Serum thymidine kinase activity in clinically healthy and diseased horses: A potential marker for lymphoma

•Rapidly proliferating cells have higher levels of activity of thymidine kinase (TK).•TK activity in serum is a potential biomarker for the presence of neoplasia.•Serum TK activity in horses with lymphoma was significantly higher than in healthy horses or horses with other tumours.•Serum TK activity is a potentially useful biomarker for equine lymphoma.

Serum thymidine kinase (sTK) activity is a tumour marker used as a prognostic indicator for lymphoma in humans, dogs and cats. The aim of this study was to evaluate the clinical utility of sTK as a biomarker for lymphoma in horses. Serum samples were collected from clinically normal horses (n = 37), horses with lymphoma (n = 23), horses with non-haematopoietic neoplasia (n = 9) and horses with inflammatory disease (n = 14). sTK was measured using a radioenzyme assay. A reference cut-off value of <2.7 U/L (mean + 2 standard deviations, SDs) was established using data from clinically normal horses. sTK activity (mean ± SD) was 26.3 ± 91.5 U/L (range 0.8-443 U/L) for horses with lymphoma, 2.3 ± 1.4 U/L (range 0.6-5.7 U/L) for horses with non-haematopoietic neoplasia and 1.5 ± 0.6 U/L (range 0.6-2.8 U/L) for horses with inflammatory disease. Horses with lymphoma had significantly higher sTK activity than horses without clinical signs of disease (P <0.01), horses with inflammatory disease (P <0.01) and horses with non-haematopoietic neoplasia (P <0.05). sTK activity is a potentially useful biomarker for equine lymphoma.