Evaluation of serum thymidine kinase 1 and serum amyloid A as tumor biomarkers in equids: a comparative cross-sectional study

Serum thymidine kinase 1 (TK1) has been identified as a potential biomarker for lymphoma in horses, but has been minimally studied in other equine tumor types, while serum amyloid A (SAA), an acute-phase protein, lacks systematic evaluation for equine tumors. This study assessed the utility of TK1 and SAA as biomarkers for various equine neoplasms. Serum samples utilized in this study were obtained from the tumor biobank at the Institut Suisse de Médecine Equine (ISME). The tumor-bearing cohort included 90 horses and one donkey with lymphoma (n = 14), (adeno)carcinoma (n = 8), hemangiosarcoma (n = 4), other solid internal tumors (n = 8), ovarian granulosa cell tumor (n = 11), sarcoid (n = 14), squamous cell carcinoma (n = 17), and melanoma (n = 15). Additionally, 149 control horses with (n = 107) and without (n = 42) non-neoplastic diseases were included. TK1 activity was measured in all serum samples (n = 240) using tritium-labeled thymidine, while SAA was assessed in a subset (n = 112) using enzyme-linked immunosorbent assay (ELISA). Serum concentrations of both biomarkers were compared between the tumor and control groups. Subsequently, receiver operating characteristic (ROC) curve and cut-off analyses were performed, and post-test probabilities were calculated for neoplastic disorders with known pre-test probabilities (lymphoma, sarcoid, melanoma). Serum TK1 activity was significantly elevated in all tumor subgroups compared to control horses with and without non-neoplastic comorbidities with moderate to large effect sizes, except for (adeno)carcinoma, hemangiosarcoma and ovarian granulosa cell tumor. SAA concentrations did not significantly differ between tumor-bearing equids vs. controls with non-neoplastic comorbidities. In the ROC curve analysis, serum TK1 exhibited good to excellent diagnostic accuracy for all tumor types, except for (adeno)carcinomas. In sarcoid and melanoma, TK1 testing markedly shifted post-test probabilities, supporting its potential as a reliable diagnostic tool. TK1 shows promise as a non-invasive adjunct diagnostic tool for different types of equine neoplasias, though findings are limited by small sample sizes in some tumor groups and warrant validation in larger cohorts. SAA was not a reliable tumor biomarker in this study. This study serves as a starting point for improving non-invasive tumor diagnostics in horses, with the goal of minimizing the need for invasive procedures and thereby promoting animal welfare.