Data source
This analysis utilized data from the IBM MarketScan® Research Databases (01/01/2010–07/31/2017), which include the Commercial Claims and Encounters database and the Medicare Supplemental and Coordination of Benefits database. The databases comprise claims from approximately 75 million individuals covered by 100 payers. The databases cover all census regions of the US and provide eligibility-related information as well as outpatient medical claims, inpatient medical claims, and outpatient drug dispensing claims information. Data were de-identified and the databases were fully compliant with the Health Insurance Portability and Accountability Act and thus, no ethics board review was required.
Study design
A retrospective longitudinal cohort study design was used. The index date for patients with AS, PsA, PsO, or RA was randomly assigned among service dates with a claim related to these CIDs. The rationale was to capture a prevalent population of patients with CIDs and to minimize the potential bias associated with the selection of the most or least recent dates associated with such claims, which could lead to the inclusion of patients with new-onset or advanced CID. When evaluating the incidence of IBD, a non-CID population was also studied to put in perspective the incidence of IBD in patients with CIDs to that of the general population. For the non-CID population, the index date was randomly assigned among all dates for which a service was provided. The baseline period was defined as the 12-month period prior to the index date. The observation period was defined as the 12-month period following the index date (i.e., patients were censored 12 months after the index date; Fig. 1).
Study sample
To be included in the study sample, patients were required to be ≥18 years of age as of the index date and have ≥12 months of continuous health plan enrollment pre- and post-index date. Patients in the CID cohort were required to have ≥2 claims with the same diagnosis among the following CIDs: AS (International Classification of Diseases, Ninth Revision, Clinical Modification [ICD-9 CM] code 720.0; International Classification of Diseases, Tenth Revision, Clinical Modification [ICD-10 CM] code M45.x), PsA (ICD-9 CM code 696.0; ICD-10 CM code L40.5x), PsO (ICD-9 CM code 696.1; ICD-10 CM codes L40.0-L40.4, L40.8, or L40.9), or RA (ICD-9 CM code 714.0; ICD-10 CM codes M05.1-M05.9, or M06). Patients with ≥2 claims with a diagnosis of IBD (i.e., Crohn’s disease [ICD-9 CM code 555.x; ICD-10 CM code K50.x] or ulcerative colitis [ICD-9 CM code 556.x; ICD-10 CM code K51.x]) during the 12-month baseline period were excluded. Moreover, patients with cancer treated with rituximab or ofatumumab during the baseline period were excluded because it could not be determined using claims data whether these agents were prescribed to treat cancer or CID. Patients with a transplant procedure were also excluded because of the high costs usually associated with this procedure, which may skew results. The non-CID cohort was composed of individuals with no claim with a diagnosis of AS, PsA, PsO, or RA at any time. Similar to the CID cohort, patients in the non-CID cohort were additionally required to be ≥18 years of age as of the index date and have ≥12 months of continuous health plan enrollment pre- and post-index date. Patients with cancer treated with rituximab or ofatumumab, with a transplant procedure, or with ≥2 claims with a diagnosis of IBD during the 12-month baseline period were excluded.
Study cohorts
Patients with CIDs were classified into six cohorts: (1) all patients with CIDs (i.e., AS, PsA, PsO, or RA); (2) patients with RA; (3) patients with PsO and PsA; (4) patients with PsO but without PsA; (5) patients with AS; and (6) patients with AS, PsA, or PsO. Eligible patients with claims for more than one CID were classified into each eligible CID cohort (i.e., one patient could be present in more than one cohort). Each of the six cohorts mentioned above was analyzed separately.
Study measurements
Baseline characteristics included age, gender, type of insurance plan, region of residence, year of index date, Quan-Charlson Comorbidity Index (Quan-CCI), extra-articular manifestations (EAMs), gastro-related conditions, drug use, all-cause HRU, and all-cause costs.
Patients developing IBD were identified by the presence of ≥2 claims with a diagnosis of IBD during the course of the observation period. The one-year incidence rate (IR) of IBD was calculated for each cohort. The numerator was defined as the number of patients with ≥2 claims with an IBD diagnosis during the period of evaluation (i.e., 1 year), and the denominator was defined as the total number of patients in the cohort studied. As a sensitivity analysis, the one-year prevalence of IBD among patients with or without a prior IBD diagnosis was also reported for each cohort. The numerator and denominator definitions used were the same as the ones used to measure incidence, except that patients who had a prior IBD diagnosis were not excluded from the numerator and denominator.
HRU outcomes included number of inpatient admissions, number of days of inpatient stay, number of days with outpatient services, number of days with emergency room (ER) visits, and number of days with durable medical equipment use. The number of days with a surgery was also evaluated.
All-cause total healthcare costs were stratified by medical and pharmacy costs. Medical costs were further stratified by inpatient, outpatient, ER, and durable medical equipment costs. Costs related to surgery were also evaluated. By definition, all-cause healthcare costs included any healthcare costs incurred during the follow-up period, regardless of whether they were related to CIDs, IBD (e.g., endoscopies, colonoscopies), or other comorbidities.
Statistical analysis
Baseline characteristics were summarized using means and standard deviations (SDs) for continuous variables and proportions for categorical variables. The one-year IR of IBD was reported as a proportion of patients.
The rates of HRU between patients with and without IBD among the six cohorts were compared using generalized linear models (GLMs) with a Poisson distribution adjusting for age, gender, region, type of insurance plan, Quan-CCI, and type of CID (i.e., RA, PsA, PsO, or AS); incidence rate ratios (IRRs) with corresponding 95% confidence intervals (CIs) and p-values were reported. GLMs with normal distribution were used to compare costs between patients with and without IBD among the six cohorts, adjusting for age, gender, region, type of insurance plan, Quan-CCI score (a score derived from a number of comorbidities [e.g., hypertension, diabetes, chronic pulmonary disease] and their associated risk of in-hospital mortality [29]), and type of CID (i.e., AS, PsA, PsO, or AS); mean yearly cost differences (MYCDs) and the corresponding 95% CIs and p-values were reported. Non-parametric bootstrap procedures were used to evaluate statistical significance and 95% CIs. In a sensitivity analysis, costs were evaluated in the subset of patients with AS, RA, PsA, or PsO who received DMARDs and/or corticosteroids.
Cost and HRU outcomes were evaluated per-patient-per-year (PPPY). Cost outcomes were inflated to 2017 US dollars (USD) using the medical component of the US Consumer Price Index.