NAC Phase IIB: A Multi-Center, Phase IIB, Randomized, Placebo-controlled, Double-Blind Study Of The Effects Of N-Acetylcysteine On Redox Changes and Lung Inflammation In Cystic Fibrosis Patients: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for NAC Phase IIB: A Multi-Center, Phase IIB, Randomized, Placebo-controlled, Double-Blind Study Of The Effects Of N-Acetylcysteine On Redox Changes and Lung Inflammation In Cystic Fibrosis Patients (NCT00809094), a randomized phase 2 study. The primary outcome is Change in the Logarithm of the Level of Human Neutrophil Elastase (HNE) Activity Measured in Sputum.

2. Clinical Question

This was a proof-of-concept study of a biological measure related to lung inflammation in cystic fibrosis. Its primary question concerned the change in sputum human neutrophil elastase activity, not whether participants experienced a particular clinical benefit. The registry also lists lung-function measures as secondary outcomes, including change in FEV1 percent predicted for age, FEV1 in liters, and FEF 25-75% in liters per second and as percent predicted. The posted primary result should therefore be read as an estimate about the specified sputum measure over the stated follow-up, rather than as a complete assessment of all possible clinical outcomes.

The record identifies a randomized, parallel-group comparison with placebo. The trial is marked completed and lists Stanford University as sponsor. Additional details about eligibility, recruitment settings, and the characteristics of participants are not reported in the registry record.

3. Trial Design

The registry describes a multicenter, randomized, placebo-controlled Phase IIB trial of oral N-acetylcysteine in cystic fibrosis. Its title calls the study double-blind, while the masking entry is recorded as triple. The identities of the parties covered by the masking designation, and how masking was maintained, are not reported in the registry record. The parallel-group design assigned participants to N-acetylcysteine or placebo, with both interventions administered TID for 24 weeks. The active-treatment description specifies 900 mg per administration.

The registry lists 70 subjects randomized. It reports that 65 completed the Week 12 visit and 62 completed the study. The primary endpoint description says its analysis used the subset of the ITT population with complete case data. It also reports 6 withdrawals in the NAC group and 2 in Placebo. The available description ends while beginning to state the reasons for withdrawal; the full explanation is not reported in the registry record. Completion figures and withdrawal counts are useful context, but they do not by themselves establish which participants contributed data to each endpoint or analysis.

The primary endpoint was change in the logarithm of sputum human neutrophil elastase activity, measured in log10 mcg/mL, from enrollment to the end of the 24-week trial. The specified treatment contrast subtracts change in the placebo group from change in the active-treatment group. The posted arm summaries identify Study Drug and Placebo, while the intervention descriptions identify N-acetylcysteine and placebo. The record lists four secondary lung-function outcomes and 7 outcome measures overall. It reports 1 analysis, but details of any additional statistical procedures are not reported in the registry record.

The study is marked completed. The registry lists a start in 2008-11 and primary completion in 2011-02. Its full protocol, detailed statistical analysis plan, and explanations for the selected enrollment are not reported in the registry record. Those omissions limit what can be concluded about planned procedures beyond the information entered in the trial record.

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:

Analysis population for the primary outcome, as defined in the registry: The primary endpoint analyses described uses the subset of the ITT population with complete case data. 70 subjects randomized, 65 completed the Week 12 visit and 62 completed the study. There were 6 withdrawals in the NAC group and 2 in Placebo. Five out of 6 subjects in the NAC group withdrew due to subject decision. Missing data was not imputed.

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 the Logarithm of the Level of Human Neutrophil Elastase (HNE) Activity Measured in Sputum, measured over From enrollment to end of the 24-week trial.

The primary endpoint measures change in sputum human neutrophil elastase activity, expressed as the logarithm of the level in log10 mcg/mL. The registry defines the period as enrollment to the end of the 24-week trial and defines the treatment contrast as the change in the active-treatment group minus the change in the placebo group. Human neutrophil elastase is the biological measure selected for this proof-of-concept study of redox changes and lung inflammation in cystic fibrosis.

Using a logarithm expresses the measurement on a transformed scale. As a general statistical point, transformations can be useful when the original measurement has a distribution or scale for which analysis on the untransformed scale is less suitable. The registry record does not report why the logarithm was selected, whether any transformation checks were performed, or how measurements below assay limits, if any, were handled.

The endpoint focuses on a laboratory measure rather than a direct clinical event or symptom. Its value is that it targets a biological process the study set out to examine. Its interpretation is correspondingly narrow: a group contrast in this endpoint does not by itself establish a difference in lung function or another clinical outcome. FEV1 and FEF 25-75% measures are listed as secondary outcomes, but their numerical results are not included in the registry record described for the posted primary comparison.

The endpoint description states that the primary analysis used complete case data from the ITT population. That qualification matters because the endpoint result need not represent every randomized participant if a measurement was missing. The registry reports completion and withdrawal information, but does not provide the full reasons for missing endpoint data or a detailed account of how the analysis population was assembled.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

The registry reports the primary comparison as a two-sided t-test and gives the parameter as a net mean difference. The two-sided description means the analysis considered departures in either direction, rather than testing only for an increase or only for a decrease. A t-test is a general method for comparing means while accounting for uncertainty in estimated means. The record does not report additional detail about the exact test implementation, such as variance assumptions, covariate adjustment, or checks of model conditions.

What does the net difference represent?

The endpoint definition sets the contrast as change in the active-treatment group minus change in the placebo group. A positive contrast therefore means the estimated change was higher in the active-treatment group on the endpoint's stated scale; a negative contrast means it was lower. This direction alone does not establish whether a change is clinically favorable. The registry does not report a threshold defining a clinically important difference for this biomarker.

The posted net mean difference is 0.21. Its 95% confidence interval extends from -0.07 to 0.48, and the reported p-value is 0.14. The interval gives a range of values compatible with the estimate under the analysis assumptions. Because the interval includes the null value, it does not exclude no difference. The p-value describes how unusual a result at least as inconsistent with the null as the observed result would be under the null model and the stated test assumptions. It is not the probability that the null hypothesis is true, nor does it measure the size or clinical importance of an effect.

Why report a confidence interval as well as a p-value?

A p-value condenses evidence against a null hypothesis into a single measure, while a confidence interval communicates the estimated direction, scale, and precision of the contrast. The interval can make clear that a result is uncertain even when a point estimate looks notable. Conversely, a p-value alone does not show which effect sizes remain compatible with the data. Here, the interval includes values on both sides of the null, so the direction of the underlying contrast is not settled by the reported estimate. The registry does not state a prespecified decision threshold for the primary comparison.

Arm-level confidence intervals and the confidence interval for the contrast answer different questions. The former describe uncertainty around each arm's mean change; the latter describes uncertainty around the between-group difference. The overlap or non-overlap of arm intervals is not a substitute for the direct contrast and its uncertainty. The registry reports the direct net difference, so that is the appropriate estimate for the primary comparative question.

What does complete case analysis imply?

The primary endpoint description says the analysis used the subset of the ITT population with complete case data. In general, an intention-to-treat population refers to participants analyzed according to their randomized assignments, while a complete case analysis uses only participants with the data required for that analysis. When data are missing, a complete case result can differ from the result that would have been obtained had outcomes been available for all randomized participants. The size and direction of any such difference depend on why data are missing and how missingness relates to outcomes. The registry does not report a missing-data model, imputation procedure, or sensitivity analysis for this endpoint.

The record reports 70 subjects randomized, 65 completing the Week 12 visit, and 62 completing the study. It also reports 6 withdrawals in the NAC group and 2 in Placebo. These participant-flow details provide context but cannot replace a breakdown of endpoint observations by randomized group and visit. The available primary endpoint description is incomplete where it begins to discuss withdrawal reasons. Additional reasons and the exact endpoint analysis counts are not reported in the registry record.

How should planning illustrations be understood?

Planning illustrations based on an observed effect are not observed power and are not a post hoc demonstration that the trial was adequately powered. The registry's stated illustration assumptions are a two-sided significance level of 0.05, equal allocation, and a normal approximation, with the effect to detect expressed as a fraction of the observed effect. Such illustrations show how planning requirements would vary under those assumptions. They do not establish the assumptions used when this trial was designed, and the registry does not report its original sample-size justification. The illustrations should not be read as evidence about the probability that this completed study detected an effect.

More generally, a statistical method relies on assumptions about the data and the analysis. A t-test comparison of means is most straightforward to interpret when its assumptions are reasonably appropriate for the analyzed observations. The record does not report diagnostics or enough methodological detail to independently assess those assumptions. The logarithmic endpoint scale may be relevant to distributional behavior, but the rationale and checks are not reported. These uncertainties do not invalidate the posted result; they describe limits on how much can be independently assessed from the registry record.

10. Analyses Reported by the Sponsor

The registry reports a two-sided t-test for the net mean difference, with an estimate of 0.21, a 95% confidence interval from -0.07 to 0.48, and p = 0.14. These are the reported inferential results for the primary comparison. The arm summaries list the Study Drug estimate as 0.03 and the Placebo estimate as -0.17, each with its own 95% confidence interval. The contrast is defined as active-treatment change minus placebo change.

The record does not flag a discrepancy between the reported analysis and a recalculated result. There is no separate recalculated comparison described in the registry record, so no alternative inferential estimate should be substituted for the posted test. A reconstruction from summary values, if considered, would be an unadjusted calculation and could differ from a study analysis because of data handling or methods not represented in group summaries. No such separate result is reported in the registry record.

The reported interval includes the null value, and the p-value is 0.14. The comparison therefore remains uncertain rather than establishing a clear difference. The registry does not provide enough detail to determine whether the reported test included any adjustments beyond the method named, or how all missing values were handled. The reported analysis is the principal inferential result to interpret, alongside those limitations.

11. Multiplicity, Missing Data and Analysis Populations

The registry lists 7 outcome measures and identifies four secondary lung-function outcomes: change in FEV1 percent predicted for age, FEV1 in liters, FEF 25-75% in liters per second, and FEF 25-75% as percent predicted. It reports 1 analysis. The primary outcome concerns change in log sputum human neutrophil elastase activity. The registry does not describe how these outcomes were organized into a testing hierarchy, whether any secondary outcomes were formally tested, or whether a multiplicity adjustment was planned or used.

Multiplicity is a general statistical concern that arises when several hypotheses are tested. With more tests, the chance of at least one apparently notable result can increase if each test is interpreted without accounting for the others. Whether and how to address that issue depends on the study's aims and prespecified analysis plan. The statistical plan and a full account of outcome testing are not reported in the registry record, so the role of multiplicity in interpreting the secondary results cannot be assessed from the record.

The primary endpoint analysis used the subset of the ITT population with complete case data. The record reports 70 subjects randomized, 65 who completed the Week 12 visit, and 62 who completed the study. It also reports 6 withdrawals in the NAC group and 2 in Placebo. These registry entries indicate that the randomized population and the population with complete study follow-up were not necessarily the same, but the exact number with analyzable primary endpoint data by arm is not fully reported in the available endpoint description.

The primary endpoint text begins to describe reasons for withdrawal but is cut off before the explanation is complete. Full reasons for missing observations, timing of all missing measurements, and whether missingness differed across outcome visits are not reported in the registry record. The record also does not describe imputation, weighting, or sensitivity analyses for missing endpoint values. As a general matter, complete case results are most straightforward to interpret when the analyzed cases adequately represent the intended population for the question. Whether that condition holds cannot be determined from the details reported.

These omissions matter because missingness and multiplicity concern different sources of uncertainty. Missing data affect which observations contribute to the estimate; multiple testing affects how evidence is interpreted across outcomes or hypotheses. Neither concern can be resolved by the reported p-value for the primary comparison alone.

12. Safety Results

The registry reports serious adverse events as affected participants divided by participants at risk.

Other (non-serious) adverse events

Participants with at least one non-serious event at or above the registry's 5% reporting threshold.

Most frequent serious adverse event terms

Most frequent non-serious adverse event terms

Serious events with the highest arm-level rate: pulmonary exacerbation (50.0%).

The registry lists serious adverse events for the Study Drug and Placebo groups. It reports 15 participants affected among 36 at risk in the Study Drug group and 17 affected among 34 at risk in the Placebo group. Pulmonary Exacerbation is listed as a top serious adverse event term with 50.0%. The record does not explain the denominator or method used for that percentage, so it should not be treated as a directly comparable group-specific rate.

These counts describe the serious adverse event information entered in the registry; they do not establish whether an event was caused by a study intervention. Interpreting safety comparisons requires attention to event definitions, follow-up, exposure, participant characteristics, and how events were collected. Those details are not reported in the registry record. The figures are also counts of participants affected, not necessarily counts of every event, and the record does not clarify recurrent events in the listed summary.

Deaths are not given as a numerical count in the registry entry. A missing death value is not evidence that no deaths occurred. The serious adverse event figures should therefore be read as descriptive registry reporting, not as a complete causal assessment or a basis for comparing safety beyond the information recorded.

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

The primary result concerns the estimated between-group difference in change in sputum human neutrophil elastase activity over the 24-week trial. The net mean difference is 0.21, and the 95% confidence interval extends from -0.07 to 0.48. The reported two-sided p-value is 0.14. Because the interval includes the null value, the result does not establish a clear difference between the groups on this endpoint. The interval also leaves a range of possible effects compatible with the data, so it should not be interpreted as showing that the true effect is exactly the point estimate.

This result does not demonstrate that the groups are equivalent. A failure to distinguish the groups statistically is not the same as evidence that any possible difference is sufficiently small to be unimportant. An equivalence or non-inferiority conclusion would require a prespecified margin and an analysis designed for that purpose; such a margin and analysis are not reported in the registry record.

The finding is specific to the primary biomarker endpoint, its measurement period, and the analysis population described. It does not establish effects on FEV1, FEF 25-75%, symptoms, exacerbations, or other outcomes. The registry lists lung-function measures as secondary outcomes, but the numerical secondary results are not reported in the registry record for this account. Nor does a group average reveal whether an individual participant's measurement increased or decreased.

The study used complete case data from the ITT population for the primary endpoint analysis. The record reports completion and withdrawal information, but does not fully describe missing measurements or their handling. This limits assessment of how representative the analyzed cases were of all randomized participants. The statistical plan and the original sample-size justification are also not reported, so the intended precision and planned approach cannot be reconstructed from the registry record.

The most defensible conclusion is therefore limited: the registry reports an uncertain estimate for the primary sputum biomarker contrast, with a confidence interval that includes no difference. The result is not a clinical recommendation, does not resolve every outcome of interest, and does not establish an effect in either direction with high precision. Safety figures are separately descriptive and do not establish causation.

14. Why This Trial Matters Statistically

This trial illustrates why a clinical study's main statistical result should be interpreted through its estimand, analysis population, and uncertainty together. The endpoint is a change in a logarithmically expressed sputum biomarker, and the contrast is explicitly defined as active-treatment change minus placebo change. Without preserving that definition and scale, it is easy to misstate what the estimate represents or imply a clinical meaning that the endpoint alone cannot support.

The reported point estimate is accompanied by a confidence interval that includes the null value. That combination illustrates the distinction between an estimated effect and the precision with which it is known. The point estimate gives a central summary, while the interval shows that the data are compatible with effects on either side of no difference. A p-value should not replace that assessment of precision, and a result that does not distinguish groups statistically should not be described as proof of equivalence.

The complete case description also highlights the importance of analysis populations. Randomization establishes the intended comparison, but incomplete follow-up means that the primary analysis may not include all randomized participants. Completion figures alone cannot show how missing endpoint data affected the result. Clear reporting of endpoint-specific counts, missing-data handling, and sensitivity analyses would help readers assess this uncertainty.

Finally, the trial provides a useful example of the boundary between a biological endpoint and clinical outcomes. The primary question concerns a sputum measure related to inflammation, while lung-function measures are listed separately. Statistical evidence about a biomarker does not automatically answer questions about those other outcomes. The registry's limited reporting of the analysis plan and secondary results reinforces the need to keep conclusions tied to the outcomes and estimates actually reported.

15. Limitations

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

The registry record does not provide the full protocol or statistical analysis plan. It therefore does not report the rationale for the endpoint scale, the prespecified significance threshold for the primary comparison, or whether the test included any covariate adjustment. The reported method is a two-sided t-test, but its implementation and assumptions are not described in enough detail for an independent assessment.

The primary endpoint analysis is described as a complete case analysis within the ITT population. The record gives participant completion and withdrawal information, but does not provide a complete endpoint-specific accounting of missing observations by group. The withdrawal description is incomplete, and the full reasons are not reported in the registry record. Missing-data methods and sensitivity analyses are also not reported.

The trial title describes the study as double-blind, while the masking entry says triple. The registry does not identify which parties were masked or explain this difference in terminology. The record is also limited in participant characteristics and eligibility detail. Those omissions constrain assessment of how the enrolled population relates to other populations with cystic fibrosis.

The listed serious adverse event information is descriptive. The registry does not explain the denominator behind the 50.0% Pulmonary Exacerbation entry, does not provide a numerical death count, and does not establish causality for the reported events. Full secondary outcome results are not reported in the information available for this account, despite the listing of four secondary lung-function measures and 7 outcome measures overall. Finally, the original sample-size justification and intended precision are not reported; planning illustrations based on the observed effect cannot replace that missing design information.

16. Related Tutorials

17. Related Statistical Calculators

18. Sources

• ClinicalTrials.gov: NAC Phase IIB: A Multi-Center, Phase IIB, Randomized, Placebo-controlled, Double-Blind Study Of The Effects Of N-Acetylcysteine On Redox Changes and Lung Inflammation In Cystic Fibrosis Patients — NCT00809094.
• Registry record last updated 2013-04-22; status last verified 2013-04.
• Results first posted 2013-03-15.
• Data retrieved from ClinicalTrials.gov on 2026-11-09.
• Group sizes, means, event counts and sponsor-reported p-values are reproduced as posted. Effect estimates, confidence intervals and p-values not attributed to the sponsor are independently calculated from the posted summary statistics.

19. Record Summary

The registry describes a completed, randomized, parallel-group, placebo-controlled Phase IIB study of oral N-acetylcysteine in people with cystic fibrosis. The active-treatment description specifies 900 mg TID for 24 weeks, while placebo tablets were administered TID for 24 weeks. The title calls the study double-blind; the masking entry records triple, with the masked parties not reported in the registry record.

The primary endpoint was change in log sputum human neutrophil elastase activity from enrollment to the end of the 24-week trial. The reported net mean difference was 0.21, with a 95% confidence interval from -0.07 to 0.48 and a two-sided p-value of 0.14. The interval includes the null value, and the estimate remains uncertain. The primary analysis used complete case data from the ITT population.

The registry reports 70 subjects randomized, 65 completing the Week 12 visit, and 62 completing the study, with 6 withdrawals in the NAC group and 2 in Placebo. It lists four secondary lung-function outcomes and serious adverse event counts, but important details about missing data, secondary analyses, safety denominators, and the statistical plan are not reported in the registry record.