the intravenous group and 8.08 ug/g for the intraosseous group. Standard deviation is not the same as the standard error. In general, the standard error describes the sampling uncertainty in an estimated mean or difference, whereas standard deviation describes variation among the individual measurements. The registry's recalculation lists standard errors of 0.245 and 2.332 for the respective group means. These quantify uncertainty in the estimated means under the calculation, rather than the range in which individual measurements fall.

What does the confidence interval tell the reader?

In general, a confidence interval gives a range of values compatible with the data and the method's assumptions, and conveys uncertainty around an estimate. The 95% interval for the mean difference is 29.9 to 39.1 ug/g. It is an interval for the difference in group means, not a prediction interval for an individual concentration. It also does not include every source of uncertainty, such as uncertainty arising from unreported aspects of sampling or trial conduct.

A narrow interval can indicate greater precision than a wide interval for the same quantity, but precision is not the same as clinical importance. Interpreting clinical importance would require a meaningful concentration threshold or a link to outcomes; neither is reported in the registry record. The interval is calculated on the concentration scale, which makes its limits directly interpretable in ug/g.

What is a p value testing?

In general, a p value describes how unusual data at least as incompatible with a specified null hypothesis would be if that null hypothesis and the statistical model were true. It is not the probability that the null hypothesis is true, and it does not measure the size or clinical importance of an effect. The registry lists an ANOVA p value of 0.001 for its superiority analysis. The simple unadjusted recalculation reports p <0.001. The record does not detail the ANOVA specification or explain why the reported formats differ.

ANOVA is a method for comparing group means. In general, it evaluates whether observed differences among means are larger than expected from within-group variation under a model. With the two groups in this trial, the reported result is a comparison of the intravenous and intraosseous groups. More detailed information about model assumptions, diagnostics, transformations, or adjustments is not reported in the registry record. For an unadjusted comparison of means, the pattern and distribution of the measurements can matter, particularly in a small sample; participant-level values and diagnostic results are not reported.

Why does variation matter?

In general, over-dispersion means that the observed variability is greater than a model assumes. When a model understates variability, standard errors can be too small and confidence intervals can be too narrow. The term is commonly used for count-data models, but the broader lesson applies across statistical methods: uncertainty estimates depend on how well the model represents variation in the data. The posted standard deviations differ between the two groups, with more variation reported in the intraosseous group. That difference makes it important to know how the analysis handled group variability. The registry identifies the method as ANOVA but does not report diagnostics or further details on its variance assumptions. No conclusion about a model violation can be drawn from the summaries alone.

The recalculated standardized effect is 6.005, based on the pooled standard deviation of 5.745. A standardized effect expresses a mean difference relative to a measure of spread, rather than in ug/g. It can support comparisons across scales, but it does not replace the original-unit estimate. In this case, the group standard deviations are unequal in the posted summaries, so the standardized value should be treated as a supplementary summary of these data, not as an independent clinical threshold.

General statistical interpretation also depends on study design and data completeness. Randomized allocation can support a comparison between assigned groups, but the registry record does not detail sequence generation, allocation concealment, adherence, exclusions, or the analysis population. Missing-data methods and participant-level records are not reported. The estimates should therefore be understood in light of what is posted: a randomized parallel-group comparison, two group summaries, an ANOVA p value, and an unadjusted recalculation from those summaries.

10. Confidence Intervals and Precision

The estimated mean difference in fat vancomycin concentration was 34.5 ug/g, with a 95% confidence interval from 29.9 to 39.1 ug/g. This interval expresses the precision of the estimated difference under the recalculation's assumptions. The interval is on the original concentration scale and concerns the contrast between group means. It is not an interval containing 95% of individual concentrations, and it does not show the likely range of results for a future participant.

The trial included 12 participants in each group. The posted standard deviations were 0.85 ug/g for the intravenous group and 8.08 ug/g for the intraosseous group. The smaller group size and the observed variation are relevant to the uncertainty of the estimate. The confidence interval is a useful description of statistical precision, but it does not account for every potential source of uncertainty. For example, sampling procedures, analysis assumptions, missing data, and the analysis population are not fully described in the registry record.

The interval does not cross zero, and the registry's ANOVA analysis reports p = 0.001; the unadjusted recalculation gives p <0.001. These statistics address evidence about a difference in means under their respective analysis assumptions. They do not indicate whether a difference of this size is clinically meaningful. The record does not report a concentration threshold for adequacy or a clinical outcome linked to the measured concentration. Precision in estimating the concentration difference should not be confused with precision about infection outcomes or the effect of the approaches on patients.

11. Planning a Similar Trial: Sample Size

This is a planning illustration, not a re-evaluation of the original trial. It asks: if a new study wanted to detect a difference like the one observed, or a smaller one, how large would it need to be? It is deliberately not “observed power” of the finished trial, which is not a meaningful quantity.

The sample-size figures are planning illustrations expressed as participants per arm, not total events. They are calculated using a two-sided significance level of 0.05, equal allocation between the two arms, and a normal approximation. The effect to detect is set to the stated fraction of the observed effect. The registry record does not report the original trial's sample-size justification or design assumptions.

The 80% power column gives the illustrated participants per arm for a design with 80% power under those assumptions; the 90% power column gives the corresponding figure for 90% power. In general, power is the probability that a specified analysis will reject its null hypothesis when a particular effect is present, assuming the planning model. It is not a guarantee that a trial will produce a statistically significant result. The sample-size table lists 1 participant per arm for 80% power at 100% of the observed effect, and 1 for 90% power at that effect fraction. For planning at 75% of the observed effect, the figures are 1 per arm for 80% power and 2 per arm for 90% power. At 50% of the observed effect, they are 2 per arm for 80% power and 3 per arm for 90% power.

These very small outputs arise from applying the stated calculation to the large observed effect relative to the posted variability. They should be read as results of an idealized planning illustration, not as a recommended enrollment target. Smaller effects need more participants to detect under otherwise similar assumptions. This is not observed power: it is not a calculation of the probability that the completed trial detected its result after the data were observed. The original sample-size rationale and any planning assumptions used for the trial are not reported in the registry record.

12. How the Sample Size Is Derived

Power is the probability that a study of a given size rejects the null hypothesis when a specific true effect exists. It depends on four things: the effect to detect, the variability of the outcome, the significance level (here a two-sided α of 0.05, so z_1−α/2 = 1.960) and the desired power (z_1−β = 0.842 for 80% and 1.282 for 90%).

Formula

n is the number of participants in each arm, Δ is the difference in means to detect and σ is the common standard deviation.

Worked calculation for this trial

1. Observed difference Δ = 34.500; pooled standard deviation σ = 5.745.
2. n = 2 × (1.96 + 0.842)² × 5.745² / 34.500² = 0.4, rounded up to 1 participants per arm for 80% power.

The smaller-effect rows in the table above repeat the same calculation with the effect multiplied by 0.75 and 0.5 (for ratios, applied to the logarithm of the ratio).

R code

alpha <- 0.05
z_a   <- qnorm(1 - alpha / 2)          # two-sided
delta <- 34.5                      # observed difference in means
sigma <- 5.74494995626594                     # pooled standard deviation
n_per_arm <- function(power, frac = 1) {
  ceiling(round(2 * (z_a + qnorm(power))^2 * sigma^2 / (delta * frac)^2, 6))
}
for (f in c(1, 0.75, 0.5))
  cat(sprintf("%3.0f%% of observed effect: n per arm = %s (80%% power), %s (90%% power)\n", 100 * f,
              format(n_per_arm(0.80, f), big.mark = ","), format(n_per_arm(0.90, f), big.mark = ",")))

# Check against R's built-in function (t-based, so very slightly larger)
cat("power.t.test:", round(power.t.test(delta = delta, sd = sigma, power = 0.80)$n, 1), "\n")

# Simulation: exact two-sample t-test using simulated means and variances
set.seed(3506347)
n    <- n_per_arm(0.80)
reps <- 20000
m_diff <- rnorm(reps, delta, sigma * sqrt(2 / n))             # difference in sample means
s2     <- sigma^2 * rchisq(reps, 2 * (n - 1)) / (2 * (n - 1))   # pooled sample variance
tstat  <- m_diff / sqrt(s2 * 2 / n)
cat(sprintf("Simulated power at n = %s per arm: %.3f (%d replicates)\n", format(n, big.mark = ","), mean(abs(tstat) > qt(0.975, 2 * (n - 1))), reps))

Output from running the code

100% of observed effect: n per arm = 1 (80% power), 1 (90% power)
 75% of observed effect: n per arm = 1 (80% power), 2 (90% power)
 50% of observed effect: n per arm = 2 (80% power), 3 (90% power)
power.t.test: 2 
Simulated power at n = 1 per arm: NA (20000 replicates)

The simulation draws many sets of group means and variances at the planned sample size and counts how often the two-sample t-test rejects the null. It should land close to the target power; at very small required sample sizes it can fall below it, because the normal approximation is rough there.

13. Analyses Reported by the Sponsor

The registry reports the sponsor's analysis as an ANOVA superiority analysis comparing the two groups, with p = 0.001. It does not provide an ANOVA estimate or confidence interval in that analysis entry. Separately, the unadjusted recalculation from the posted group summaries gives a mean difference of 34.5 ug/g, a 95% confidence interval from 29.9 to 39.1 ug/g, and p <0.001.

The registry record does not explain why the sponsor's p value and the recalculated p value are presented in different formats. The recalculation is not a replacement for the analysis described in the registry; it is a simple comparison based on the posted means, standard deviations, and group sizes. The registry does not report additional details about the ANOVA model, such as whether any adjustments were included. The direction and scale of the recalculated difference are useful for interpreting the group summaries, while the sponsor's ANOVA result is the analysis identified as the trial's reported analysis.

Neither p value establishes clinical importance. The registry does not report a threshold for an adequate tissue concentration or a patient outcome analysis that would connect the concentration difference to clinical benefit.

14. Multiplicity, Missing Data and Analysis Populations

The registry record lists two outcome measures and two analyses, but it reports zero posted secondary outcomes. The primary outcome is mean vancomycin concentration in fat. Details for the other listed outcome measure are not reported in the registry record. The two analyses are not described in enough detail to establish whether they correspond to separate outcomes, analysis approaches, or other comparisons.

Multiplicity arises when several outcomes, comparisons, time points, or subgroup analyses are tested. In general, conducting many tests can increase the chance of at least one apparently positive result by chance, and prespecified approaches may address that issue. Whether any multiplicity adjustment was planned or used is not reported in the registry record. The record also does not report an analysis plan that would clarify which outcome was primary before results were examined, or how the listed outcomes and analyses were handled together.

The registry identifies the reported primary comparison as an ANOVA superiority analysis. It does not report details of the analysis population, exclusions, treatment adherence, or whether participants were analyzed according to assigned group. Although allocation is listed as randomized, the analysis population and the handling of any protocol deviations are not reported in the registry record.

Missing-data methods and the amount or pattern of missing outcome data are not reported in the registry record. The posted group counts are 12 per arm, but those counts alone do not establish whether all randomized participants had evaluable primary outcome measurements or whether any missing values were imputed. The registry also does not describe sensitivity analyses for missing data, subgroup analyses, covariate adjustment, or interim analyses. These omissions limit assessment of how robust the reported result is to analysis choices, but they do not establish that any particular procedure was or was not used.

15. 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.

The serious adverse event figures report 0 affected participants among 12 at risk in the Vancomycin 15mg/kg IV group and 0 among 12 at risk in the Vancomycin 500mg Intraosseous group. The registry also reports 0 participants affected by other adverse events in each group, and 0 deaths in each group.

These counts describe events recorded for the groups in this trial. They do not show that the risk of an event is zero, nor do they establish that the groups have equivalent safety. With 12 participants in each group, an event that did not occur in the observed participants could still occur in a larger population. The record does not report event definitions, the period over which events were collected, severity grading for non-serious events, or details of how events were monitored. The registry lists no top serious-event terms.

Safety interpretation is therefore limited to the posted counts and the trial's size and reporting detail. The adverse-event figures should not be read as a recommendation about treatment or as proof that either administration method is free of risk.

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

The central result is a difference in mean vancomycin concentration in fat between the two randomized groups. The intravenous group mean was 6.0 ug/g, and the intraosseous group mean was 40.5 ug/g. The estimated mean difference was 34.5 ug/g, with a 95% confidence interval from 29.9 to 39.1 ug/g. The registry reports an ANOVA p value of 0.001, while the simple unadjusted recalculation gives p <0.001. These results describe the measured tissue concentrations in the trial.

The finding does not by itself establish that either approach prevents infection, improves recovery, or changes another clinical outcome. The primary endpoint is a tissue concentration, and a concentration threshold that defines adequacy is not reported in the registry record. The record also does not report a patient outcome comparison connecting concentration to later infection or other clinical events. The result should not be extended from this measurement to claims about clinical effectiveness.

The data also do not establish the full profile of concentrations in individual participants. Means and standard deviations summarize groups, but do not show each observation or how values were distributed. The greater standard deviation in the intraosseous group indicates more observed spread in that group, but the record does not provide participant-level values or diagnostic results. Nor do the zero reported adverse-event counts establish that adverse events cannot occur.

The trial was randomized and included 24 participants, with 12 assigned to each group. That design provides a direct comparison in the enrolled sample, but the record does not fully describe allocation procedures, the analysis population, missing data, or the original sample-size rationale. These limitations matter when assessing the stability and reach of the result. The finding is best understood as evidence about a particular tissue-concentration endpoint under the trial's recorded conditions. Whether it applies to other populations, surgical settings, dosing practices, or clinical outcomes is not reported in the registry record.

17. Why This Trial Matters Statistically

This trial is statistically interesting because its primary outcome is a continuous tissue concentration, and the reported group means are separated relative to the observed variability. That makes it possible to discuss a mean difference on the original measurement scale, a confidence interval for that difference, and a standardized effect. The concentration scale is directly interpretable in ug/g, while the standardized effect expresses the mean difference relative to pooled variation.

The comparison also illustrates why both an estimate and its uncertainty matter. A p value alone does not convey the size of the observed difference or the range of values compatible with the calculation. The estimated difference and confidence interval describe the concentration contrast; the group standard deviations show that observed variation differed between groups. In general, unequal variability can matter for how a statistical model estimates uncertainty, so the model assumptions and diagnostics are useful context. Those details are not reported in the registry record.

Another statistical lesson is that a biomarker or tissue measurement is not interchangeable with a patient outcome. A clear difference in measured concentration does not answer whether the difference matters clinically. That question would require a stated threshold or a link between concentrations and outcomes, neither of which is reported in the registry record.

Finally, the trial's small enrollment places the numerical result in context. Planning illustrations based on the observed effect yield very small participant counts under their assumptions, but these are not the original design rationale and are not observed power. They show how strongly a planning calculation can depend on the effect used. For interpretation, the actual sample, uncertainty, measurement definition, analysis details, and limits of the endpoint all remain important.

18. 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 trial enrolled 24 participants, with 12 in each group. This is a limited sample for characterizing the range of possible measurements and for assessing uncommon adverse events. The posted standard deviations differ between the groups, and the record does not provide participant-level data or analysis diagnostics to examine the distributions in more detail.

The registry provides two descriptions of measurement timing: approximately 60 minutes after surgical incision and at surgical closure. It does not report further clarification of the sampling time, tissue collection, laboratory assay, or handling of samples. These details can matter when interpreting a concentration measurement, particularly when comparing results across settings.

The intervention description for the intraosseous arm is truncated in the registry record. Although the record specifies the dose, route, saline volume, and tourniquet inflation to 300 mmHg, additional procedural information is not reported. The protocol's specific restriction or duration of tourniquet use is also not reported. The registry does not provide the randomization procedure, allocation concealment details, masking of laboratory personnel, eligibility criteria beyond the listed condition, or baseline characteristics.

The primary outcome is a tissue concentration rather than a clinical outcome. The record does not report a concentration threshold for adequacy, infection outcomes, or a relationship between measured concentration and subsequent outcomes. The original sample-size rationale, missing-data handling, analysis population, and any multiplicity strategy are not reported. Adverse-event counts are posted, but event definitions and monitoring duration are not. These trial-specific omissions limit assessment of reproducibility, applicability, and the broader clinical meaning of the concentration result.

19. Related Tutorials

20. Related Statistical Calculators

21. Sources

• ClinicalTrials.gov: Regional Prophylactic Vancomycin With Restricted Tourniquet Time in Primary Total Knee Replacement — NCT03506347.
• Registry record last updated 2020-01-27; status last verified 2020-01.
• Results first posted 2019-12-17.
• 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.

22. Record Summary

The registry record describes a completed randomized, parallel-group study with no masking and enrollment of 24 participants. It lists PHASE2 and PHASE3, and identifies Mayo Clinic as the sponsor. The comparison was Vancomycin 15mg/kg IV versus Vancomycin 500mg Intraosseous, with 12 participants in each group.

The primary outcome was mean vancomycin concentration in fat, measured in ug/g. The reported means were 6.0 ug/g for the intravenous group and 40.5 ug/g for the intraosseous group. The recalculated mean difference was 34.5 ug/g, with a 95% confidence interval from 29.9 to 39.1 ug/g. The registry reports an ANOVA p value of 0.001.

Serious adverse events, other adverse events, and deaths were each reported as 0 in both groups. The registry does not report the original sample-size rationale, detailed analysis plan, missing-data methods, or clinical outcome results that connect tissue concentration to infection.