To investigate and compare the accuracy and usefulness of diagnostic tests for rheumatoid factor (RF).
In a cross-sectional study sera derived from patients admitted to the Section of Rheumatology were tested for presence of RF using either nephelometry or the Waaler test. Diagnostic sensitivity and predictive values of the tests were calculated and compared. The accuracy of the tests was compared using receiver-operating characteristics (ROC) methodology.
Good agreement was found between the tests (kappa approximately 0.7). At cut-off 19 IU/mL nephelometry showed the highest sensitivity (82.4%) and specificity (95.9%) for rheumatoid arthritis (RA). In comparison, the Waaler test had a sensitivity of 60.3% and specificity of 95.9% at cut-off titer 128. The tests showed nearly equal performance characteristics when predicting SS.
Although both tests exhibit good performance characteristics, nephelometry has a higher accuracy when predicting RA and SS. The common practice of using both tests for detection of RF is not recommended.