Pharmacy & Therapeutics - May 2008 - (Page 289) Effect of Persistence with Drug Therapy on Myocardial Re-infarction Study Sample We extracted all pharmacy and medical claims of AMI patients (according to International Classification of Disease, Ninth Revision, and Clinical Modification [ICD-9-CM] code 410) between January 1, 2002, and December 31, 2003, from Medicaid MCOs. All patients had to fill at least one of three possible drug prescriptions (for statins, beta blockers, or calciumchannel blockers) within one month of the initial MI diagnosis. Both statins and beta-blocking agents are supported as first-choice drugs for the secondary prevention after an MI by class 1 level of evidence, whereas calcium-channel blockers are not recommended as a first-line drug for secondary prevention (class 2 level of evidence).1,4,6 We excluded patients with an ICD-9-CM 410 code during the first three months of the study period (January 1, 2002, to March 31, 2002) to increase the likelihood of identifying those patients with a first MI. To be included in the study, patients also had to be 18 years of age or older, had to be continuously enrolled during the study period, had to have both medical and pharmacy claims, and had to participate for at least one month of follow-up after the initial MI. However, if a patient’s refill gap exceeded the predetermined grace period, the patient was classified as nonpersistent. Disease Severity and Comorbidity To control for sociodemographic characteristics and severity of disease, we included variables of age, sex, race, and the presence of other cardiovascular conditions. These included hypertension (code 401.xx), diabetes (code 250.xx), renal disease (codes 581–585.xx), hyperlipidemia (code 272.xx), and comorbid heart disease (including other forms of ischemic heart diseases and angina pectoris, codes 411-414.xx and 429.2). We further categorized the initial MI as subendocardial (or non-transmural). A non-transmural MI (codes 410.70 and 410.71) affects only the inner one-third to one-half of the heart muscle; a transmural MI damages almost the entire wall of the heart (codes 410.80, 410.81, 410.90, and 410.91). Through our baseline analysis, we found that more than 90% of our study subjects had hypertension as a comorbid condition. We recorded the number of concomitant antihypertensive drugs being taken by those patients. Outcome Assessment A previous study of older patients (mean age, 62.6) by Brener et al. showed that the combined death or re-infarction rate at 30 days was about 7%.12 Another study by De Luca et al. found a 30-day re-infarction rate of 3.8%.13 To define an episode of re-infarction in our present study, we identified a subsequent claim, occurring after at least 30 days, with a diagnosis of MI or sudden death (code 798) as an outcome event. We considered the absence of such claims during the follow-up period as indicating that the patient remained in remission. Assessment of Persistence The initial study drug prescribed after an MI was considered the index drug. For each patient, we analyzed prescription claims records for persistency of the index drug. Each individual had a certain grace period of three times the day’s supply of the previous prescription to obtain an additional refill. If the patient refilled the index drug by the end of the grace period during the study period, he or she was classified as persistent. Patients with ICD-9-CM code 410 from January 1, 2002, through December 31, 2004 1,198 First three-month washout 240 Patients with AMI after April 1, 2002 958 Statistical Analysis To analyze differences in baseline characteristics by outcome status, we used chi-square (χ2) tests. Cox proportional hazards models were constructed by statistical tests on proportionality to adjust for age, sex, race, concomitant heart disease, hypertension, diabetes, renal disease, hyperlipidemia, types of MI, and the number of concurrent antihypertensive drugs being taken. The proportional hazards assumption was not violated (i.e., the interaction variable was not significant). Statistical significance was defined as a two-tailed P value of less than 0.05. SAS software, version 9.1 was used to perform all analyses. Patients who were not continuously enrolled 87 Continuously enrolled patients 871 Patients without the study drug within one month after initial diagnosis or with follow-up <30 days 356 RESULTS Baseline Characteristics We initially identified 1,198 patients who had AMI-related medical claims with an ICD-9-CM code 410 in the primary, secondary, or tertiary diagnosis. Of these, 327 patients were excluded because of a diagnosis of AMI during the first three months of the study period or because they were not continuously enrolled. Of the remaining 871 patients, 515 had prescriptions for a statin, a beta blocker, or a calcium-channel blocker within Cohort study patients prescribed one of the drugs of interest (statin, beta blocker, or calcium-channel blocker) 515 Figure 1 Cohort determination. AMI = acute myocardial infarction; ICD = International Classification of Diseases. Vol. 33 No. 5 • May 2008 • P&T® 289
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