Serum thymidine kinase is a tumour marker for canine and feline malignant lymphoma. Thymidine kinase (TK1) is a cytosolic enzyme associated with cellular proliferation. It markedly increases in activity during the G1 and S phases of the cell cycle. Thymidine kinase enters the circulation from cycling cells or perhaps from cells dying during replication. In essence, then, TK is a marker of the volume of replicating cells in the animal and consequently an indirect measure of neoplastic burden.

The relationship between serum TK and a significant increase in cell replication allows the test to be considered as a diagnostic adjunct for neoplasia such as malignant lymphoma. However, suggestions for its use focus more on prognosis and therapeutic monitoring.

Those lymphoma patients with the highest TK activities are likely to have the highest neoplastic burden or replication rate and these may have a worse prognosis than those lymphoma patients with lower levels.

Thymidine kinase can also be used during the initial phase of chemotherapy to document a decrease from pre-treatment values, indicating the “success” of early therapy and informing the remaining protocol. Frequent monitoring of TK during or between chemotherapeutic cycles should also identify relapse (perhaps pre-clinically) or help differentiate the origin of a clinical deterioration.

Thymidine kinase may not have perfect diagnostic specificity when used as a diagnostic test for lymphoma because of the possible association with other conditions of increased cell turnover including inflammation. However, in the absence of leukocytosis the likelihood of a neoplastic origin is much increased.

Reference:
VonEuler, Einarsson, Olsson, Lagerstedt and Erikson (2004) Serum Thymidine Kinase Activity in Dogs with Malignant Lymphoma: A potent marker for prognosis and monitoring the disease. Journal of Veterinary Internal Medicine 18:696-702