Cytidine deaminase may be a useful marker in differentiating elderly onset rheumatoid arthritis from polymyalgia rheumatica/giant cell arteritis.

Objective: PMR/GCA is a relatively common inflammatory disease in the elderly population. Clinical differentiation from a polymyalgic onset of RA in the elderly can be difficult. We have examined in a preliminary study the hypothesis that serum cytidine deaminase (CD) may be valuable in the differential diagnosis of these disorders.

Methods: CD was assayed by a spectrophotometric method in 20 patients with active PMR/GCA, both before and after treatment with prednisolone, and was compared with serum CD levels in 20 patients with active RA.

Results: CD levels were within the normal range (< 10 units/ml) in 36 of the 40 samples from patients with PMR/GCA: The mean CD in pre-treatment samples was 8.64 units/ml (SD 7.09), and after treatment 7.20 units/ml (SD 3.53). The mean serum CD in the RA patients was 21.33 units/ml (SD 8.94), significantly higher than in PMR/GCA (p < 0.0001).

Conclusion: Serum CD levels were significantly different when proven PMR was compared with established, long-standing RA. Therefore, serum CD could be a useful diagnostic marker for differentiating PMR/GCA from active RA in older patients.