Omadacycline Versus Linezolid for the Treatment of ABSSSI (EudraCT #2013-003644-23): Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Omadacycline Versus Linezolid for the Treatment of ABSSSI (EudraCT #2013-003644-23) (NCT02378480), a randomized phase 3 study. The primary outcome is Number of Participants With Early Clinical Response.

2. Clinical Question

The trial addresses an early clinical-response question in adults with ABSSSI. Its primary endpoint is a participant count, not a direct measure of the size of each participant’s improvement. The registry definition combines lesion-size change with survival and absence of rescue antibacterial therapy. It also allows an indeterminate classification when response cannot be adequately inferred, including when an assessment is unavailable for a specified reason.

The registry record names additional clinical-response outcomes at the post-therapy evaluation visit and in the clinically evaluable post-therapy population, as well as an adverse-event outcome. However, the primary outcome is not reduced to one comparison in the record because it has several categories or time points. A result for the primary comparison, including the category counts and an estimate of the difference between treatments, is not reported in the registry record. Therefore, the record as described does not establish which treatment had a higher early-response count or how precisely any difference was estimated.

3. Trial Design

This was a phase 3, randomized, triple-masked, parallel-group study comparing omadacycline with linezolid for treatment of adults with ABSSSI. The registry lists an enrollment of 655 and marks the study as completed. Omadacycline was given intravenously or as tablets, as was linezolid. The record identifies Paratek Pharmaceuticals Inc as sponsor.

The study began in 2015-06, and primary completion was recorded in 2016-04. These dates describe the registry milestones; they do not, by themselves, explain the length of follow-up for each participant. The primary endpoint timeframe is Screening and 48 to 72 hours after the first dose of test article. Other outcomes listed include investigator assessments of clinical response at the post-therapy evaluation visit and adverse events.

Randomization assigns participants to study groups by chance, helping reduce systematic differences in measured and unmeasured characteristics at the time of assignment. A parallel-group design means participants are assigned to one treatment group rather than receiving both treatments in sequence. Masking is intended to limit the influence of treatment knowledge on trial conduct or outcome assessment. Although the record labels masking as triple, it does not explain which roles were masked or how masking was maintained.

The registry record does not report the randomization procedure, allocation ratio, treatment duration, detailed follow-up schedule, eligibility criteria beyond the primary population definition, or the operational details of masking. It also does not report a sample-size justification or a full statistical analysis plan. The enrollment figure is the study-level registry figure and should not be treated as the denominator for every endpoint: the primary endpoint is defined for the mITT population, and the record describes other analysis populations as well.

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.

Before assignment: Participants who met inclusion criteria and did not meet exclusion criteria were randomly assigned to a treatment group, and received their first dose of test article within 4 hours after randomization. All participants were expected to present with ABSSSI severe enough to require a minimum of at least 3 days of intravenous treatment.

Overall Study

Reasons for not completing this period:

Analysis population for the primary outcome, as defined in the registry: Modified Intent-to-Treat (mITT) Population: all randomized participants without a baseline sole Gram-negative ABSSSI pathogen

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: Safety Population: all randomized participants who received test article

6. Primary Endpoint

The registry lists the primary outcome as Number of Participants With Early Clinical Response, measured over Screening; 48 to 72 hours after the first dose of test article.

The primary endpoint counts participants with an early clinical response. The registry defines success as survival with at least a 20% reduction in the size of the primary ABSSSI lesion compared with Screening, without use of rescue antibacterial therapy. The assessment is made 48 to 72 hours after the first dose. Participants whose response cannot be adequately inferred because they were not assessed for a specified reason may be classified as indeterminate.

This composite definition captures several clinically relevant features in one early assessment: lesion-size change, survival, and whether rescue antibacterial therapy was needed. Using a defined assessment window makes the timing of the comparison explicit. Counting participants is also a straightforward way to present a binary response outcome: each participant is classified according to the endpoint rules rather than contributing a continuous measurement of lesion change to the primary count.

That simplicity has limits. A count alone does not describe the magnitude of lesion reduction among responders, and the endpoint combines different reasons for not meeting the success definition. Indeterminate classifications are not equivalent to observed successes or failures; their handling can affect interpretation and must be described in the analysis plan. The registry gives the endpoint definition and population but does not report the primary-response category counts or a single treatment comparison. Thus, the endpoint’s meaning can be described, but its comparative result and precision are not reported in the registry record.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

Clinical trial results are most informative when the outcome definition, analysis population, group counts, treatment contrast, and uncertainty are presented together. The registry record describes the primary outcome as a count of participants with early clinical response, but does not report a single comparison for that outcome. The following concepts explain how such outcomes are commonly summarized; they do not identify an analysis performed in this trial unless the registry record says so.

What does a participant count represent?

A binary outcome places each participant into a defined category, such as meeting or not meeting a response definition. A count reports how many participants fall into a category. To understand the frequency of that category, readers also need the relevant denominator: the number of participants assessed in the analysis population. A treatment comparison may then express the contrast as an absolute difference in response proportions or another prespecified measure. The registry record does not provide the primary category counts or a single contrast, so no such treatment estimate can be interpreted from it.

Counts depend on clear endpoint rules. Here, early response incorporates survival, lesion-size reduction from Screening, and no rescue antibacterial therapy. The response may also be classified as indeterminate when assessment is unavailable for a specified reason. How indeterminate outcomes are handled in the analysis matters: excluding them, treating them as nonresponses, or using another prespecified approach can lead to different interpretations. The analysis plan and handling of these classifications are not reported in the registry record.

Why report an effect estimate and confidence interval?

An effect estimate describes the observed treatment contrast using a defined scale. A confidence interval describes the statistical uncertainty around that estimate under the assumptions of the analysis. It helps readers judge which effect sizes are compatible with the data, rather than relying only on whether a significance threshold was crossed. A confidence interval does not measure every source of uncertainty, and it cannot remedy limitations in study conduct, missing data, or outcome definition. No primary treatment estimate or confidence interval is reported in the registry record.

A p-value, when used, addresses how compatible the observed data are with a specified statistical null model. It is not the probability that a treatment is effective, nor does it measure the size or clinical importance of an effect. The record does not give a single primary-outcome p-value. Without the estimate, its uncertainty, and the analysis details, a significance statement alone would be insufficient for judging the result.

What does the planning illustration mean?

The registry’s planning assumptions describe an illustrative sample-size calculation based on the observed effect, not the trial’s own design. The stated assumptions are a two-sided significance level of 0.05, equal allocation between groups, and a normal approximation; the effect to detect is a stated fraction of the observed effect. Such a calculation is a planning illustration: it asks how many participants per arm would be required under those assumptions to target a chosen power for an assumed effect. It is not an observed-power calculation and does not show the probability that the completed trial detected its true effect.

The calculation depends on the effect used as its starting point and on the stated model assumptions. If the observed effect is uncertain, planning from it is also uncertain. An illustrative calculation should not be mistaken for a protocol target or evidence that the trial was planned around that effect. The registry record does not report the trial’s sample-size justification, and no specific planning values are stated in the registry record.

The registry notes that time-to-event planning displays use the total number of events under the Schoenfeld method, while other planning displays use participants per arm. This distinction is general: time-to-event calculations often depend on the event count, whereas calculations for other outcome types may be expressed as participant counts. The primary endpoint is a participant-count outcome, not a time-to-event endpoint.

Why can event counts be difficult to compare?

Safety summaries may count participants with at least one event, individual event occurrences, or events within specific categories. Those are not interchangeable quantities. A participant with multiple events may be counted once in a participant-level summary but more than once in an event-level summary. Interpretation also depends on the definitions used, observation time, exposure, and whether events were actively solicited or reported spontaneously. The registry provides participant counts for serious and other adverse events, but fuller event-level details and a comparative safety analysis are not reported in the registry record.

Statistical methods should be judged in relation to the endpoint, analysis population, and question. The registry supplies some definitions and selected counts, but not the full model specification, adjustment strategy, missing-data procedure, or primary comparison. Those details should not be inferred from the study design label or from the fact that the study was randomized and masked.

10. Multiplicity, Missing Data and Analysis Populations

Multiplicity arises when a study evaluates several outcomes, time points, populations, or treatment comparisons. If many hypotheses are tested, the chance of at least one apparently positive result can rise unless the analysis plan addresses that issue. The registry lists four outcome measures and three analyses, including clinical-response assessments at post-therapy evaluation and in a clinically evaluable population, as well as adverse-event information. However, the statistical plan for managing multiplicity is not reported in the registry record. The number of listed outcomes alone does not reveal which were formally tested or how their results were intended to be interpreted.

The primary outcome has several categories or time points and is not reduced to one comparison. This makes it especially important to know which contrast was designated as primary and how categories, time points, and indeterminate classifications were handled. Those details are not reported in the registry record, and no single primary-outcome estimate is available for interpretation.

The primary population is the mITT population, defined as all randomized participants without a baseline sole Gram-negative ABSSSI pathogen. The registry also names a clinically evaluable post-therapy population for a secondary outcome. Such populations answer different questions: an mITT-type population retains a broad randomized-group perspective subject to its stated exclusions, while a clinically evaluable analysis focuses on participants meeting its evaluability criteria. The detailed criteria and the number analyzed in each population are not reported in the registry record.

The endpoint description acknowledges that some participants may not be assessed and may receive an indeterminate classification. The registry record does not report how missing or indeterminate observations were handled in the analysis, whether sensitivity analyses were conducted, or whether missingness differed by group. These omissions limit assessment of how robust the primary comparison would be to incomplete outcome data.

11. 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 2% reporting threshold.

Most frequent serious adverse event terms

Most frequent non-serious adverse event terms

Serious events with the highest arm-level rate: cellulitis (0.6%), subcutaneous abscess (0.6%), wound infection (0.6%), cardiac arrest (0.3%), cardiac failure (0.3%).

The registry reports serious adverse events for 12 participants in the omadacycline group and 8 in the linezolid group. Other adverse events are reported for 104 participants and 101 participants, respectively. Deaths are reported for 1 participant in the omadacycline group and 2 in the linezolid group. These figures are participant counts; they are not, by themselves, estimates of a causal treatment effect.

For context, the registry lists Cellulitis, Subcutaneous Abscess, and Wound Infection at 0.6% each, and Cardiac Arrest, Cardiac Failure, and Sepsis at 0.3% each, among its serious adverse-event terms. The record does not report a group attribution for these term percentages in the registry record. The terms and percentages should therefore not be interpreted as a head-to-head comparison.

Counts do not show how many events occurred per participant, when events occurred, their duration, or their relationship to treatment. A full comparison also depends on denominators, exposure and follow-up, event definitions, and uncertainty estimates. Although group figures at risk are listed, the registry record does not provide a comparative statistical analysis or sufficient detail to infer whether the observed differences reflect treatment effects. These summaries describe what was recorded; they do not establish that either treatment is safer.

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

The registry record establishes that a randomized, triple-masked, parallel-group phase 3 study compared intravenous or tablet omadacycline with intravenous or tablet linezolid in adults with ABSSSI. It defines early clinical response using survival, lesion-size reduction, and absence of rescue antibacterial therapy, with assessment in the stated early window. It also reports selected adverse-event counts and identifies other clinical-response and safety outcomes.

Those details do not provide the primary comparison needed to judge the relative early-response results. The primary outcome is not reduced to one comparison in the record, and the response counts, treatment-effect estimate, confidence interval, and primary p-value are not reported. Without those elements, neither the direction nor the size or precision of a comparative response result can be established. The trial’s design and endpoint definition should not be mistaken for evidence of a particular outcome.

The safety counts are also descriptive. Differences in reported counts do not, on their own, establish a treatment-related increase or decrease in risk. The registry record does not provide a complete comparative safety analysis, including uncertainty estimates and the detailed context needed to interpret event rates.

More broadly, trial results apply to the population and endpoint definitions used in the study. The primary population excludes randomized participants with a baseline sole Gram-negative ABSSSI pathogen, while a separate clinically evaluable population is named for a secondary outcome. The record does not supply all eligibility and analysis details needed to determine how closely a particular patient or clinical setting matches the study population.

The appropriate conclusion from the record is limited: it describes the trial design, the early-response definition, and selected safety counts, but does not report enough of the primary comparative analysis to establish a treatment difference. No clinical recommendation follows from these registry entries alone.

13. Why This Trial Matters Statistically

This trial illustrates why a registered study design and a reported treatment result are different kinds of information. Randomization, masking, and a parallel-group structure describe how a comparison was organized. They do not disclose the observed outcome distribution or the uncertainty around a treatment contrast. For statistical interpretation, the endpoint counts and an explicit estimate are essential.

The primary endpoint also shows how a binary response can depend on multiple components. A participant must meet the lesion-size criterion, survive, and avoid rescue antibacterial therapy to qualify as a success; an assessment can be indeterminate when response cannot be inferred. Each component contributes to the endpoint’s meaning, while the treatment of indeterminate cases affects the analysis. Clear definitions and transparent handling of missing outcomes are therefore central to reproducibility.

The record names more than one response outcome and population. Comparisons across time points or analysis populations can be informative, but they do not automatically have the same evidential status as the primary comparison. Prespecified priorities and multiplicity procedures help distinguish confirmatory findings from additional analyses. Those statistical-plan details are not reported in the registry record.

Finally, safety counts emphasize the need to distinguish descriptive summaries from comparative inference. Counts are useful for identifying what was recorded, but a sound comparison also needs denominators, follow-up context, event definitions, and uncertainty. The record offers selected counts without a complete comparative analysis. These limitations make careful separation of design, endpoint, and result especially important when reading the registry.

14. 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 report a single primary-outcome comparison. It gives the early clinical-response definition and timeframe but not the category counts, treatment-effect estimate, confidence interval, or primary p-value. As a result, comparative efficacy and the precision of any difference cannot be assessed from the recorded information.

The record states that the primary outcome has several categories or time points and is not reduced to one comparison. It does not explain the intended primary contrast, how the categories were analyzed, or how indeterminate outcomes were handled. The reasons for missing assessments are described in the endpoint definition, but the analysis approach for missing data and any sensitivity analyses are not reported.

Although the study is identified as randomized, triple-masked, and parallel-group, the randomization method, allocation ratio, masking roles, and detailed masking procedures are not reported. The registry record also does not report the sample-size justification, full statistical analysis plan, multiplicity strategy, treatment duration, detailed follow-up schedule, or numbers analyzed in each outcome population.

Safety information is limited to selected participant counts and listed serious-event terms with percentages. Detailed event-level data, group attribution for the listed term percentages, exposure context, and a comparative analysis are not reported in the registry record. These limitations mean that the safety counts should not be used as a standalone comparative risk assessment.

15. Related Tutorials

16. Related Statistical Calculators

17. Sources

• ClinicalTrials.gov: Omadacycline Versus Linezolid for the Treatment of ABSSSI (EudraCT #2013-003644-23) — NCT02378480.
• Registry record last updated 2019-03-21; status last verified 2019-03.
• Results first posted 2019-03-21.
• 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.

18. Record Summary

The registry describes a completed phase 3 randomized, triple-masked, parallel-group study of omadacycline versus linezolid for adults with ABSSSI. Enrollment was 655. Both treatments were administered intravenously or as tablets. The primary endpoint was the count of participants with early clinical response in the mITT population, assessed at 48 to 72 hours after the first dose using a definition based on survival, lesion-size reduction, and no rescue antibacterial therapy.

The record lists post-therapy clinical-response outcomes and adverse-event outcomes. It reports serious adverse events in 12 omadacycline participants and 8 linezolid participants, other adverse events in 104 and 101 participants, and deaths in 1 and 2 participants, respectively. These are descriptive counts.

The primary outcome is not reduced to one comparison, and the registry record does not report a primary treatment-effect estimate or its precision. Details of the statistical plan, missing-data handling, and multiplicity procedures are also not reported.