Comparison of Tetracaine 0.5% and Lidocaine 2% Jelly for Topical Phacoemulsification Cataract Surgery: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Comparison of Tetracaine 0.5% and Lidocaine 2% Jelly for Topical Phacoemulsification Cataract Surgery (NCT00827073), a randomized na study. The primary outcome is Change in Ln(Bacterial Colony Count) From Pre-antibiotic Administration to Post Study Medication Swabs.

2. Clinical Question

3. Trial Design

The registry describes a randomized, single-masked, parallel-group study. Participants were assigned to one of two medication arms: tetracaine 0.5% drops or lidocaine 2% jelly. The registry lists enrollment as 40 and reports 20 participants in each arm for the primary outcome. It does not report who was masked, how randomization was generated or implemented, or whether allocation was concealed.

The study concerned topical phacoemulsification cataract surgery. The brief description says eye-surface swabs were taken before and after topical anesthesia was administered, with the primary outcome timeframe specified as a pre-antibiotic swab and a post-study-medication swab before surgery. The arm descriptions state that Betadine 5% was used on the operative eye after the assigned anesthetic. The registry record does not provide further procedural details about timing or sequence beyond those descriptions.

The registry lists the study as completed, with a start date of 2008-11 and primary completion date of 2010-07. The listed sponsor is Hermann Eye Center. Phase is recorded as not applicable. The registry record does not report the trial's original sample-size justification, a detailed protocol, or a prespecified statistical analysis plan. It lists two outcome measures and no posted analyses. Those omissions limit what can be concluded about how the design and analysis were planned, beyond the randomized, single-masked, parallel comparison recorded in the registry.

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 Change in Ln(Bacterial Colony Count) From Pre-antibiotic Administration to Post Study Medication Swabs, measured over (1) Pre-antibiotics swab, and (2) Post-study medication (pre surgery).

The primary endpoint is the change in the natural log of bacterial colony count from the pre-antibiotic administration swab to the post-study-medication swab collected before surgery. It is a continuous measure, summarized as a mean with a standard deviation. Cultures were processed within 3 hours of acquisition: samples were vortexed for 30 seconds, and 100µl aliquots were plated on 5% sheep blood and chocolate agar. The plates were incubated with 5% carbon dioxide at 35˚ C for 72 hours, then read for colony count; isolates were identified using routine microbiological methods.

In general, a logarithmic transformation can make highly uneven count data easier to summarize and compare, because it compresses the scale at higher values. The outcome therefore describes change on the log-count scale, not a direct measure of Betadine concentration or penetration. Bacterial colony count is relevant to the stated study purpose because the registry describes comparing counts and species in eye-surface swabs before and after topical anesthesia. However, the posted primary endpoint is the change in log colony count; a separate quantitative result for Betadine penetration or bacterial species is not reported in the registry record.

7. Results Posted in the Registry

• Lidocaine 2% Jelly: mean -0.52 versus -0.14 on control; difference -0.38 (95% CI -1.24 to 0.48), standardized effect d = -0.27; z-test p = 0.388.

8. Statistical Interpretation

9. Statistical Methods Explained

What does the mean change represent?

The endpoint is a continuous measure: change in the natural log of bacterial colony count between the specified swabs. A mean summarizes the average observed change within an arm. The reported values were -0.14 for tetracaine and -0.52 for lidocaine. In general, a negative change indicates a lower value at the later measurement than at the earlier measurement on the analyzed scale. Because the scale is the natural log of the count, these mean changes should be interpreted on that scale; they are not raw colony counts.

A standard deviation describes how dispersed individual outcome values are around their arm's mean. It is not the uncertainty in the mean itself, and it is not a confidence interval. The reported standard deviations were 0.67 and 1.85. In general, a larger standard deviation indicates more variation among observed values. The wider reported spread in the lidocaine arm is important context when reading its mean and the between-arm estimate. The registry record does not provide individual-level data or additional distributional summaries.

How should the mean difference be read?

The reported mean difference compares the lidocaine arm with the tetracaine arm. Its estimate was -0.38. A negative value indicates that the observed mean change was lower in the lidocaine arm than in the tetracaine arm on the log-count scale. The estimate is an unadjusted difference between arm means, not a ratio of raw bacterial counts and not an estimate adjusted for participant characteristics. The registry record does not report an adjusted analysis.

In general, a comparison estimate is most informative when read alongside its uncertainty interval and the variability of the underlying data. The point estimate gives one best estimate under the stated comparison, but it does not show how much uncertainty surrounds that estimate. The interval and p-value help describe that uncertainty; neither should be treated as a stand-alone measure of clinical importance.

What does the confidence interval say?

The reported confidence interval runs from -1.242 to 0.482. In general, a confidence interval is calculated using a method that, under its assumptions, would cover the true parameter at its stated confidence level across repeated samples. It does not mean that a particular range of effects is guaranteed, nor that there is a specified probability that the true value lies within this one interval. Here, the interval crosses zero, so the data are compatible with no mean difference as well as with differences in either direction. The breadth of the interval reflects uncertainty in the estimated contrast.

How should the p-value be understood?

The unadjusted p-value was 0.388. In general, a p-value measures how unusual data at least as incompatible with a null hypothesis as the observed data would be if that null hypothesis and the analysis assumptions were true. It is not the probability that the null hypothesis is true, and it does not measure the size or importance of an effect. The p-value should be considered together with the estimate and confidence interval. The registry record does not report a multiplicity adjustment or a detailed model specification.

What are the implications of the count scale?

Colony counts are counts, while the primary endpoint analyzes their natural logarithm. In general, count data can be strongly skewed, and a logarithmic transformation may reduce the influence of very large values and make mean-based analysis more practical. The transformed outcome remains a continuous scale for the reported mean comparison. This does not guarantee that model assumptions were met. The registry record does not report diagnostic checks, the handling of zero counts, or an alternative count model, so those details cannot be assessed.

Over-dispersion is a general concern when count variability exceeds what a simple count model expects; if present and not addressed, standard errors and inferential conclusions can be unreliable. That is a general statistical consideration, not a reported finding in this study. Because the registry reports log-transformed outcomes with means and standard deviations, and does not report a count-model analysis or diagnostics, there is no basis to claim that over-dispersion was assessed or was present.

The registry identifies random allocation, single masking, and a parallel design. In general, randomization is intended to make assigned groups comparable on average, while masking can reduce some sources of bias when the masked parties and procedures are appropriate. The record does not identify who was masked or describe the randomization procedure, so the effectiveness of these safeguards cannot be evaluated from the registry information alone. The posted comparison is unadjusted and should be read as a comparison of the reported arm summaries.

10. Confidence Intervals and Precision

The estimated mean difference for lidocaine compared with tetracaine was -0.38 on the natural-log colony-count change scale. Its reported confidence interval extends from -1.242 to 0.482. The interval is wide enough to include zero and estimates on both sides of zero. Thus, the data do not pin down the direction of the underlying mean difference with precision: the estimate favors a lower mean change in the lidocaine arm, but values consistent with a higher mean change are also within the interval.

In general, a confidence interval conveys uncertainty around an estimate under the assumptions of its method. A narrow interval indicates greater precision than a wide interval, but precision is not the same as clinical importance or freedom from bias. The interval should be interpreted on the log-count scale and in the context of the reported group variability, especially the standard deviation of 1.85 in the lidocaine arm compared with 0.67 in the tetracaine arm.

The unadjusted p-value was 0.388, consistent with the interval including zero. This does not establish equivalence, and it does not show that a meaningful difference is absent. Rather, the posted results leave considerable uncertainty about the size and direction of the mean contrast. The registry record does not report a prespecified precision target or an adjusted confidence interval, so the posted interval is the basis for describing the uncertainty.

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, not total events. They illustrate sample sizes for a two-arm comparison using the observed effect as a starting point; they are not the trial's original enrollment target or a record of how the study was designed. For planning for 100% of the observed effect, the figures are 211 participants per arm for 80% power and 282 participants per arm for 90% power. For planning for 75% of the observed effect, they are 375 and 501 participants per arm, respectively. For planning for 50% of the observed effect, they are 842 and 1,127 participants per arm, respectively.

In plain terms, the 80% power column gives the modeled number per arm for a design with an 80% chance of detecting the specified effect under the stated assumptions; the 90% column gives the corresponding number for a 90% chance. These are planning probabilities, not a statement about what the completed study achieved. The planning assumptions are a two-sided significance level of 0.05, equal allocation between the arms, and a normal approximation. The effect to detect is the stated fraction of the observed effect. Smaller target effects require more participants in these illustrations, because they are harder to distinguish from sampling variation. This is not observed power.

The registry record does not report the trial's own sample-size justification or original design assumptions. These figures should therefore be read as hypothetical planning illustrations based on the observed estimate, not as proof that the study was underpowered or as evidence about its actual design choices. A planning calculation also depends on whether its assumptions and target effect are suitable for a future study.

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 = -0.380; pooled standard deviation σ = 1.391.
2. n = 2 × (1.96 + 0.842)² × 1.391² / -0.380² = 210.4, rounded up to 211 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 <- -0.38                      # observed difference in means
sigma <- 1.3912943613772                     # 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(827073)
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 = 211 (80% power), 282 (90% power)
 75% of observed effect: n per arm = 375 (80% power), 501 (90% power)
 50% of observed effect: n per arm = 842 (80% power), 1,127 (90% power)
power.t.test: 211.4 
Simulated power at n = 211 per arm: 0.799 (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 two outcome measures and no posted analyses. It identifies the primary outcome as change in the natural log of bacterial colony count, but the registry record does not report a detailed statistical analysis plan, a hierarchy of outcomes, or a plan for controlling multiplicity. Multiplicity arises when several outcomes, time points, subgroups, or comparisons are tested: without an appropriate plan, the chance of at least one chance finding can increase. This is a general concern, not evidence that a particular adjustment was required or omitted in the trial's conduct.

The primary comparison is reported with an unadjusted p-value of 0.388. The registry record does not say whether this was the only inferential test conducted, whether additional tests were prespecified, or whether any multiplicity procedures were used. It also does not report results for a distinct secondary outcome. Therefore, the posted primary result can be described, but the full set of analyses and their relationship to any prespecified plan cannot be assessed.

Missing-data handling is not reported in the registry record. Although 20 outcome observations are listed in each arm, that does not establish whether any randomized participants lacked outcome data, how incomplete swabs or cultures were handled, or whether all randomized participants were included in the analysis. The registry does not report an analysis population definition such as intention-to-treat or per-protocol, nor does it describe imputation or sensitivity analyses. No conclusion about attrition or missingness should be inferred from the arm summaries alone.

These omissions matter because the handling of multiple outcomes and incomplete data can affect interpretation. They do not invalidate the posted comparison by themselves, but they limit assessment of whether the reported analysis followed a prespecified plan and whether it represents all randomized participants. Further details on those procedures are not reported in the registry record.

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 adverse-event record lists 0 serious adverse events and 0 other adverse events among 20 participants at risk in the tetracaine 0.5% drop arm. It also lists 0 serious adverse events and 0 other adverse events among 20 participants at risk in the lidocaine 2% jelly arm. No serious adverse-event terms are listed.

These are the event figures recorded for the trial, not evidence that no adverse events could occur. The registry record does not describe the collection window, event definitions, severity grading, or how events were assessed. Death counts are not reported in the registry record. The listed zeros should therefore be read as reported counts within the registry's adverse-event fields, not as proof of absence of risk or as a complete characterization of safety.

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

The posted results describe a randomized comparison of two topical anesthetic regimens on change in the natural log of bacterial colony count. The mean change was -0.14 with tetracaine and -0.52 with lidocaine; the estimated mean difference for lidocaine compared with tetracaine was -0.38. The interval from -1.242 to 0.482 includes zero, and the unadjusted p-value was 0.388. These results leave uncertainty about the direction and size of the underlying difference. They do not establish that the regimens are equivalent, and they do not prove that a difference exists.

The endpoint is a microbiological measure based on swab cultures. Although the stated purpose concerns Betadine 5% penetration, the posted primary outcome is bacterial colony count, not a direct measurement of penetration. The registry record does not report a separate quantitative penetration result or a posted result for bacterial species. Conclusions should stay within the measure that was actually reported.

The outcome is on a natural-log scale, so the means and mean difference are not raw colony counts. The results do not establish what the comparison would be in other settings, with different procedures, or in populations not described in the registry. The registry record also does not report details needed to evaluate missing-data handling, analysis populations, the masking process, or multiplicity procedures. Those limits qualify interpretation of the posted summary statistics.

In general, a nonsignificant p-value is not evidence of no effect, and an interval crossing zero does not demonstrate equivalence. The appropriate reading is that the study's posted data yield an imprecise estimate with uncertainty spanning both directions. The figures summarize the observed comparison; they are not medical advice and do not determine an individual treatment choice.

16. Why This Trial Matters Statistically

This trial is statistically instructive because the outcome is a laboratory count transformed to a continuous log scale. That choice affects what the reported means represent: they describe changes in log colony count, rather than direct changes in raw counts. Understanding the analyzed scale helps prevent the mean difference from being mistaken for a raw-count difference or a direct measurement of Betadine penetration.

The results also show why an estimate should not be interpreted alone. The mean difference is -0.38, but the confidence interval extends from -1.242 to 0.482 and includes zero. The unadjusted p-value is 0.388. Together, these statistics communicate uncertainty rather than a definitive answer about the direction of the underlying mean contrast. The different arm standard deviations—0.67 for tetracaine and 1.85 for lidocaine—also make variability a relevant part of the statistical reading.

Finally, this record illustrates the limits of registry summaries. It identifies randomization, masking, and the primary outcome, but does not report key details such as the original sample-size justification, analysis population, handling of missing data, or multiplicity plan. Those omissions do not change the values reported, but they constrain how fully the analysis can be evaluated. Clear reporting of design and analysis decisions is important for distinguishing a result's numerical uncertainty from uncertainty about the methods used to obtain it.

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 gives limited information about trial conduct. It describes the study as randomized and single-masked but does not report the randomization method, allocation concealment, or who was masked. It does not provide detailed eligibility criteria or baseline characteristics, so the enrolled population cannot be characterized beyond the listed cataract condition and surgical context.

The primary endpoint uses change in log colony count, while the stated study purpose refers to Betadine 5% penetration. The registry record does not report a separate direct measurement of penetration, or a posted result for bacterial species, despite the description that isolates were identified. This limits conclusions to the outcome and comparison that are actually posted. The record also does not provide the underlying participant-level data, so distributions, outliers, and the effect of the different reported standard deviations cannot be examined.

The registry does not report how missing or unusable swabs and cultures were handled, define an analysis population, or describe sensitivity analyses. It does not provide a detailed statistical analysis plan, multiplicity procedures, or the trial's original sample-size justification. The comparison is reported as unadjusted, and no adjusted analysis is reported. These limits make it difficult to evaluate the analysis beyond the summary values and to assess whether all planned outcomes and analyses are represented.

The adverse-event fields list zero events in both categories and arms, but the registry record does not report event definitions, ascertainment procedures, or the observation period. Death counts are not reported in the registry record. These reporting limits should be considered when reading the safety figures.

18. Related Tutorials

19. Related Statistical Calculators

20. Sources

• ClinicalTrials.gov: Comparison of Tetracaine 0.5% and Lidocaine 2% Jelly for Topical Phacoemulsification Cataract Surgery — NCT00827073.
• Registry record last updated 2014-07-22; status last verified 2014-07.
• Results first posted 2014-06-23.
• 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

NCT00827073 is a completed, randomized, single-masked, parallel-group study sponsored by Hermann Eye Center. It compared tetracaine 0.5% drops with lidocaine 2% jelly in topical phacoemulsification cataract surgery. Enrollment was 40, with 20 participants listed in each arm for the primary outcome.

The primary outcome was change in the natural log of bacterial colony count from a pre-antibiotic swab to a post-study-medication, pre-surgery swab. The mean changes were -0.14 for tetracaine and -0.52 for lidocaine. The estimated mean difference for lidocaine compared with tetracaine was -0.38, with a confidence interval from -1.242 to 0.482 and an unadjusted p-value of 0.388. The interval includes zero, leaving uncertainty about the direction and size of the difference.

The registry record lists zero serious and zero other adverse events in each arm. It does not report deaths, detailed analysis methods, missing-data handling, or the original sample-size justification.