Thymidine kinase 1 activity in serum and body cavity fluids of horses with lymphoma.

BACKGROUND: The accuracy of serum thymidine kinase 1 (TK1S) as a biomarker of lymphoma in horses is variable. The value of measurement in body cavity fluid (TK1F) has not been established. HYPOTHESIS/OBJECTIVES: (1) TK1F and TK1S will be higher in horses with lymphoma compared to diseased controls (DCs); (2) TK1F will be higher than TK1S; and (3) there will be a strong correlation between both measurements. ANIMALS: Twenty animals in the lymphoma and 31 animals with nonneoplastic, gastrointestinal/respiratory disorders in DC group. METHODS: This was a cross-sectional study using prospectively collected data and samples obtained during routine diagnostic evaluation at referral hospitals. Thymidine kinase 1 was measured using chemiluminescence immunoassay and presented as median (and interquartile range). Nonparametric tests compared TK1F and TK1S between and within groups and Kendall's Tau assessed correlation between TK1F and TK1S. RESULTS: Median TK1F was 7.48 (2.20-48.23) U/L in horses with lymphoma vs 2.50 (0.59-4.83) U/L in the DC group (P = .007); median TK1S was 4.26 (0.46-9.37) U/L vs 0.40 (0.40-0.75) U/L (P < .001), respectively. Both biomarkers showed good discrimination (area under the curve > 0.70) between lymphoma and the DC group, with optimal cutoff values of 5.7 and 2.5 U/L, respectively. Thymidine kinase 1 activity in body cavity fluid (3.62 [0.85-9.86] U/L) was significantly higher than TK1S (0.50 [0.40-2.09] U/L; P < .001) and there was no significant correlation between them within groups. CONCLUSIONS AND CLINICAL IMPORTANCE: Thymidine kinase 1 activity in body cavity fluid and TK1S provide different information and should be interpreted independently. They proved useful in lymphoma diagnosis, but not as a standalone test.