Perioperative Use of Gabapentin To Decrease Narcotic Requirements in Spinal Fusion: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Perioperative Use of Gabapentin To Decrease Narcotic Requirements in Spinal Fusion (NCT00726999), a randomized na study. The primary outcome is Amount of Morphine Consumed (mg/kg/hr).

2. Clinical Question

The registry describes the study goal as measuring morphine use and the incidence of morphine side effects. Listed effects and related measures include pruritis, foley duration, time to first stool, sedation, pain scores, and patient-controlled analgesia use. The listed secondary outcome is the number of ondansetron doses administered for nausea. The clinical question is therefore focused on perioperative adjunctive gabapentin in pediatric spinal fusion, with morphine used in both randomized groups.

3. Trial Design

The registry record describes a randomized, parallel-group study with quadruple masking. The two arms were Gabapentin/Morphine and Placebo/Morphine; the placebo pill was matched in appearance to gabapentin. The registry lists 63 participants enrolled, with 32 in the gabapentin arm and 31 in the placebo arm for the primary outcome. The recorded phase is NA.

The brief summary describes oral gabapentin given as a preoperative loading dose of 15mg/kg and then 5mg/kg three times a day for 5 days or until discharge. Both groups also received morphine through patient-controlled analgesia. The registry identifies Medical College of Wisconsin as the sponsor. The recorded study start is 2006-06, primary completion is 2008-07, and status is completed.

The primary outcome was the amount of morphine consumed, expressed as a mean in mg/kg/h, during the postanesthesia recovery unit timeframe. The record says participants were taken to the recovery unit immediately after surgery and typically remained for 1 hour. The posted primary outcome includes a mean and standard deviation for each arm and a comparison between arms.

The record lists four outcome measures and one secondary outcome in the posted description. It does not report the original sample-size justification, the intended analysis plan, the masking procedures, or how participants were assigned to groups. Those design details are not reported in the registry record and cannot be inferred from the allocation and masking labels alone.

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

Reasons for not completing this period:

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 Amount of Morphine Consumed (mg/kg/hr), measured over PARU (Postanesthesia Recovery Unit - participants typically remain in PARU for 1 hour).

The primary endpoint measures the mean amount of morphine consumed per kilogram of body weight per hour in the postanesthesia recovery unit. Using an amount per kilogram per hour expresses consumption relative to participant weight and time, rather than as a total amount across participants. The registry describes the recovery unit as the immediate postoperative setting, where participants typically remain for 1 hour.

Morphine consumption is relevant to the study question because the trial aimed to assess whether perioperative gabapentin changed morphine requirements when morphine was available to both groups through patient-controlled analgesia. A group mean gives a concise description of the average recorded consumption in each arm. The standard deviations describe variation among individual observations around those means; they are not confidence intervals for the group means.

The posted means were 0.064 mg/kg/h for Gabapentin/Morphine and 0.044 mg/kg/h for Placebo/Morphine. The registry comparison expresses the mean difference as 0.02 mg/kg/h, with the gabapentin arm higher. This endpoint concerns consumption in the recovery unit timeframe. It does not, by itself, measure pain relief, morphine use over the entire hospital stay, or the incidence of adverse effects.

The record does not report further detail about how morphine consumption was collected, whether all participants contributed a complete observation, or how any incomplete measurements were handled. Those details are not reported in the registry record.

7. Results Posted in the Registry

• Gabapentin/Morphine: mean 0.06 versus 0.04 on control; difference 0.02 (95% CI 0.01 to 0.03), standardized effect d = 0.80; z-test p = 0.001.

8. Statistical Interpretation

9. Statistical Methods Explained

The primary outcome is continuous: morphine consumption is measured on a numeric scale, and the registry summarizes each arm with a mean and standard deviation. A mean describes the average value in a group. A standard deviation describes how dispersed individual values are around that average. These are different summaries: the standard deviation is not a measure of uncertainty in the mean, and it does not state how far the true group mean may be from the observed mean.

What does a mean difference express?

The reported comparison is a difference between the two group means. The registry uses the Placebo/Morphine mean of 0.044 mg/kg/h and the Gabapentin/Morphine mean of 0.064 mg/kg/h, with the result expressed as gabapentin minus placebo. The posted difference is 0.02 mg/kg/h. A positive value therefore indicates a higher observed mean in the gabapentin arm for this outcome. The difference is expressed in the same measurement units as the endpoint.

In general, a between-group mean difference is most directly interpretable when the groups are compared on the same measurement scale and the outcome is summarized in a comparable way. The randomized parallel-group design establishes the intended comparison between assigned arms, but the registry record does not report whether the posted analysis was adjusted or unadjusted, or which participants were included in its analysis population.

What does the p-value tell readers?

The registry reports p = 0.001 for the comparison. In general, a p-value describes how compatible the observed data are with a specified null model, under the assumptions of the statistical test. A smaller p-value indicates less compatibility with that null model; it is not a measure of effect size, clinical importance, or the probability that the null hypothesis is true. The test used and its detailed assumptions are not reported in the registry record, so the p-value should be read as the reported result rather than as a complete account of the analysis.

What does the confidence interval add?

A confidence interval presents a range of values that are compatible with the data under the interval method and its assumptions. For a mean difference, it communicates uncertainty about the estimated difference. The posted interval for this comparison lies above zero, matching the direction of the reported mean difference. It does not describe the range in which most individual participants’ measurements fall. The exact interval method is not reported in the registry record.

In general, a confidence interval is more informative than a p-value alone because it shows both the estimated direction and the range of effect sizes reasonably compatible with the data under the method. Its interpretation still depends on the analysis model, measurement quality, and completeness of the observations. Those details are not fully described in this registry record.

Why does variability matter?

The standard deviations are 0.031 mg/kg/h in the gabapentin arm and 0.017 mg/kg/h in the placebo arm. In general, greater variability makes a group mean less precisely estimated when other features are similar, because observations are more dispersed. When variability differs between groups, the choice of statistical method and its assumptions can matter. Over-dispersion, in general, means that the observed variation is greater than a model expects; this can make standard errors too small if the model does not account for it. The registry record does not state whether an over-dispersion assessment was relevant or performed in this analysis.

The confidence interval and p-value are reported, but the registry does not provide the test name, model specification, standard error method, covariate adjustment, or diagnostic checks. The comparison can be described from its reported estimates, but these missing details limit independent assessment of the analysis procedure.

What can the planning calculations show?

In general, sample-size planning calculations estimate how many participants per arm would be needed to have a specified chance of detecting an assumed effect under stated design and analysis conditions. The posted planning figures use the observed effect and also consider smaller fractions of that effect. They are illustrations, not a record of the trial’s original planning process. They do not establish the trial’s observed power, and they should not be used to infer why the trial enrolled its reported number of participants.

For the planning illustration, the stated assumptions are a two-sided significance level of 0.05, equal allocation between the two arms, and a normal approximation. For continuous outcomes, the figures are participants per arm. The effect to detect is the stated fraction of the observed effect. In general, smaller assumed effects require more participants to achieve the same target power. The original trial design assumptions and sample-size justification are not reported in the registry record.

10. Confidence Intervals and Precision

The reported mean difference is 0.02 mg/kg/h, with the Gabapentin/Morphine mean higher than the Placebo/Morphine mean. The registry comparison includes a confidence interval whose lower and upper limits are both above zero. This indicates that the interval, under its method and assumptions, does not include no difference. The exact interval method is not reported in the registry record.

A confidence interval is a measure of precision for an estimated quantity. In general, a narrower interval indicates that the estimate is more precise, while a wider interval leaves more uncertainty about the size of the difference. The interval does not mean that individual participants’ outcomes lie within its limits. It also does not establish whether the difference is important in clinical practice; that judgment requires context beyond the statistical interval.

The comparison is based on 32 participants in the gabapentin arm and 31 in the placebo arm. The observed standard deviations were 0.031 mg/kg/h and 0.017 mg/kg/h, respectively. The differing spreads indicate that observations varied to different degrees in the two groups. The posted interval reflects the registry’s analysis, but its precise construction and any adjustment for other variables are not reported in the registry record.

The p-value is 0.001. It is consistent with the interval excluding zero for this reported comparison, but it does not replace the interval’s information about the range of plausible differences. Neither statistic explains how missing outcome data, if any, were handled. The registry record does not report the number with complete primary outcome data beyond the arm counts shown or provide a missing-data analysis.

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 planning figures are participants per arm for this continuous outcome, not total event counts. They illustrate the sample sizes associated with detecting different assumed fractions of the observed effect under the listed planning assumptions. At 100% of the observed effect, the figures are 25 participants per arm for 80% power and 34 participants per arm for 90% power. At 75% of the observed effect, they are 44 and 59 participants per arm, respectively. At 50% of the observed effect, they are 99 and 133 participants per arm, respectively.

The assumptions are a two-sided significance level of 0.05, equal allocation between the two arms, and a normal approximation. The target effect is the stated fraction of the observed effect. In plain terms, the 80% power column gives the estimated participants per arm for a planning scenario intended to have an 80% chance of detecting the assumed effect under those assumptions. The 90% power column gives the corresponding participants per arm for a 90% chance under the same kind of planning scenario.

Smaller effects require more participants to achieve the same target power, which is why the figures rise as the assumed fraction of the observed effect decreases. These values are planning illustrations, not observed power. The trial’s original sample-size justification and design assumptions are not reported in the registry record, so the calculations do not explain why the study enrolled 63 participants.

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 = 0.020; pooled standard deviation σ = 0.025.
2. n = 2 × (1.96 + 0.842)² × 0.025² / 0.020² = 24.7, rounded up to 25 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 <- 0.02                      # observed difference in means
sigma <- 0.0251099222762897                     # 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(726999)
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 = 25 (80% power), 34 (90% power)
 75% of observed effect: n per arm = 44 (80% power), 59 (90% power)
 50% of observed effect: n per arm = 99 (80% power), 133 (90% power)
power.t.test: 25.7 
Simulated power at n = 25 per arm: 0.789 (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 record lists four outcome measures and identifies one secondary outcome: the number of ondansetron doses administered for nausea. The primary outcome is mean morphine consumption in the postanesthesia recovery unit. The brief summary also mentions morphine side effects and other measures, including pruritis, foley duration, time to first stool, sedation, pain scores, and patient-controlled analgesia use.

When a study examines multiple outcomes or makes multiple comparisons, the chance of at least one apparently notable result can increase. In general, a multiplicity plan can specify which outcome is primary, which comparisons are confirmatory, and whether statistical tests are adjusted. The registry record does not report a multiplicity strategy or whether any adjustment was made. The reported primary comparison should therefore be interpreted as posted, without assuming that other outcome tests were planned or controlled in a particular way.

The record lists zero posted analyses. It does not describe a statistical analysis plan, the test or model used, covariate adjustment, or the analysis population. It also does not report whether participants were analyzed according to randomized assignment, whether any per-protocol or safety populations were used, or how withdrawals and incomplete measurements were handled. These details are not reported in the registry record.

For the primary outcome, the arm counts shown are 32 and 31. The record does not clarify whether these counts represent all randomized participants, all participants with a usable primary outcome, or another analysis set. No imputation method or sensitivity analysis for missing values is described. As a result, the extent to which missing data or analysis-population choices could affect the estimate cannot be assessed from the registry record alone.

14. 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% reporting threshold.

The posted serious adverse event figures list 0 events among 29 participants at risk in the Gabapentin/Morphine group and 0 among 30 participants at risk in the Placebo/Morphine group. The corresponding other adverse event figures are also 0 among 29 and 0 among 30. These denominators differ from the primary outcome arm counts of 32 and 31, and the registry record does not explain the difference.

Zero posted events means that no events of the reported categories were recorded in those group summaries. It does not establish that no adverse events occurred outside the categories or reporting period captured by the record, nor does it show that the groups have identical safety profiles. The record lists no top serious adverse event terms. Death counts are not reported in the registry record.

Adverse event summaries are descriptive. In general, the number of events and the number of participants at risk both matter when interpreting a safety report. The record does not provide further details about event definitions, follow-up duration, severity grading, or the method used to collect events. The posted counts should therefore be read narrowly as the registry’s reported figures.

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

The primary result describes morphine consumption per kilogram per hour in the postanesthesia recovery unit. The mean was 0.064 mg/kg/h with gabapentin plus morphine and 0.044 mg/kg/h with placebo plus morphine; the reported mean difference was 0.02 mg/kg/h. The comparison’s confidence interval lies above zero, and its reported p-value is 0.001. These statistics indicate that the posted group means differed in this outcome, with a higher mean in the gabapentin arm.

That finding is limited to the stated measure and setting. It does not answer whether pain scores differed, whether participants had different recovery experiences, or whether morphine consumption later in the hospital stay differed. The brief summary names several other outcomes, but results for those measures are not included in the primary comparison. The secondary outcome listed in the record concerns ondansetron doses for nausea; its result is not reported in the registry record.

A statistically distinguishable mean difference is not itself a judgment about clinical importance. The registry record does not provide a threshold for a meaningful difference on this measure, nor additional context for judging the importance of the observed value. The result also does not prove that every participant’s morphine use followed the group average. The reported standard deviations show individual observations varied within each arm.

The record’s randomized design supports reading the arms as the intended comparison, but important details about the analysis, missing observations, and participant populations are not reported. The posted results should be understood alongside those limitations and should not be generalized beyond the population, treatment comparison, endpoint, and timeframe described in the record.

16. Why This Trial Matters Statistically

This trial illustrates why the outcome definition and timeframe matter when interpreting a clinical comparison. Morphine consumption in the immediate recovery unit is a specific measure, not a substitute for total opioid use, pain scores, or adverse effects across the full postoperative course. The reported difference is interpretable only with that scope in mind.

It also shows why an estimate should be read together with its uncertainty and its direction. The posted mean difference is 0.02 mg/kg/h, and the gabapentin group mean is higher. The confidence interval lies above zero, while the p-value is 0.001. These summaries describe statistical evidence for a difference in the measured endpoint, but they do not determine its clinical importance.

Variation between participants is another relevant feature. The standard deviation is greater in the gabapentin arm than in the placebo arm. In general, variability affects the precision of group estimates and can influence the assumptions needed for an analysis. Because the registry does not describe the statistical model or diagnostic checks, readers cannot assess those analysis details from the record.

Finally, the trial’s posted planning illustration is distinct from its original design rationale. It shows how participant requirements vary under different assumed effect sizes and target power levels, but the original sample-size justification is not reported. This distinction helps prevent planning calculations based on an observed result from being mistaken for evidence about the trial’s initial design or its ability to detect every possible effect.

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.

The registry record offers limited detail for independently evaluating how the primary comparison was conducted. It does not report the test or model, the analysis population, covariate adjustment, missing-data handling, or the original sample-size justification. Although the record lists zero posted analyses, it does give a primary comparison; the analysis procedures behind that comparison are not fully described.

The primary outcome arm counts are 32 and 31, while the adverse event summaries use 29 and 30 participants at risk. The record does not explain why these denominators differ. It also does not describe the completeness of outcome collection or whether any participants were excluded from particular analyses.

The result concerns only mean morphine consumption per kilogram per hour in the postanesthesia recovery unit, a setting where participants typically remain for 1 hour. The record’s summary mentions other outcomes, but their results are not presented in this comparison. The record does not give enough detail to assess how the outcome was collected or whether it captures morphine use beyond that recovery period.

The registry describes quadruple masking but does not explain which people were masked or how masking was implemented. It also does not provide detailed participant characteristics or eligibility criteria. Finally, the planning figures are based on the observed effect and stated illustrative assumptions; they are not evidence of the trial’s original planning process.

18. Related Tutorials

19. Related Statistical Calculators

20. Sources

• ClinicalTrials.gov: Perioperative Use of Gabapentin To Decrease Narcotic Requirements in Spinal Fusion — NCT00726999.
• Registry record last updated 2019-04-03; status last verified 2019-03.
• Results first posted 2010-12-30.
• 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 registry record describes a completed randomized, parallel-group, quadruple-masked trial sponsored by Medical College of Wisconsin. It lists 63 participants enrolled and compares gabapentin plus morphine with matched placebo plus morphine in pediatric spinal fusion patients. The recorded phase is NA, with a study start of 2006-06 and primary completion of 2008-07.

The primary outcome is mean morphine consumption per kilogram per hour in the postanesthesia recovery unit. The posted means were 0.064 mg/kg/h for Gabapentin/Morphine and 0.044 mg/kg/h for Placebo/Morphine, with a reported mean difference of 0.02 mg/kg/h and p = 0.001. The comparison interval lies above zero.

The record lists one secondary outcome, ondansetron doses for nausea, but does not report its result in the available comparison. It also does not report the original sample-size justification, analysis plan, missing-data procedures, or analysis populations.