Study Oversight

The study was designed by the authors and approved by the local institutional review board and the Tennessee Bureau of TennCare and Department of Health, all of which waived the requirement for individual informed consent. The sponsors had no role in the study conduct or reporting.

Study Cohort

The study cohort consisted of persons enrolled in the Tennessee Medicaid program19,20; all data on patients in the study were appropriately deidentified. Computerized Medicaid data, which were linked to death certificates and to a statewide hospital-discharge database, provided information on Medicaid enrollment, medical care encounters, and dates and causes of death. Antibiotics and other medications that patients had taken were identified from Medicaid pharmacy files.19,21-23

The cohort included patients who had been prescribed azithromycin between 1992 (when azithromycin was introduced in the United States) and 2006 and met the eligibility criteria on the date on which the prescription was filled. These criteria were formulated to exclude persons at high risk for death from causes unrelated to a short-term effect of proarrhythmic medication (Tables 1 and 2 in the Supplementary Appendix, available with the full text of this article at NEJM.org). Eligible cohort members were 30 to 74 years of age, had no life-threatening noncardiovascular illness, had not received a diagnosis of drug abuse or resided in a nursing home in the previous year, and had not been hospitalized in the prior 30 days. To ensure adequate data for study variables, cohort membership also required at least 365 days of Medicaid enrollment and regular use of medical care.

The study also included matched control periods (of similar length to the courses of antibiotic therapy) during which there was no use of study antibiotics. For each qualifying azithromycin prescription, we identified four such control periods, which were frequency-matched24 according to a propensity score that was calculated from 153 covariates (Table 3 in the Supplementary Appendix). The persons in the control group had to satisfy the eligibility criteria on the day that the control period began and could not have used any study antibiotics during the prior 30 days.

To attempt to control for confounding by indication, we also included as additional control groups patients who took three other antibiotics: amoxicillin (including amoxicillin with clavulanate potassium), ciprofloxacin, and levofloxacin. Amoxicillin, the primary control antibiotic, has indications that are similar to those of azithromycin and has not been shown to have adverse cardiac effects.8 The indications for ciprofloxacin and levofloxacin overlap those of azithromycin. Ciprofloxacin is thought to have minimal adverse electrophysiological effects, although there are case reports of torsades de pointes.1,25 Levofloxacin, which is considered to have greater proarrhythmic potential than ciprofloxacin,1,25 has been implicated in numerous case reports of torsades de pointes.18

A single person could have multiple prescriptions of the study antibiotics and also could have a control period with no use of study antibiotics. However, for each person, these periods did not overlap, and the end point (death) occurred only once. Thus, assumptions of statistical independence were not violated (see the Supplementary Appendix).

Study End Points

The primary study end points were cardiovascular death (see the Supplementary Appendix) and death from any cause. We hypothesized that the incidence of cardiovascular death should be increased if azithromycin is proarrhythmic, particularly in a cohort chosen to reduce the likelihood of out-of-hospital deaths from serious illnesses. We included an analysis of death from any cause to guard against differential misclassification of deaths related to use of a study antibiotic. Given the study hypothesis, we also analyzed sudden cardiac deaths, identified with an independently validated computerized definition that is based on multiple sources of data (see the Supplementary Appendix) and that has a positive predictive value of 88%.26

Statistical Analysis

The study unit of analysis was the course of antibiotic therapy, which was defined as a fixed period, beginning with the date on which the prescription was filled, during which patients would have been advised to take the antibiotic. This should correspond to the period of greatest risk of adverse cardiac effects, given that the case reports for azithromycin suggest an acute mechanism.11-17 Because the usual duration of treatment varies according to the specific study antibiotic, we analyzed two periods: the 5-day period that is generally recommended for azithromycin and the 10-day period most commonly suggested for the other study antibiotics. The 10-day analyses for azithromycin included an interval during which patients were unlikely to be taking the drug (days 6 through 10); these days were considered separately in several analyses. Although these periods usually had a fixed duration (5 or 10 days), the data were censored if the patient filled a subsequent prescription for a study antibiotic or ceased to meet the eligibility criteria.

The analysis estimated the cumulative incidence, or risk, of death during a course of antibiotic therapy. The unadjusted cumulative incidence was calculated by means of the product-limit method. The relative risk of death between the groups, defined by use of the study antibiotics, with adjustments for characteristics of the subjects, was calculated with the hazard ratio from Cox regression models (see the Supplementary Appendix).

Each study comparison was adjusted for an extensive set of covariates (reflecting status on the date on which the prescription was filled) that were possibly associated with both the use of the study antibiotic and the risk of death (Table 3 in the Supplementary Appendix). This adjustment used the propensity score27 (the conditional probability of having a prescription for a study antibiotic, given the covariates). Specific propensity scores were estimated for each pairwise comparison (Tables 4 and 5 in the Supplementary Appendix). The propensity scores for comparisons between study antibiotics included the recorded antibiotic indication (see the Supplementary Appendix).

To check for misspecification of the propensity-score regression models, we evaluated whether the covariate distributions were balanced across the study groups. For the azithromycin group and the control group of persons not taking antibiotics, this distribution was unadjusted, because the propensity-score matching should ensure balance. For amoxicillin, the distribution was adjusted for the propensity score, with the use of a modified method for weighting by inverse probability of treatment28 that standardized the distribution to that for azithromycin (Table 6 in the Supplementary Appendix).29 We also checked for overlap of the distribution of propensity scores (Tables 4 and 5 in the Supplementary Appendix).

To provide a summary measure of the risk of cardiovascular death, we calculated a risk score for cardiovascular disease.30 This score estimated the probability of cardiovascular death (in the absence of use of a study antibiotic) as a function of the indicators of coexisting conditions (see the Supplementary Appendix).

We estimated the difference between the cumulative incidence of cardiovascular death during a 5-day course of azithromycin and the incidence during a similar period of amoxicillin use. We defined the additional risk per course of azithromycin therapy as (HR a −1)×I 0 , where HR a was the hazard ratio for azithromycin versus amoxicillin and I 0 was the unadjusted cumulative incidence of cardiovascular death for patients taking amoxicillin. The risk difference was also calculated according to the deciles of cardiovascular risk as defined by the risk score for cardiovascular disease.

We performed alternative analyses that tested the validity of several study assumptions. These included a repeated-measures analysis testing the validity of treating the prescription periods as independent observations and an analysis stratified by propensity-score deciles. All analyses were performed with SAS software, version 9.3 (SAS Institute). All reported P values are two-sided.