Introduction: In this retrospective, register-based population study, we evaluated if anti-citrullinated protein antibodies (ACPA) is a better choice than immunoglobulin M rheumatoid factor (IgM RF) in primary care when rheumatoid arthritis (RA) is suspected, as it determines predictive values in real-life settings. Furthermore, the study described ordering patterns to investigate the benefit of repeated testing.
Methods: Test result, requisitioning unit, test date and the patient's social security number were collected from the Department of Clinical Immunology at Odense University Hospital in 2007-2016 and merged with patient diagnoses from the Danish National Patient Registry.
Results: Overall, 5% were diagnosed with RA. IgM RF remained the preferred test during the entire period. Test sensitivity was 61% for IgM RF and 54% for ACPA. The test specificity was 88% for IgM RF and 96% for ACPA. Positive predictive value (PPV) was higher for ACPA than for IgM RF (30% versus 12%) and negative predictive value (NPV) was equal (99%) in primary care. Few individuals seroconverted from negative to positive (ACPA 2% and IgM RF 5%) and positive to negative (ACPA 3% and IgM RF 6%).
Conclusions: ACPA has a higher PPV for RA than IgM RF, whereas their NPV is identical. ACPA is the better choice when testing for RA in primary care. Seroconversion is rare, and it is only rarely relevant to retest.
Funding: The Department of Clinical immunology at Odense University Hospital funded the study.
Trial registration: not relevant.
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