An evidence-based approach to laboratory tests in usual care of patients with rheumatoid arthritis.

Laboratory tests often are regarded as the most important information in clinical care by patients and doctors, and dominate clinical decisions in many chronic diseases such as diabetes and hyperlipidemia. Most patients with rheumatoid arthritis (RA) have a positive test for rheumatoid factor or anti-cyclic citrullinated peptide antibodies (ACPA), or an elevated erythrocyte sedimentation rate (ESR) or C-reactive protein (CRP). However, about a third of RA patients, have negative tests for rheumatoid factor or ACPA, and more than 40% have a normal ESR or CRP at presentation ('false-negative' results). Furthermore, many normal people have a positive test for rheumatoid factor or ACPA but do not have RA, even among those with extensive musculoskeletal pain ('false-positive' results). Abnormal laboratory tests are the most significant predictor of high levels of radiographic progression, and therefore regarded as indicators of 'poor prognosis RA'. By contrast, laboratory tests are far less predictive of severe long-term outcomes such as work disability and premature mortality than functional difficulties reported on a patient questionnaire. A patient questionnaire score is abnormal in 89% of RA patients at presentation, and therefore more useful than ESR or CRP to document subsequent clinical improvement or deterioration. In clinical practice, patient questionnaire scores and RAPID3, an index of physical function, pain, and patient global estimate of status, identify incomplete responses to methotrexate more effectively than ESR. Improved understanding of the limitations of laboratory tests in diagnosis and management of individual patients with RA (and all rheumatic diseases) could improve patient care and outcomes.