Thomson's MarketScan Commercial Claims and Encounters Research Database and the Medicare Supplemental from January 1, 1999 through June 30, 2007 was the data source for this analysis. Marketscan is composed of claims submitted to health plans which have contracts with large private employers or with public organizations in the United States. The longitudinal database covers, at the patient level, all inpatient, outpatient, and prescription claims, as long as employees stay enrolled. It consists of employer- and health plan sourced data. Nearly 18 million individuals are included in the 2006 database: employees, their spouses, and dependents. Healthcare for these individuals is provided under a variety of fee-for-service, fully capitated (i.e. set amount per person), and partially capitated health plans. Medical claims are linked to outpatient prescription drug claims and person-level enrollment information. This is one of the largest collections of patient data in the US with over four billion patient records. It includes 77 contributing employers and 12 contributing health plans, with 126 unique carriers, and Medicaid data from eight states ; Medicaid covers individuals and families with low income and resources. Elderly are well represented, through the inclusion of groups covered by Medicare, the US social insurance program for people who are aged 65 years and over.
MarketScan research databases meet or exceed requirements of the US Health Insurance Portability and Accountability Act (HIPAA) of 1996. The MarketScan databases underwent a statistical analysis by a third party confirming that HIPAA requirements for fully de-identified data sets were met. Thus, data use met HIPAA criteria for anonymous and aggregate research analysis and reporting of data derived from clinical records not requiring specific patient consent or ethical approval.
Cases were defined as persons with at least two claims for RA [codes ICD 9: 714.0*, 714.1*, 714.2*, and 714.3*.] that were non-diagnostic (i.e. not blood, lab, radiological claims) with active insurance status on June 30, 2007. At least one diagnosis of RA had to be before July 1 2006 and the index date was defined as the earliest claim containing the diagnosis. The control group was patients who had at least two claims for eczema/dermatitis [codes ICD 9: 690.*, 691.*,692.*] with the same criteria as the cases for insurance status and timing of the claims. Controls were matched 1:1 to the cases, by age, gender and health plan [Medicare or not] using propensity scores.
Propensity matching was done using a logistic regression model on dependent variable RA = 0,1 where 1 = in RA cohort and with explanatory variables: age, sex, and health plan (Medicare or not). The propensity score is the probability of being in the RA cohort. Then, one control was selected for each case based on a best match algorithm, using 8 digits of the probability value, then 7 digits, etc.  Each control was used only once. SAS version 8.2 was used for all analyses.
All co-morbidities with a level 5 - i.e. the most detailed - ICD9 (diagnostic) code were identified in the case and control groups in the one-year window from July 1, 2006 through June 30, 2007. A one-year period prevalence was calculated for each co-morbidity for both case and control groups. Occurrence of each co-morbidity was counted only once for each patient.
Also, a set of relative risks (RR) was calculated for patients aged at least 16 years old as of RA index date, by dividing the one-year period prevalence of each co-morbidity in the RA group with the corresponding prevalence in the controls. Thus, the RR represents here a ratio of one-year prevalences. After this, the various co-morbidities were rank-ordered by magnitude of RR; 95% confidence intervals were added.
ICD9 codes covering arthropathies and related disorders [710-719], dorsopathies [720-724] and rheumatism, excluding the back [725-729] were excluded from the analysis (as likely related/part RA), also mechanical complications of internal orthopedic device, implant and graft [996.4]. In order to obtain stable estimates, rank-ordering was done for non-rare diagnoses (occurring in at least 20 persons in control group).