Anti-carbamylated protein (anti-CarP) antibodies are present in approximately one-fourth of the patients who are seronegative for both rheumatoid factor and anti-citrullinated protein antibody and who may therefore have psoriatic arthritis. The investigators hypothesized that detection of anti-CarP antibodies in serum may be useful for diagnosis of psoriatic arthritis.

Inclusion Criteria:

• "Controls" : Patients with a psoriasis confirmed by a dermatologist and with absence of psoriatic arthritis confirmed by a rheumatologist.
• "Cases" : Patients with psoriatic arthritis fulfilling the classification criteria for psoriatic arthritis (CASPAR), with a diagnosis confirmed by a rheumatologist and associated with personal or familial psoriasis.

Exclusion Criteria:

• "Controls": Presence of any chronic rheumatic inflammatory disease such as psoriatic arthritis.