AbstractBackgroundPsoriasis usually precedes the onset of psoriatic arthritis by an average of 10 years in about 70% of patients. Receptor activator of nuclear factor kappa‐B ligand (RANKL) is a natural and necessary surface‐bound molecule, which is a ligand for osteoprotegrin and functions as a key factor for osteoclast differentiation and activation.ObjectiveEvaluation of the serum level of RANKL in psoriasis and psoriatic arthritis and its correlation with severity of the disease as a trial to predict the occurrence of psoriatic arthritis in such patients.MethodThis study included 80 subjects; 40 patients had chronic plaque psoriasis, 20 patients had chronic plaque psoriasis with psoriatic arthritis, and 20 were healthy controls. Patients with psoriasis were divided into three subgroups according to psoriasis area severity index score: mild, moderate, and severe. Serum RANKL levels were estimated for all subjects using enzyme‐linked immunosorbent assay.ResultsSerum RANKL level in psoriatic arthritis was significantly higher compared to both patients with plaque psoriasis and control groups. Serum RANKL level significantly correlated to the severity of psoriasis, with a very high correlation coefficient in groups I and II. The diagnostic performance of serum RANKL level for the early prediction of psoriatic arthritis in patients with psoriasis was >170 pg/ml.ConclusionSerum RANKL level could be considered as a useful diagnostic marker for the early prediction of psoriatic arthritis in patients with moderate and severe psoriasis. Assessment of soluble RANKL in psoriasis could identify those at increased risk for psoriatic arthritis, and anti‐RANKL agents may be effective in decreasing incidence of psoriatic arthritis.