The Diet Gout Trial: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for The Diet Gout Trial (NCT03569020), a randomized na study. The primary outcome is Uric Acid Level.

2. Clinical Question

3. Trial Design

The Diet Gout Trial (NCT03569020) is listed as a completed, randomized, double-blind crossover study sponsored by Johns Hopkins University. Its listed conditions are gout and hyperuricemia. Enrollment was 43. The registry record lists the study start as 2018-08-07 and primary completion as 2019-07-23. A phase is not applicable.

In a crossover design, participants receive more than one intervention in sequence, so each participant may contribute observations under multiple conditions. The primary-outcome description states that all 43 participants underwent both interventions, and that the data are presented by first-period assignment. The sequence titles are Dietitian-Directed Diet Then Self-Directed Diet and Self-Directed Diet Then Dietitian-Directed Diet. Their posted group sizes are 22 and 21, respectively. The outcome values described in the record are baseline values grouped by sequence assignment; the record does not report the timing or results of a treatment-period comparison.

The dietitian-directed intervention combined food-purchase support and dietitian education. The self-directed period asked participants to continue their typical diet for 4 weeks and did not include a subsidy. The registry record does not report the length of the dietitian-directed period, a washout period, or further details of the masking procedures. It also does not report the original design assumptions, including the planned sample-size rationale. Randomization and double masking are registry design labels; further implementation details are not reported in the registry record.

The listed secondary outcomes are systolic and diastolic blood pressure, fasting blood glucose, low density lipoprotein cholesterol, and body mass index. The record does not provide results for these outcomes in the posted statistics.

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.

Period 1

Period 2

Analysis population for the primary outcome, as defined in the registry: This was a crossover design. All 43 participants underwent both interventions. The data is presented by first period assignment.

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: This was a crossover design. Baseline characteristics are presented by participants' period 1 assignment.

6. Primary Endpoint

The registry lists the primary outcome as Uric Acid Level, measured over Baseline.

The primary outcome is serum uric acid, also called serum urate, measured in mg/dL. Uric acid is the analyte the trial was designed to examine in adults with gout and hyperuricemia. The registry states that it was measured in serum and characterizes the outcome as a mean of a continuous measure. A continuous outcome can take values across a numerical scale; reporting its mean describes the average level in the group, while a standard deviation describes variation among measurements.

The record labels the primary outcome timeframe as baseline. That detail is essential for interpretation: the posted means describe levels at baseline, arranged by first-period assignment. They do not describe a change from baseline, a post-diet level, or a treatment-period contrast. The record’s study objective concerns the DASH diet’s effects on uric acid, but an objective does not substitute for a reported result at the relevant follow-up time. A treatment effect on uric acid is not reported in the registry record.

Serum uric acid is relevant to the study question because the registered population has gout and hyperuricemia, and the trial is focused on uric acid levels. The posted baseline measure can describe the groups’ starting values. It cannot establish whether the dietary interventions changed those values. The registry record does not report a detailed measurement protocol, laboratory procedures, or the timing of any treatment-period serum samples.

7. Results Posted in the Registry

• Self-Directed Diet Then Dietitian-Directed Diet: mean 8.03 versus 8.23 on control; difference -0.20 (95% CI -0.66 to 0.26), standardized effect d = -0.26; z-test p = 0.392.

8. Statistical Interpretation

9. Statistical Methods Explained

The central statistical distinction in this record is between a baseline group comparison and a comparison of outcomes after treatment. The primary outcome is serum uric acid measured at baseline, with results grouped by first-period sequence assignment. The posted means and mean difference therefore describe how the two sequence groups stood at baseline. Although the study is a crossover trial in which participants underwent both interventions, that design feature does not turn a baseline comparison into a treatment-effect estimate.

What does a mean describe?

For a continuous outcome, the mean is the arithmetic average of measurements in a group. The registry reports baseline means of 8.23 mg/dL and 8.03 mg/dL for the two assigned sequences. A mean is useful for describing the group’s central level, but it does not show the full distribution. The accompanying standard deviations, 0.74 mg/dL and 0.79 mg/dL, describe how dispersed individual measurements are around their group mean. They are not confidence limits and do not indicate the range of observed values.

The reported mean difference is -0.2 mg/dL, with the self-directed-first sequence compared against the dietitian-directed-first sequence. A negative sign indicates that the reported mean for the first-named comparison group is lower than the reference group’s mean. The registry’s comparison is an unadjusted summary; a model specification or adjustment set is not reported in the registry record. No further arithmetic is needed to interpret the posted estimate.

What does a confidence interval communicate?

A confidence interval expresses uncertainty around an estimated quantity under a statistical model and its assumptions. For the baseline mean difference, the reported 95% interval is -0.658 to 0.258 mg/dL. Its endpoints indicate that the data are compatible with a range of possible baseline differences, including zero. In general, a confidence interval is not a statement that a particular probability applies to the fixed, true parameter after the data have been observed. Rather, the confidence level describes the long-run behavior of the interval procedure under its assumptions.

The interval is about the estimated difference between group means, not the distribution of individual measurements. Its width reflects uncertainty in the estimate, which depends on factors such as outcome variability, the amount of information, and the analysis model. The registry does not report the detailed model or assumptions behind the posted interval, so those cannot be assessed from the record.

How should the p-value be read?

The unadjusted p-value is 0.392. In general, a p-value describes how surprising data at least as inconsistent with a specified null model would be if that model and the analysis assumptions were true. It is not the probability that the null hypothesis is true, nor is it the probability that the result occurred by chance. The registry’s planning assumptions refer to a two-sided significance level of 0.05, but the trial’s own prespecified testing procedure is not reported. The p-value should therefore be read alongside the estimate and confidence interval, and in light of the fact that the outcome is baseline.

The word unadjusted indicates that this posted p-value is not described as adjusted for multiple comparisons or other covariates. It does not establish whether the trial investigators performed other analyses, or whether other results were adjusted. The registry record does not list analysis entries or give a detailed statistical analysis plan.

Why does crossover structure matter?

In general, crossover trials can use within-participant information because the same participant receives multiple interventions. This can reduce the influence of stable differences between individuals when an analysis compares outcomes measured under each intervention. However, appropriate analysis depends on the measurements and design details, including period and sequence handling. The registry states that participants underwent both interventions, but the posted primary outcome is baseline and grouped by sequence. It does not report paired outcome measurements under the two diets or an analysis accounting for period, sequence, or within-person correlation. No crossover treatment-effect model can be inferred from the baseline summaries.

Over-dispersion is a general issue for count outcomes, where the observed variability exceeds a simple model’s expectation. It is not the relevant feature of this primary outcome, which is continuous and summarized by means and standard deviations. Other distributional concerns may matter for continuous data, but the registry does not report diagnostic checks or modeling details.

Finally, the registry’s planning illustration uses a normal approximation, equal allocation, a two-sided significance level of 0.05, and effects defined as fractions of the observed effect. Those are assumptions for the illustration, not evidence of the original trial’s statistical plan. The original planning rationale and the trial’s full analysis methods are not reported in the registry record.

10. Confidence Intervals and Precision

The posted baseline mean difference is -0.2 mg/dL, with a 95% confidence interval from -0.658 to 0.258 mg/dL. This interval describes uncertainty in the estimated difference between the two first-period assignment groups, under the analysis that generated the posted result. It includes zero, so the data are compatible with no baseline mean difference as well as with differences in either direction within the reported interval.

The interval should not be confused with the range of individual uric acid values. Individual variability is described separately by the standard deviations of 0.74 mg/dL and 0.79 mg/dL. Nor does the interval quantify uncertainty about a diet effect: the posted outcome is measured at baseline, and a treatment-period contrast is not reported in the registry record.

Precision concerns how narrowly an estimate is located. A wider interval leaves more uncertainty about the size and direction of the baseline difference; a narrower interval would indicate greater precision for that particular comparison. Statistical compatibility with zero does not show that the groups are exactly alike, and it does not establish equivalence. Equivalence requires a prespecified margin and an analysis designed to assess whether plausible differences fall within that margin; neither is reported in the registry record.

The p-value of 0.392 gives another summary of the baseline comparison under a null model, but it does not replace the effect estimate or its interval. The interval is more informative about the range of differences compatible with the data than a threshold-based label alone. Since detailed modeling and missing-data information are not reported, the interval’s assumptions cannot be independently evaluated from the registry record.

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 planning figures are participants per arm, not total event counts. They are an illustration based on the observed effect, not the trial’s original design calculation. The listed assumptions are a two-sided significance level of 0.05, equal allocation between the two arms, and a normal approximation. The effect to detect is specified as a fraction of the observed effect. The trial’s own original design assumptions and sample-size justification are not reported in the registry record.

For planning based on 100% of the observed effect, the illustration gives 230 participants per arm for 80% power and 308 participants per arm for 90% power. For 75% of the observed effect, it gives 409 participants per arm for 80% power and 547 participants per arm for 90% power. For 50% of the observed effect, it gives 919 participants per arm for 80% power and 1,230 participants per arm for 90% power.

In plain language, 80% power means that, if the assumed effect and model were correct, a study of the stated size would have an 80% chance of detecting that effect using the specified test procedure. The 90% column uses the same planning idea but targets a higher chance of detection under those assumptions. It is not a statement about the probability that an effect is true. These figures are planning illustrations, not observed power.

The smaller the effect one wants to detect, the more participants the illustration requires. The estimates also depend on the stated assumptions; a different analysis or variance structure could lead to different planning needs. Because this study is a crossover trial while the illustration is described in two-arm terms, the illustration should not be read as a reconstruction of the trial’s actual design calculation.

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 = -0.200; pooled standard deviation σ = 0.765.
2. n = 2 × (1.96 + 0.842)² × 0.765² / -0.200² = 229.5, rounded up to 230 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 <- -0.200000000000001                      # observed difference in means
sigma <- 0.764798714153425                     # 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(3569020)
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 = 230 (80% power), 308 (90% power)
 75% of observed effect: n per arm = 409 (80% power), 547 (90% power)
 50% of observed effect: n per arm = 919 (80% power), 1,230 (90% power)
power.t.test: 230.5 
Simulated power at n = 230 per arm: 0.802 (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. Multiplicity, Missing Data and Analysis Populations

The registry lists five secondary outcomes: systolic blood pressure, diastolic blood pressure, fasting blood glucose, low density lipoprotein cholesterol, and body mass index. Results for these outcomes are not reported in the posted statistics. The record’s listed outcome-measure count is eight, but the registry record does not provide a full results account for every outcome measure.

When many outcomes or analyses are tested, the chance of at least one apparently unusual result can increase. Statistical approaches to multiplicity can control that risk, for example by adjusting thresholds or interpreting outcomes within a prespecified hierarchy. The registry record does not report a multiplicity strategy, an outcome hierarchy, or a complete analysis plan. The reported p-value is labeled unadjusted; it should not be assumed to account for other outcomes or analyses.

Missing-data handling is also not reported in the registry record. The primary-outcome description says all 43 participants underwent both interventions, but this statement does not specify whether every planned measurement was available, how missing observations were handled, or whether participants were excluded from any analysis. The record does not describe an intention-to-treat population, a per-protocol population, or another analysis population. Those populations have different interpretations, particularly in a crossover trial, and should not be inferred from enrollment or intervention completion alone.

The registry does not list analysis entries or provide details about covariate adjustment, model selection, or sensitivity analyses. It is therefore not possible to determine from this record whether the posted baseline comparison was the only analysis conducted or how it fits into the investigators’ full analysis strategy. These omissions limit assessment of selective reporting, multiplicity, and robustness. They do not justify assuming that analyses were or were not performed outside the registry record.

14. 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 registry reports separate adverse-event summaries for the Dietitian-Directed Diet and Self-Directed Diet. For each intervention, 43 participants are listed as at risk for serious adverse events and 0 as affected. For each intervention, 43 are listed as at risk for other adverse events and 0 as affected. The listed death count is 0 for each intervention.

These figures are counts in the registry record, not proof that no adverse events occurred outside the definitions, monitoring, or reporting represented there. The record does not list serious adverse-event terms, and it does not describe the event-collection procedures, severity criteria, or duration of safety follow-up. The absence of reported events should therefore be read as zero events recorded in these categories, not as a guarantee of no risk.

Because the study used a crossover design, participants underwent both interventions; the intervention-specific at-risk counts should not be added to create a count of unique participants. The registry’s figures describe each intervention’s safety summary separately. They do not provide comparative safety modeling or detailed information about event timing.

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

The posted statistics describe baseline serum uric acid levels in the two groups defined by first-period assignment. The Dietitian-Directed Diet Then Self-Directed Diet group had a mean of 8.23 mg/dL, and the Self-Directed Diet Then Dietitian-Directed Diet group had a mean of 8.03 mg/dL. The reported difference was -0.2 mg/dL, with a 95% confidence interval from -0.658 to 0.258 mg/dL and an unadjusted p-value of 0.392.

Those numbers do not tell whether the dietitian-directed diet lowered uric acid, whether the self-directed diet changed it, or whether the diets differed after exposure. The primary outcome is explicitly identified as baseline, and the registry presents it by first-period assignment. A baseline comparison is descriptive of the groups before the intervention periods; it is not a substitute for results measured after each diet.

The interval includes zero and allows for possible baseline differences on either side of zero. This means the posted comparison does not establish a clear baseline separation, but it also does not demonstrate exact equality or equivalence. The result’s precision and interpretation are limited to the comparison as reported, and detailed analysis assumptions are not reported in the registry record.

The study’s stated purpose was to examine the effects of the DASH diet on uric acid in adults with a history of gout. That purpose is not itself a finding. Results for a treatment-period uric acid contrast, the five listed secondary outcomes, and a complete statistical analysis are not reported in the registry record. The safety summaries list no serious adverse events, other adverse events, or deaths for either intervention, but they do not establish that an intervention is free of risk.

In short, the posted data support a description of baseline levels and their uncertainty by sequence group. They do not answer the main treatment-effect question using a reported post-intervention estimate.

16. Why This Trial Matters Statistically

This trial illustrates why the outcome timeframe and design structure must be read together. A registry may identify a clinically relevant primary outcome and a crossover design, yet the posted primary statistic may describe only a baseline measurement. If that distinction is overlooked, a baseline group comparison can be mistaken for an effect of the intervention.

The crossover feature is important because, in general, the same participant receives both interventions, allowing comparisons to use within-person information. That potential advantage depends on outcome measurements under each intervention and an analysis that appropriately handles the repeated observations and design structure. The posted primary outcome is baseline and organized by first-period assignment; it does not provide that within-person treatment comparison. A design label alone is not enough to establish what the reported estimate measures.

The record also demonstrates the value of reading effect estimates together with uncertainty. The mean difference and confidence interval convey the direction and range of the baseline contrast, while the p-value provides a related test statistic. None of these quantities, considered alone, can answer a different question from the one actually analyzed. Here they address the baseline contrast, not a diet effect.

Finally, the planning illustration emphasizes that required sample size depends on the effect targeted and the assumptions used. It gives larger participant requirements for smaller target effects and a higher power target. These are planning quantities, not evidence that the trial was designed around the same assumptions or that its observed result had a particular power. The original planning rationale is not reported in the registry record.

17. Limitations

• The posted summary statistics are the only data these results rest on; patient-level data, full protocols and statistical analysis plans are not available.
• Recalculated results are simple, unadjusted calculations from posted summaries. They do not replace the sponsor's prespecified analysis.

A central limitation of the posted record is that the primary outcome is reported at baseline, rather than as a treatment-period estimate. Although the trial is described as a crossover and all 43 participants are said to have undergone both interventions, the registry record does not report serum uric acid results under each diet or a within-participant treatment comparison. The baseline means cannot answer whether uric acid changed during either intervention.

Important crossover details are not reported in the registry record, including the duration of the dietitian-directed period, whether there was a washout period, and the statistical handling of sequence and period. The intervention description is incomplete, and further information about adherence, dietary assessment, and implementation is not reported. The registry identifies the study as double-blind but does not explain how masking was implemented for dietary interventions.

The record does not provide detailed eligibility criteria, participant characteristics, participant flow, missing-data handling, or analysis-population definitions. It also does not report a full statistical analysis plan, multiplicity approach, or the original sample-size assumptions. These gaps limit evaluation of representativeness, potential bias, and the robustness of the posted comparison.

Results for the listed secondary outcomes are not reported in the posted statistics. The adverse-event summaries report zero events in the listed categories, but event definitions and monitoring procedures are not described. These limitations concern what can be concluded from the registry record and do not establish that additional study information does not exist elsewhere.

18. Related Tutorials

19. Related Statistical Calculators

20. Sources

• ClinicalTrials.gov: The Diet Gout Trial — NCT03569020.
• Registry record last updated 2021-03-30; status last verified 2019-11.
• Results first posted 2021-03-30.
• 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.

21. Record Summary

The Diet Gout Trial (NCT03569020) is listed as a completed randomized, double-blind crossover study sponsored by Johns Hopkins University. It enrolled 43 adults with gout and hyperuricemia, and the registry states that participants underwent both the dietitian-directed and self-directed diet interventions.

The primary outcome is mean serum uric acid at baseline, reported by first-period assignment. The posted mean difference is -0.2 mg/dL, with a 95% confidence interval from -0.658 to 0.258 mg/dL and an unadjusted p-value of 0.392. This is a baseline comparison, not a reported treatment-period effect. The registry lists five secondary outcomes, but their results and a full statistical analysis plan are not reported in the registry record.