Analysis of methotrexate treatment effect in a longitudinal observational study: utility of cluster analysis.

We studied 235 patients with rheumatoid arthritis (RA) beginning therapy with methotrexate utilizing a k-means clustering algorithm. Four groups were identified: mild RA (Group 3), very severe RA (Group 4), and 2 groups intermediate in severity (Groups 1 and 2). Group 2, the largest of the clusters (n = 89), appeared to have greater tolerability of RA as measured by severity and psychological variables, and took the drug almost twice as long as other groups, although improvement was not greater nor side effects fewer. All groups improved over a mean of 1.9 years, and the degree of improvement was not related to the initial severity classification. Improvement occurred almost equally in all clusters, and the relative ranking of the groups was maintained at study closure.