The invention provides a method and system for developing and using diagnoses of lymphosarcoma in feline subjects using thymidine kinase (TK) and haptoglobin (HP) as biomarkers. The invention provides a method for obtaining the level of each biomarker and computing an index for a feline subject. The invention provides predefined index ranges to which the index value may be matched in order to determine whether the subject has a high probability of being affected by lymphosarcoma, even when the subject shows apparent symptoms that may be common with inflammatory bowel disease.