The thymidine kinases are enzymes involved in DNA synthesis (converting deoxythymidine to thymidine monophosphate) and are therefore present in high levels in rapidly proliferating cells.

Serum thymidine kinase (sTK) activity is recognised as a tumour marker in humans with various neoplasms and is used as a prognostic indicator as well as for assessing response to treatment and relapse in leukaemia, lymphoma (Hodgkin's and non-Hodgkin's) and myelodysplastic syndrome. sTK has also been assessed as a tumour marker for indicating prognosis and predicting relapse in dogs with lymphoma. Limited studies have been performed on tumour markers in cats. A serum marker that could assist in diagnosis, provide prognostic information and indicate response to treatment in cats with lymphoma would be clinically valuable.

The aim of this study was to measure sTK activity in clinically normal cats, cats with inflammatory disease, non-haematopoetic neoplasms and lymphoma, in order to evaluate whether sTK is a useful marker for lymphoma in cats.

Serum samples were collected from 61 cats. The study groups included 22 healthy cats, six with lymphoma (prior to treatment), 26 with inflammatory diseases and two with non-haematopoetic neoplasms. The inflammatory groups included six cats with urinary tract infections, seven with leukocytosis and 13 with gastro-intestinal disease.

Enzyme activity was measured using an enzyme linked immunosorbant assay (Divitum®, Biovica, Sweden).

Mean sTK activity was 2.2ng/L (range 0.1-14.7, SD 3.8) for healthy cats, 176.4ng/L (range 9.3-783.7, SD 301.9) for cats with lymphoma, 0.3ng/l (range 0.2-0.4, SD 0.2) for cats with non-haematopoetic neoplasia, and 6.9ng/L (range 0.1-85.9, SD 17.5) for cats with inflammatory diseases. A U-Mann Whitney test for non-parametric data was used to assess the significance of differences between the groups. Healthy cats and cats with lymphoma had significantly different sTK activity (P<0.001). Cats with inflammatory disease and cats with lymphoma also had significantly different sTK activity (P=0.001).

This study shows that measurement of feline sTK activity has the potential to be a useful tumour marker in feline lymphoma. sTK may therefore be useful in indicating prognosis and for predicting relapse in cats in remission from lymphoma. Further study is required, including measuring sTK in a greater number of normal cats, cats with treated and untreated lymphoma and other neoplastic and inflammatory diseases.