Both algorithms indicate substantial increases in the number of hospital discharges for IDU-related IE nationwide between periods 3 and 4. To our knowledge, this is the first time such a trend in IDU-related IE has been reported nationwide, although another study reported an increase in such hospitalizations in New York City during the period 1996–1999 [4
Possible causes of the observed nationwide increase merit consideration. First, it is unlikely that the observed increase is an artifact of changing IE diagnostic criteria. The Duke criteria have been the standard diagnostic tool for IE since 1994 [8
]. Modifications proposed to the Duke criteria in 2000 likely produced greater diagnostic specificity with little change in sensitivity among IDUs [8
Second, available data indicate that the increase in the number of IDU-related IE hospitalizations was not produced by an increase in the size of the at-risk population (i.e., IDUs). An ongoing National Institute on Drug Abuse–funded study suggests that the number of past-year IDUs nationwide remained relatively constant during the study period (unpublished data from the Community Vulnerability and Response to IDU-related HIV project 5R01DA013336).
Third, the observed increase in the number of IDU-related IE does not appear to be a function of increasing prevalence of 2 factors associated with IE among active IDUs: HIV-related immunosuppression and cocaine injection. We found that cases of IDU-related IE among patients with HIV infection or AIDS decreased markedly during the study period—a finding consonant with past research that may reflect the advent of HAART and HIV-positive individuals’ improved access to addiction treatment [7
]. Likewise, drug treatment admissions data, which are often used as a marker of population trends in substance use, indicate that the number of treatment admissions of persons who injected cocaine was relatively stable during periods 3 and 4 [9
Given the rising prevalence of community-acquired methicillin-resistant Staphylococcus aureus infection in the United States, we investigated whether this bacterium might have contributed to the observed increase in the number of IDU-related IE hospital discharges. Our analyses indicate that, although codiagnoses of community-acquired methicillin-resistant S. aureus infection among IDU-related IE hospital discharges indeed increased during the study period, this increase did not coincide with the increase in IDU-related IE hospital discharges observed here.
Possibly, the increase in the number of IDU-related IE is part of a broader increase in the number of IE overall, particularly given recent increases in the number of people who are vulnerable to IE (e.g., elderly individuals and people with prosthetic valves) [6
]. Our algorithms, however, excluded such vulnerable populations, and we suspect that, because of distinct causal mechanisms, the incidence of IDU-related IE follows a different temporal trend from that of non-IDU-related IE [1
By reviewing (1) the known causes of IDU-related IE and (2) documented changes occurring during the study period in the contexts in which IDUs use drugs, we identified the mechanisms that may have recently increased the incidence of IE among active IDUs. First, increasing methamphetamine use may be such a mechanism, because drug treatment admissions of persons who currently use methamphetamine increased nationwide during the period 1996–2003, with the steepest increase occurring during 2001–2002 (admissions increased by 23% during these 2 years, from 133,971 in 2001 to 164,916 in 2002, and then increased again by 7% in 2003, to 176,764) [9
]. Methamphetamine use (regardless of the mode of administration) induces a host of cardiac complications that are similar to those induced by cocaine, including vasospasm and injury to the myocardial surface [10
]. Hypothetically, methamphetamine injection may increase IE risk through pathophysiologic mechanisms similar to those proposed for cocaine. A second mechanism may be increasing injection frequency among heroin injectors. The mean price of heroin decreased by 40%, from $403 per pure gram in 1995 to $241 per pure gram in 2002 [11
]. Dramatic price decreases may have allowed increased injection frequency among individuals who were already injecting heroin. Increased injection frequency provides more opportunity to introduce skin flora into the circulatory system.
The primary limitation of our study was the absence of codes denoting IDU in the International Classification of Disease, Ninth Revision, Clinical Modification system. We addressed this limitation by constructing 2 algorithms to identify cases of interest; one was designed to be specific, and the other was designed to be sensitive. Both algorithms suggest that IDU-related IE hospital discharges increased during the period 2002–2003—a concordance that reveals convergent validity.
Research regarding whether the 2 mechanisms proposed above or other mechanisms (acting alone or in combination) have produced the observed increase in the number of IDU-related IE cases is needed so that effective interventions can be established to stem IE among IDUs. Meanwhile, organizations serving IDUs could educate IDUs and health care providers about IE symptoms, so that affected individuals could obtain medical care as early as possible.