IIMs are a rare condition, and little is known about the extent of their incidence and prevalence in the U.S. population. In this study, we used the MarketScan Commercial, Medicare, and Medicaid administrative claims databases to identify incidence and prevalence of IIMs based on medical claims with an ICD-9-CM diagnosis indicating DM, PM, or interstitial myositis. Administrative claims analyses, such as the one performed here, have the benefit of data contributed by very large populations seeking medical care, which can be used to identify rare conditions and shed additional light into incidence and prevalence. The limitation of these methods is the flip side of the benefit coin, in that unless patients seek care for their IIM, they will not be correctly identified as incident or prevalent. In addition, misdiagnosis or recording errors can result in under- or over-reporting of the condition. Another limitation in using administrative claims databases for identifying incidence and prevalence is that the results are sensitive to the methods used to identify the population denominator.
Due to the dynamic nature of the employer-based and Medicaid administrative claims databases used here, the annual values are most informative as snapshots providing a range of estimates among a fluctuating population, rather than indicative of trends in incidence or prevalence of myositis in the U.S. population. Another consideration particular to employer-based databases is that employees with chronic conditions may be less likely to change employers and risk changes to their healthcare coverage, and therefore may stay in the data longer than other employees at the same employer.
Both the current study and one using very similar methods (1 inpatient or 2 outpatient claims for PM, DM, or interstitial myositis) in another U.S. managed care administrative claims database [19
] reported notably higher incidence of IIMs than the 0.1 to 1 per 100,000 py reported in community-based studies using smaller populations [2
This study identified age- and sex-adjusted incidence rates of 4.27 (commercial/Medicare) and 5.23 (Medicaid) per 100,000 py for the period 2004–2008, while Furst et al. [19
] reported adjusted incidence rates of 6.57 (95% CI, 6.20–6.94) per 100,000 py for combined IIMs. For DM, we identified adjusted incidence rates of 1.52 and 1.70 per 100,000 py in the commercial/Medicare and Medicaid databases respectively, which were relatively comparable to the 1.38 per 100,000 py reported by Furst et al. [19
Incidence rates for PM were also comparable at 2.46 (commercial/Medicare) and 3.53 (Medicaid) in this study versus 3.79 per 100,000 py in Furst et al [19
These rates were similar to the prevalence of PM of 3.45 per 100,000 py identified by Wilson et al. [10
Interstitial myositis varied among the two studies, in that it was the least common of the three types of IIM, with adjusted incidence of 0.73 and 0.78 per 100,000 py in the two databases compared to 1.69 per 100,000 py reported in Furst et al.
in which DM rather than interstitial myositis was the least common type of myositis. Due to very low counts of interstitial myositis, along with limited consistent clinical information about the condition and its diagnosis, incidence and prevalence for that sub-type should be interpreted with caution.
In contrast to incidence estimates, the prevalence of IIMs identified by medical claim (20.62 to 25.32 per 100,000 py in commercial/Medicare and 15.35 to 32.74 in Medicaid) in this study is quite comparable to that identified using a contemporary administrative claims database in Quebec (combined DM and PM annual prevalence in 2003 of 21.5 (95% CI 19.4-23.9) per 100,000 py [16
The prevalence of IIMs identified in this study is, however, somewhat higher than prevalence of combined DM, PM and interstitial myositis given in Furst et al. [19
] of 9.54 in 2003 and 13.61 in 2008, albeit the lower prevalence in Furst et al. could be associated with a younger population (9% of prevalent cases were
years in their IIM cohort vs. 24%
years in the commercial/Medicare and 16% of prevalent cases in the Medicaid IIM cohorts). Notably, all three administrative claims studies reported annual prevalence that was nearly 2- to 6-fold higher than that reported in one earlier study from 2003 [17
It is possible that with increased awareness of IIMs, the condition may be increasingly recognized and diagnosed, and thus showing up more frequently in administrative claims databases over the past decade than in previous decades.
When the 2004–2008 age- and sex-adjusted incidence rates for the two databases are projected to the U.S. population ages 18
years and older, the expected number of incident cases per year ranges from 8,919 to 10,937 (based on the commercial/Medicare and Medicaid incidence rates respectively, standardized to the 2000 U.S. Census). Using the combined sensitivity analysis (e.g., IIM incidence along with one or more of the three sensitivity criteria), the expected number of incident cases per year among adults in the U.S. ranges from 6,871 (commercial/Medicare incidence rates) to 7,312 (Medicaid incidence rates).
Extrapolating the annual prevalence of persons seeking medical care for IIMs reported here to the U.S. population (as of the 2000 U.S. Census) aged 18
years and older from the commercial/Medicare cohort results in a range of an estimated 43,000 to 53,000 treated cases per year, or, after employing any one of the three sensitivity criteria, an estimated 38,000 to 45,000 treated cases per year. If the Medicaid annual prevalence is used, estimated annual U.S. IIM prevalence ranges from approximately 32,000 to 68,000 using IIM claims only, or 26,000 to 60,000 treated cases using any of the three sensitivity criteria.
This study is unique in its incorporation of three payer groups: commercially insured, Medicare enrollees (with employer paid supplemental insurance), and Medicaid enrollees. The different incidence and prevalence identified in the commercial/Medicare versus Medicaid populations in this analysis may be a result of differences in patients’ healthcare-seeking behaviors as well as in reimbursement and practice patterns rather than the underlying difference in disease incidence and prevalence among the two populations. The relative similarity in prevalence and incidence identified in separate studies conducted in different administrative claims databases indicates that these variations are common in these types of databases. Another consideration unique to the current study is the high incidence and prevalence among African Americans identified in the Medicaid database. Since race/ethnicity is unknown in the commercial and Medicare cohorts, it is unknown to what extent differences in racial composition between the two cohorts may influence the observed incidence and prevalence of myositis. Additional studies of IIM incidence and prevalence by race/ethnicity are needed.
The incidence and prevalence estimates reported here are in the range of those reported in other studies using administrative claims databases. Like those studies, the estimates reported here are based on the presence of a medical claim with the ICD-9-CM diagnoses of interest and are a function of individuals seeking medical care and physician reporting of diagnosis. This study used an algorithm to exclude rule-out diagnoses; nevertheless, administrative claims are not a substitute for a complete medical record for the study population and thus mis-diagnoses are possible. Since a one year “clean” period was required to qualify as incident, incidence may have been over-estimated if patients already had an IIM but did not receive medical care in the preceding year. In addition, prevalence was expected to be under-estimated as it was defined by evidence of a qualifying medical claim with an IIM diagnosis in each year. Patients with the condition who did not seek medical care in a given year would not be captured in the prevalence estimates. Finally, employer turnover, miscoding and misdiagnosing, care seeking behavior, and fluctuations in database membership over time can influence the results. Further studies are needed to confirm the incidence and prevalence of IIM.