Thymidine Kinase (TK) is a salvage enzyme, and therefore only present in anticipation of cell division. Therefore it will only be set free to the circulation from cells undergoing division. The enzyme is not set free from cells undergoing normal division where the cells have a special mechanism to degrade the proteins no longer needed after the cell division. In normal subjects the amount of thymidine kinase in serum or plasma is therefore very low. Tumor cells release the TK enzyme to the circulation, probably in connection with the disruption of dead or dying tumor cells. The thymidine kinase level in serum therefore serves as a measure of malignant proliferation, indirectly as a measure of the aggressiveness of the tumor.

The TK test can be used as a cancer screening and diagnostic test by general vets, ER clinics and tertiary centers. Once a dog is diagnosed with cancer, many owners chose to have a specialist administer the chemotherapy even though the general veterinarian could administer it. As a result, the general vet refers out services instead of billing them directly.

Currently, the standard of care is to have rechecks for remission status also performed by a specialist as many of the Gold Standard methods are not easily accessible by most general veterinary practices. With a TK blood test, the general vet would be able to provide the recheck service which would generate revenue for the vet. Also, given the expense and scheduling issues associated with a specialty follow up, these appointments are often delayed or missed. A simple TK blood test would likely increase recheck compliance among owners and increase the standard of care for the animal.