By The Pawline veterinary teamVeterinary-reviewed by Alastair Greenway, BVM&S MRCVS · 25 July 2026
Emerging marker
This marker has no established reference range in dogs or cats yet, so it is read as a trend rather than measured against a threshold.
TK1 is an enzyme released by rapidly dividing cells, so a raised serum level flags high cell turnover. Paired with CRP it forms a screening index used to look for occult cancer in apparently-healthy dogs — but it is read on lab-specific cut-offs, and it signals proliferation, never a diagnosis.
Thymidine kinase 1 (TK1) is an enzyme cells use when they are actively copying their DNA to divide. Quiescent tissue makes little of it; rapidly proliferating tissue makes a lot, and some leaks into the blood. That gives serum TK1 a simple, appealing logic: it is a marker of cell turnover. Because uncontrolled proliferation is the defining feature of cancer, TK1 has been developed as a screening aid for occult (hidden) malignancy — most established in canine lymphoma and, more broadly, as part of commercial cancer-screening panels.
TK1 is released by rapidly dividing cells, so a raised level flags high cell turnover — a proliferation signal, not a diagnosis.
It is often run paired with CRP — a combination sometimes marketed as a "neoplasia index" — on the reasoning that cancer frequently produces both proliferation and inflammation, and that combining the two improves screening performance over either alone.
Why this is an emerging marker
TK1 sits in the frontier layer as "emerging". It is a real commercial assay with peer-reviewed support, but it is interpreted on laboratory-specific cut-offs rather than a universal validated reference interval. So here it is:
read as a trend on one laboratory's assay;
excluded from the in/out-of-range tally and any health score;
treated as a screening flag to confirm, never as a diagnosis.
What a raised result might mean
A raised TK1 indicates elevated cell turnover somewhere in the body. In a screening context — an older or higher-risk dog that appears well — that is a prompt to look harder for an occult cancer, particularly lymphoma and other systemic malignancies. But the same logic that makes it sensitive makes it non-specific: any process with high cell turnover, including significant inflammation, infection and marrow activity, can raise it. That is exactly why it is paired with CRP and, crucially, why a raised result must be confirmed with imaging and, where indicated, biopsy. TK1 raises the question; other tests answer it.
What a normal result might mean
A normal TK1 is reassuring but not definitive: no screening test detects every cancer, and slower-growing or localised tumours may not raise it. It is best read as one reassuring input within ongoing wellness monitoring rather than a clean bill of health.
In chronic monitoring
Beyond screening, TK1 has been used to follow known cancer patients — tracking response to treatment and watching for relapse, on the logic that falling turnover reflects a shrinking tumour burden and a rising level may precede clinical relapse. Used that way, on a consistent assay, its trend can be more informative than any single value.
Cat vs dog
The evidence and commercial availability are strongest in dogs, where TK1 (alone and with CRP) has been most studied, especially in lymphoma. Feline application is less established. In both species the same honest limits apply: lab-specific cut-offs, non-specific elevation, and confirmation required.
How it's measured
TK1 is measured from a blood sample by specialist laboratories (for example within commercial canine cancer panels), reported in U/L, often alongside CRP. Because cut-offs are assay-specific, results should be compared only within the same laboratory's method and interpreted against its stated thresholds with your vet.
What to do next
A raised TK1 — especially with a raised CRP — is a reason for confirmatory investigation (imaging, and biopsy where indicated) and a careful look for inflammation that could explain it, not a reason to conclude cancer is present. As a frontier proliferation marker, its role is to prompt vigilance early; the diagnosis always comes from the follow-up.
This page is educational and is not a diagnosis. Lab results are interpreted in the full clinical context by your veterinary surgeon; a single value rarely tells the whole story.
