Objective: To determine the association between allopurinol compliance and serum urate (sUA) level; and examine the association between sUA and gout-related healthcare costs in a large managed care population.
Research design and methods: This retrospective administrative claims analysis examined subjects with gout (> or = 2 medical claims with ICD-9-CM diagnosis code 274.xx or > or = 1 claim with a gout diagnosis and > or = 1 pharmacy claim for allopurinol, probenecid, colchicine, or sulfinpyrazone) between January 1, 2002 and March 31, 2004. Each subject was observed during 1-year pre-index and 1-year post-index periods.
Main outcome measures: Outcomes were allopurinol medication possession ratio (MPR) and compliance (MPR > or = 0.80), sUA (mg/dL), and gout-related healthcare costs. 'Post-allopurinol' sUA was measured during three periods after the first observed allopurinol fill: 30-89 days; 90-149 days; > or = 150 days. A baseline sUA on or before the start of the post-index period was also identified. Outcomes were stratified by post-allopurinol or baseline sUA and compliance. Generalized linear modeling (GLM) regression measured the impact of baseline sUA on gout-related healthcare costs, controlling for demographic and health status variables.
Results: The study sample comprised 18,243 subjects with mean age of 53.9 years. In all, 55% (n = 10,073) of subjects used allopurinol. There were 1473 (8.1%) subjects with a post-allopurinol sUA and 2438 (13.4%) subjects with a baseline sUA result. Among all subjects with a post-allopurinol sUA, 45.6% were compliant; between 49.3% and 56.8% of compliant subjects had an sUA < 6.0 mg/dL compared with 22.5-27.8% of non-compliant subjects, depending on the post-allopurinol time period (all p < 0.001). GLM results showed gout-related costs associated with baseline sUA > or = 6.0 and < 9.0 mg/dL were 58% higher (95% confidence interval (CI): 1.012 -2.456; p = 0.044) than were costs for sUA < 6.0 mg/dL. There was no significant difference in gout-related costs between baseline sUA < 6.0 mg/dL and > or = 9.0 mg/dL.
Conclusions: Analysis revealed an important associations between allopurinol compliance, sUA, and gout-related costs: compliance was positively associated with favorable sUA (<6.0 mg/dL) in unadjusted comparisons. GLM showed that baseline sUA < 6.0 was inversely associated with gout-related costs relative to baseline sUA > or = 6.0 and <9.0 mg/dL. Nevertheless, a substantial portion of subjects, even compliant ones, did not achieve sUA < 6.0 mg/dL. These results should be interpreted carefully in light of study limitations, including incomplete laboratory data, the potentially incorrect inference that medications were taken as prescribed, and lack of generalizability from Medicare managed care enrollees to the broader Medicare population.