Thymidine kinase 1 (TK1) in dogs and cats

What it shows

TK1 works in the pyrimidine salvage pathway, one of the routes by which a cell makes DNA precursors. Its activity is linked with cell division. Rapidly dividing tumour cells keep TK1 high, and TK1 from disrupted tumour cells reaches the bloodstream.

In dogs, serum TK1 has been studied for diagnosis, for judging the extent of lymphoma and for monitoring. TK1 tracks remission status, so a series of results during the vet’s plan can be more informative than a single value. Older tests measure TK1 activity; some newer ones measure the amount of TK1 protein instead. It is a specialised test sent to a reference laboratory.

Higher than reference — what it may be a sign of

• Lymphoma — serum TK1 activity in dogs with lymphoma is significantly higher than in healthy dogs, and higher in advanced stages than in early ones.
• Other cancers — in one study of 253 dogs with cancer, the median TK1 was about 7.0 U/L against 1.8 U/L in 156 healthy dogs; blood-cell (haematopoietic) tumours had a higher median (19.1 U/L) than solid tumours (5.9 U/L). Lymphoma and histiocytic sarcoma were separated from health best.
• Inflammation and infection — a temporary increase has been seen in several infectious and inflammatory diseases.
• Young age — juvenile dogs had higher mean TK1 than healthy adults.

A high value is a reason for the vet to look for the cause — with an examination, cytology or a biopsy — not a diagnosis in itself.

Lower than reference

A normal TK1 does not exclude cancer. In the dog study above, a cut-off of 20 U/L had high specificity (0.98) but low sensitivity (0.27): most dogs with cancer had values below it. Dogs with T-cell lymphoma generally have lower TK1 levels. A cancer diagnosis does not rest on a biomarker alone.

Look at together with

• CRP — an inflammation marker; studies combine it with TK1, and inflammation markers can help sort out increases caused by inflammation.
• Lymphocytes and the full blood count — part of the overall picture in suspected lymphoma.
• PARR, cytology or a biopsy — tests that look at the tumour cells themselves.
• Earlier TK1 results — the trend in the same animal on the same test.

Units and conversion

Results of activity tests are given in U/L — units of enzyme activity per litre of serum; this is the unit stored and charted in the app, and no conversion is needed. A test that measures TK1 protein rather than activity reports in its own way and is read only against that test’s reference range. Results from different methods are not compared on one line.

Reference range

The reference range is on your lab’s form, and it depends on the method. For illustration only: in one feline study using a radioenzyme assay, the reference interval for healthy cats was below 5.5 U/L; in the dog study above, healthy dogs had a median of 1.8 U/L on a chemiluminescent assay. These numbers are not cut-offs for your animal; the vet reads the result against the lab’s own range and the earlier results.

Cats and dogs

Sources

1. Bryan, Frontiers in Veterinary Science 2016 — The current state of clinical application of serum biomarkers for canine lymphoma
2. Selting et al., J Vet Intern Med 2016 — Thymidine kinase type 1 and C-reactive protein concentrations in dogs with spontaneously occurring cancer (PMC)
3. Sharif et al., Frontiers in Veterinary Science 2023 — A monoclonal antibody-based sandwich ELISA for measuring canine thymidine kinase 1 protein in canine lymphoma
4. Taylor et al., J Feline Med Surg 2012 — Serum thymidine kinase activity in clinically healthy and diseased cats: a potential biomarker for lymphoma (PMC)