To estimate and compare the direct and indirect costs of illness in rheumatoid arthritis, ankylosing spondylitis, psoriatic arthritis (PsA) and systemic lupus erythematosus (SLE), and to evaluate the effect of sex, disease duration and functional status on the various cost domains.
Data of outpatients, aged 18-65, with rheumatoid arthritis (n = 4351), ankylosing spondylitis (n = 827), PsA (n = 908) or SLE (n = 844), who were enrolled in the national database of the German collaborative arthritis centres in 2002, were analysed. Data on healthcare consumption, out-of-pocket expenses and productivity losses were derived from doctors and patients. For the calculation of indirect costs, the human capital approach (HCA) and the friction cost approach (FCA) were applied.
Mean direct costs amounted to 4737 euros a year in rheumatoid arthritis, 3676 euros in ankylosing spondylitis, 3156 euros in PsA and 3191 euros in SLE. By using the HCA, total costs were calculated at 15,637 euros in rheumatoid arthritis, 13,513 euros in ankylosing spondylitis, 11,075 euros in PsA and 14,411 euros in SLE, whereas with the FCA the numbers were 7899 euros, 7204 euros, 5570 euros and 6518 euros, respectively. Costs increased with disease duration and were strongly dependent on functional status. In patients with the highest disability (<50% of full function), the total costs on applying the HCA were 34,915 euros in rheumatoid arthritis, 29,647 euros in alkylosing spondylitis, 37,440 euros in PsA and 32,296 euros in SLE.
The costs of illness are high in all four diseases, with a strong effect of functional status on total costs. Indirect costs differ by the factor 3, based on whether the HCA or the FCA is used.