Serum thymidine kinase 1 protein concentrations and presence of its autoantibody as biomarkers for screening dogs with malignant tumors

Background: An accurate and easily accessible method for diagnosing malignancies in local veterinary clinics has not yet been established. Objectives: To investigate the usefulness of serum thymidine kinase 1 (TK1) protein and its autoantibody as tumor biomarkers in dogs. Animals: Serum samples from 1702 dogs were collected from local animal hospitals and referral animal medical centers in South Korea. Methods: TK1 protein OD value and TK1 autoantibody ratio (TK1 autoantibody OD/total IgG OD) in serum samples of dogs classified into healthy controls, group with nontumor disease, group with benign and group with malignant tumors were measured using lateral flow immunochromatographic assay methods. Results: TK1 autoantibody levels were significantly higher in malignant tumor group (median 0.71) than in healthy controls (median 0.34), group with nontumor disease (median 0.34), and group with benign tumor (median 0.32, Welch t test, P <.0001). They were also significantly different among dogs with carcinomas (median 0.77), hematopoietic tumors (median 0.71), and sarcomas (median 0.56) than in healthy controls (median 0.34, post hoc Games-Howell test, P <.0001). In the receiver operating characteristic curve of TK1 protein, AUC was 0.633 (95% CI: 0.592-0.675, P <.0001). The AUC of TK1 autoantibody ratio was 0.758 (95% CI: 0.723-0.793, P <.0001). Conclusions and Clinical Importance: TK1 autoantibody is a potentially useful biomarker for differentiating between healthy and tumor-bearing dogs, better than TK1 protein measurement. However, both were inadequate when used as single biomarkers for screening dogs to discover occult malignant tumors.