To develop and validate a composite RA disease activity index using optical spectral transmission (OST)-scores obtained with the HandScan replacing tender and swollen joint counts.
RA patients from a single centre routinely undergoing HandScan measurements and at least one concurrent OST-score and DAS28 were included. Data was extracted from medical records. Linear regression analyses with DAS28 as outcome were performed to create a disease activity index (DAS-OST). OST-score, ESR and patient global assessment (PGA-)VAS, gender, age, disease duration and RF-status were evaluated as independent variables. Final models were derived, based on statistical significance of coefficients and model fit. Of the data, 2/3 was used for development and 1/3 for validation; external validation was performed in a cohort from another centre. Agreement between DAS-OST and DAS28 was assessed using the Bland-Altman plot method and intra-class correlation coefficient (ICC). Diagnostic value of DAS-OST was determined for established definitions of remission, and low (L), and high (H) disease activity (DA).
Data of 3358 observations from 1505 unique RA patients were extracted. DAS-OST was defined as: -0.44 + OST*0.03 + male*-0.11 + LN(ESR)*0.77 + PGA-VAS*0.03. The ICC between DAS-OST and DAS28 were 0.88 (95%CI 0.87-0.90) and 0.82 (95%CI 0.75-0.86) and measurement errors 0.58 and 0.87 in internal and external validation, respectively. Sensitivity for remission, LDA and HDA were 79%, 91%, 43%, and specificity 92%, 80%, 96% in external validation.
Using the HandScan, RA disease activity can be accurately estimated if combined with ESR, PGA-VAS and gender into a disease activity index (DAS-OST).