The diagnostic utility of rheumatoid factor and anticitrullinated protein antibody for rheumatoid arthritis in the Indian population.

Background: There is lack of Indian data on diagnostic utility of rheumatoid factor (RF) and anti-citrullinated protein antibody (ACPA) for diagnosis of rheumatoid arthritis (RA) and prevalence of these antibodies in patients with RA and the healthy population. The study was aimed to assess the diagnostic utility and prevalence of RF and ACPA at different titers in the Indian scenario. Method: All the patients of RA fulfilling the European League Against Rheumatism/American College of Rheumatology (EULAR/ACR) 2010 classification criteria and age and gender-matched healthy controls were included in the study. RF and ACPA were measured by nephelometry and the enzyme-linked immunosorbent assay (ELISA) method, respectively. Result: Of 803 patients (291 men and 512 women) included, the RF was positive in 566 (70.5%) study patients. The ACPA was positive in 527 (71.7%) patients of 735 of them. Among 408 healthy controls, 45 (11%) were RF positive and 19 (4.7%) were ACPA positive.At the positive cutoff level, the RF had a specificity of 87.6% (95% confidence interval [CI] = 84.4-90.8; positive likelihood ratio [LR+] 5.7). Specificity at 2 and 3 times above the upper limit of normal (ULN) increased to 96.2% (95% CI = 94.3-98.1; LR+ 15.7) and 97.1% (95% CI = 95.5-98.7; LR+ 17.1), respectively.The specificity of ACPA at the positive cutoff level was 94.4% (95% CI = 92.2-96.6; LR+ 12.7), which increased to 98% (95% CI = 96.6-99.4), at 2xULN level. The likelihood ratio for ACPA at all cutoff levels measured was more than 10. Conclusion: The sensitivity and specificity of RF and ACPA in our study population are comparable with those of other studies. ACPA at lower titers may have sufficient diagnostic utility for RA in an appropriate clinical setting.