Background/Purpose: Studies of the diagnostic utility of rheumatoid factor (RF) testing for rheumatoid arthritis (RA) conducted in early arthritis clinics in secondary care have reported sensitivity and specificity of 69% and 85% respectively. However, few studies have been conducted in primary care where the pre-test probability of RA is low. We investigated the utility of RF in primary care using the Clinical Practice Research Datalink (CPRD), a large primary care electronic database representing 8.4% of the UK population.

Methods: The CPRD was searched for patients who had a first RF test between 1^st January 2000 and 31^stDecember 2008 and had at least 2 years of follow-up data or who died within 2 years of the test date. We excluded practice-years where there was evidence for preferential recording of positive RF tests. Patients diagnosed with RA within 2 years of their first test were identified using a validated algorithm; we then calculated the sensitivity, specificity and positive and negative likelihood ratios (LR+, LR-) for a diagnosis of RA. We calculated the relative risks (RRs) of positive compared with negative RF results for patient characteristics and musculoskeletal symptoms recorded in the 6 months before the test using Poisson regression models.

Results: We found that 63,539 RF tests fulfilled the inclusion criteria; 92.3 % were negative and 68.4% were requested in females. There were 1753 incident cases of RA in patients undergoing RF testing giving an incidence rate of 1.4/100 /year (CI₉₅ 1.3, 1.5) and 1013 RA cases (57.8%) had a positive RF test. The RR of RA diagnosis increased with RF titre from RR 5.0 (CI₉₅ 4.0, 6.3) with titres of 1-9 IU/ml above the laboratory upper normal range (UNR) to RR 15.0 (CI₉₅ 11.2, 20.0) at titres 40-50 IU/ml above UNR and RR 38.8 (CI₉₅ 31.0, 48.4) at tires 500-999 IU/ml above UNR . The sensitivity and specificity of the RF test was 57.2% (CI₉₅ 54.8%, 59.6%) and 93.8% (CI₉₅ 93.6%, 94.0%) respectively. The LR+ was 9.3 (CI₉₅ 8.8, 9.8) and the LR- was 0.5 (CI₉₅ 0.4, 0.5). Patients were more likely to have a positive RF test when presenting with hand, shoulder or knee symptoms (RR 1.3 (CI₉₅ 1.2, 1.4), 1.2 (CI₉₅ 1.0, 1.3) and 1.1 (CI₉₅ 1.0, 1.2) respectively), tenosynovitis (RR 1.5, CI₉₅ 1.1, 2.0) or a history of connective tissue disease (RR 1.4, CI₉₅ 1.2, 1.7). Those presenting with neck (RR 0.8, CI₉₅ 0.7, 1.0) or back pain (RR 0.8, CI₉₅ 0.7, 0.9) were less likely to have a positive RF test. Positive RF tests were associated with an increased risk of death in the 2 years following the test (RR 1.5 (CI₉₅ 1.3, 1.7).

Conclusion: Whilst RF tests perform moderately well for identifying RA (LR+ 9.27), they do not perform well for ruling out disease if the result is negative (LR- 0.46). Forty two per cent of patients with an eventual diagnosis of RA had a negative initial RF test. We caution against the use of RF as screening tool to rule out RA in patients with symptoms consistent with RA and its indiscriminate use in patients with less specific musculoskeletal problems such as neck or back pain.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/the-diagnostic-accuracy-of-rheumatoid-factor-testing-in-primary-care/