Clinical performance of a commercially available thymidine kinase 1 assay for diagnosis of lymphoma in 42 hospitalized horses (2017‐2020)

BACKGROUND: Antemortem definitive diagnosis of lymphoma in horses is often difficult. Thymidine kinase 1 (TK1) assay is a potentially useful biomarker for lymphoma in horses. HYPOTHESIS/OBJECTIVES: To report the clinical performance of a commercially available TK1 assay for diagnosis of lymphoma in horses. We hypothesized that there would be no association between serum TK1 activity and a diagnosis of lymphoma in horses. ANIMALS: Forty‐two hospitalized horses, 14 with a definitive diagnosis of lymphoma, 4 with other neoplasia, and 24 with inflammatory disease. METHODS: Retrospective medical record review, groups were compared via Kruskal‐Wallis and Mann‐Whitney tests, and logistic regression was performed. RESULTS: Median (range) TK1 was 3 U/L (0.4‐17.7 U/L) in horses with lymphoma and 3.9 U/L (0.8‐94 U/L) in horses without lymphoma (P = .59). There was no significant difference in total protein between horses with and without lymphoma (6.6 g/dL [5.5‐8.3 g/dL] vs 6.6 g/dL [4.7‐10.4 g/dL]; P = .83). There was no significant difference in fibrinogen between horses with and without lymphoma (447 [100‐1364] mg/dL vs 433 [291‐2004] mg/dL; P = .47). On logistic regression, serum TK1 activity was not associated with a diagnosis of lymphoma (odds ratio, 0.97; 95% confidence interval, 0.9‐1.05, P = .48). CONCLUSION AND CLINICAL IMPORTANCE: Serum TK1 values were not predictive of lymphoma diagnosis in this cohort of horses.