The Self‐Administered Rheumatoid Arthritis Disease Activity Index (RADAI) is Less Accurate than DAS28‐CRP as a Disease Activity Measure in Pregnancy

OBJECTIVE: Disease Activity Score 28 (DAS28)–using the C‐reactive protein (CRP) level has been validated to determine disease activity in rheumatoid arthritis (RA) patients during pregnancy. A self‐administered questionnaire like Rheumatoid Arthritis Disease Activity Index (RADAI) has practical advantages over DAS28‐CRP, and in this study, we aimed to validate the RADAI for use during pregnancy. METHODS: Patients were derived from a prospective cohort on RA and pregnancy (Pregnancy‐induced Amelioration of Rheumatoid Arthritis study). To validate the RADAI as a disease activity measure, the disease course over time and the disease activity states were compared with the DAS28‐CRP. Furthermore, the RADAI was compared with DAS28‐CRP in predicting fertility and pregnancy outcomes. Finally, to test construct validity, correlation of both RADAI and DAS28‐CRP with a biomarker (galactosylation of immunoglobulin G [IgG]) were determined and compared. RESULTS: In total, 269 patients were analyzed in this study. Mean RADAI scores showed a great decline in disease activity in the first trimester compared with DAS28‐CRP (mean RADAI, −1.13; mean, DAS28‐CRP, −0.04). Correlations between DAS28‐CRP and RADAI scores were moderate to good (0.44 < ρ < 0.71). Agreement in disease states was low (0.26 < κ < 0.51). Time to pregnancy was different between disease states according to DAS28‐CRP (P = 0.03), but not according to RADAI (P = 0.56). Only DAS28‐CRP could predict birthweight (DAS28‐CRP β‐0.17, P = 0.04; RADAI β‐0.09, P = 0.10). Both DAS28‐CRP and RADAI were associated with galactosylation of IgG at specific time points, but only change in DAS28‐CRP was correlated with change in galactosylation of IgG from preconception to pregnancy. CONCLUSION: The RADAI could not be validated as a disease activity measure during pregnancy. DAS28‐CRP remains the gold standard of measuring disease activity in pregnancy.