Serum thymidine kinase activity in clinically healthy and diseased cats: a potential biomarker for lymphoma

The thymidine kinases are enzymes that convert deoxythymidine to deoxythymidine monophosphate and have a function in DNA synthesis. Rapidly proliferating cells will have higher levels of thymidine kinase. Serum thymidine kinase activity (sTK) is a useful tumour marker in humans and dogs, with utility as a prognostic indicator in lymphoma. In the current study serum samples were collected from 49 clinically healthy cats, 33 with lymphoma, 55 with inflammatory disease and 34 with non-haematopoietic neoplasia (NHPN). sTK was measured using a radioenzyme assay and a reference interval (1.96 x SD) was established from the clinically healthy cats (< 5.5 U/l). Mean sTK activity for healthy cats was 2.2 U/l (range 0.8-8.4, +/- SD 1.7). Mean sTK activity for cats with lymphoma was 17.5 U/l (range 1.0-100.0 SD +/- 27.4). Mean sTK activity for cats with NHPN was 4.2 U/l (range 1.0-45.0, SD +/- 8.6). Mean sTK activity for the inflammatory group was 3.4 U/l (range 1.0-19.6, SD 3.9). Cats with lymphoma had significantly higher sTK activity than healthy cats or cats with inflammatory disease (P < 0.0001) and cats with NHPN (P < 0.0002). sTK activity is a potentially useful biomarker for feline lymphoma and further study is required to assess its utility as a prognostic indicator.