Background  CD26 is a multifunctional type II transmembrane glycoprotein, which also exists as a secreted isoform, soluble CD26 (sCD26). The CD26 expression on circulating T cells is decreased in some skin diseases such as cutaneous T-cell lymphoma (CTCL) and psoriasis. It remains to be determined whether sCD26 can be used as a marker of skin diseases or not.

Objective  To investigate utility of sCD26 as a diagnostic marker of skin diseases in combination with thymus and activation-regulated chemokine (TARC).

Methods  Serum sCD26 levels were measured using enzyme-linked immunosorbent assay in 130 participants including 32 patients with atopic dermatitis (AD); 45 patients with CTCL; 26 patients with psoriasis; and 27 healthy controls.

Results  Serum sCD26 levels in patients with CTCL and psoriasis (162.1 ± 80.2 ng/mL and 125.4 ± 82.1 ng/mL respectively) were significantly lower than those of healthy controls (392.6 ± 198.7 ng/mL; < 0.01 and 0.01 respectively). In patients with CTCL, serum sCD26 levels of patients with advanced stage were 135.0 ± 51.5 ng/mL and they were significantly lower than those with early stage (193.1 ± 96.0 ng/mL; < 0.05). When we used serum sCD26 and TARC levels for diagnostic criteria, sensitivity, specificity, positive predictive value and negative predictive value for AD, CTCL and psoriasis were 65.2–73.7%, 81.4–97.6%, 65.2–94.4%, and 81.4–88.9% respectively.

Conclusion  Serum sCD26 levels, combined with serum TARC levels, are helpful in diagnosis of AD, CTCL and psoriasis.