To determine which of the following is the best method to identify PSORS1-associated psoriasis in patients with psoriatic arthritis (PsA): age at disease onset or positive family history of disease.

A total of 71 patients with PsA who met the CASPAR criteria were recruited on a randomized basis. The patients were stratified according to age at disease onset, with cutoff points at 25, 30, 35 and 40years of age. The alleles of locus Cw were analyzed by PCR-based methods, and their distribution was compared to that of 177 healthy blood donors.

HLA-Cw*0602–PSORS1- was associated with disease risk (56% vs. 18%) OR 5.8, 95%CI: 3.2-10.7, P=0.00001. A close relationship was established between this allele and onset of psoriasis under 30years of age (68% vs. 24%) OR 6.4, 95%CI: 2.3–18.2, P=0.0003. The relationship in turn lost significance above this age limit. An association was found between a family history of psoriasis and disease risk, though there was no specific cutoff point according to age at onset of the disease. Sixty-four percent of the subjects with positive family history were HLA-Cw*06 (+) compared to 44% of those without a family history, OR 2.3, 95% CI: 0.82–6.36, P=0.08.

In patients with PsA, the susceptibility effect of HLA-Cw*06 declines with increasing age of onset. Disease onset under or above 30years of age may contribute to differentiate type I vs. type II psoriasis in PsA populations, while the family history have a lesser contribution to such stratification.