title: Estimating the diagnostic accuracy of rheumatoid factor in UK primary care: a study using the Clinical Practice Research Datalink.
abstract: OBJECTIVE
  To investigate the diagnostic accuracy of RF as a test for RA in primary care and its impact on referral times using the Clinical Practice Research Datalink.
  
  
  METHODS
  We identified all patients with a first RF test recorded in the Clinical Practice Research Datalink between 1 January 2000 and 31 December 2008 and those diagnosed with RA within 2 years of testing. We calculated likelihood ratios (LRs), sensitivity, specificity and predictive values of RF for a diagnosis of RA. We compared time to hospital referral in those testing positive and negative using Kaplan-Meier failure curves and log-rank tests.
  
  
  RESULTS
  Of 62 436 first RF tests, 4679 (7.5%) were positive. There were 1753 incident cases of RA, of which 57.8% were seropositive. The positive LR for RF was 9.5 (95% CI 9.0, 10.0) and the negative LR was 0.5 (95% CI 0.4, 0.5). Sensitivity and specificity were 57.8% (95% CI 55.4%, 60.1%) and 93.9% (95% CI 93.7%, 94.1%) and the positive predictive value and negative predictive value were 21.4% (95% CI 20.3%, 22.6%) and 98.7% (95% CI 98.6%, 98.8%), respectively. Median time to first hospital contact after the first RF test in those with seropositive vs seronegative results was 54 days (95% CI 49, 58) vs 150 (95% CI 147, 152).
  
  
  CONCLUSIONS
  Only 2.8% of patients undergoing RF testing were diagnosed with RA, suggesting that RF is used to screen patients with musculoskeletal symptoms rather than those with more specific features of RA. A positive RF test may be helpful in diagnosing RA in primary care but performs badly in excluding RA and may delay referral.
authors.affiliation: Department of Rheumatology, Nuffield Orthopaedic Centre, Oxford University Hospitals NHS Trust, Oxford, anne.miller@ouh.nhs.uk.
authors.forename: Anne
authors.initials: A
authors.lastname: Miller
authors.affiliation: Department of Pharmacy and Pharmacology, University of Bath, Bath.
authors.forename: Alison L
authors.initials: AL
authors.lastname: Nightingale
authors.affiliation: Department of Pharmacy and Pharmacology, University of Bath, Bath.
authors.forename: Cormac J
authors.initials: CJ
authors.lastname: Sammon
authors.affiliation: Nuffield Department of Primary Health Care Sciences, University of Oxford, Oxford.
authors.forename: Kamal R
authors.initials: KR
authors.lastname: Mahtani
authors.affiliation: Nuffield Department of Primary Health Care Sciences, University of Oxford, Oxford.
authors.forename: Tim A
authors.initials: TA
authors.lastname: Holt
authors.affiliation: Department of Pharmacy and Pharmacology, University of Bath, Bath, Department of Rheumatology, Royal National Hospital for Rheumatic Disease, Bath and.
authors.forename: Neil J
authors.initials: NJ
authors.lastname: McHugh
authors.affiliation: Nuffield Department of Orthopaedics, Rheumatology and Musculoskeletal Science, University of Oxford, Oxford, UK.
authors.forename: Raashid A
authors.initials: RA
authors.lastname: Luqmani
mesh_terms: D000328:Adult; D000368:Aged; D001172:Arthritis, Rheumatoid / Q000097:blood* / Q000175:diagnosis* / Q000453:epidemiology; D015415:Biomarkers / Q000097:blood; D016208:Databases, Factual / Q000706:statistics & numerical data*; D005260:Female; D006801:Humans; D053208:Kaplan-Meier Estimate; D008137:Longitudinal Studies; D008297:Male; D008875:Middle Aged; D010818:Practice Patterns, Physicians' / Q000592:standards*; D011237:Predictive Value of Tests; D011320:Primary Health Care; D011379:Prognosis; D012017:Referral and Consultation; D012189:Retrospective Studies; D012217:Rheumatoid Factor / Q000097:blood*; D012307:Risk Factors; D012680:Sensitivity and Specificity; D006113:United Kingdom / Q000453:epidemiology
chemical_list: D015415:Biomarkers; D012217:Rheumatoid Factor
keywords: primary care; rheumatoid arthritis; rheumatoid factor; sensitivity; specificity