Same question, different answers: a comparison of global health assessments using visual analogue scales

To compare responses to two global health VAS of patients with rheumatoid arthritis at the same assessment within the same questionnaire.

Secondary analysis of randomised controlled trial data. Patients completed the patient global assessment VAS (PG-VAS) (horizontal 10 cm scale, left (0), right (100), no incremental markers) and EQ-5D-VAS (EQ-VAS) (vertical 20 cm scale, 100 at the top, markers at each increment of 10). Both asked "how good or bad is your health today, in your opinion, from 100 'Best imaginable health state' to 0 'Worst imaginable health state'." Agreement was assessed using intra-class correlation co-efficients (ICC) and Bland-Altman plots.

Four hundred and forty-eight patients reported median PG-VAS 66 (IQR 51, 77) and EQ-VAS 65 (IQR 50, 80) scores. Correlation of the VAS scales was moderate at baseline (ICC 0.564) and longitudinally (ICC 0.503). Bland-Altman plots suggested poor concordance of the PG-VAS and EQ-VAS; the limits of agreement were +/-32.3 on a 0-100 scale. PG-VAS scores were evenly distributed; EQ-VAS scores clustered at increments of 10; rounding did not improve agreement.

The EQ-VAS and PG-VAS scores are not interchangeable at the individual level. The EQ-VAS correlated more strongly with disease-specific and health-related quality of life measures, therefore, appears preferable.