A method for diagnosing lymphosarcoma or for differentiating between lymphosarcoma and inflammatory bowel disease in felines comprising the steps of: obtaining the concentration of thymidine kinase in an isolated blood serum sample; obtaining the concentration of haptoglobin in an isolated blood serum sample; computing an index value using said concentration of thymidine kinase and said concentration of haptoglobin; which computing comprises: obtaining a discrete value of said concentration of thymidine kinase and a discrete value of said concentration of haptoglobin; and obtaining a thymidine kinase product value by multiplying said discrete value of said concentration of thymidine kinase with a thymidine kinase weighing coefficient; obtaining a haptoglobin product value by multiplying said discrete value of said concentration of haptoglobin with a haptoglobin weighing coefficient; and summing said thymidine kinase product value and said haptoglobin product value; obtaining at least two predefined ranges of said index value wherein: one of said at least two ranges is indicative of the presence of lymphosarcoma; and one of said at least two ranges is indicative of the presence of inflammatory bowel disease; and determining that said feline subject has a high probability of having lymphosarcoma if said index value is within said range that is indicative of lymphosarcoma; or has a high probability of having inflammatory bowel disease if said index value is within said range that is indicative of inflammatory bowel disease. obtaining the concentration of thymidine kinase in an isolated blood serum sample; obtaining the concentration of haptoglobin in an isolated blood serum sample; computing an index value using said concentration of thymidine kinase and said concentration of haptoglobin; which computing comprises: obtaining a discrete value of said concentration of thymidine kinase and a discrete value of said concentration of haptoglobin; and obtaining a thymidine kinase product value by multiplying said discrete value of said concentration of thymidine kinase with a thymidine kinase weighing coefficient; obtaining a haptoglobin product value by multiplying said discrete value of said concentration of haptoglobin with a haptoglobin weighing coefficient; and summing said thymidine kinase product value and said haptoglobin product value; obtaining a discrete value of said concentration of thymidine kinase and a discrete value of said concentration of haptoglobin; and obtaining a thymidine kinase product value by multiplying said discrete value of said concentration of thymidine kinase with a thymidine kinase weighing coefficient; obtaining a haptoglobin product value by multiplying said discrete value of said concentration of haptoglobin with a haptoglobin weighing coefficient; and summing said thymidine kinase product value and said haptoglobin product value; obtaining at least two predefined ranges of said index value wherein: one of said at least two ranges is indicative of the presence of lymphosarcoma; and one of said at least two ranges is indicative of the presence of inflammatory bowel disease; and one of said at least two ranges is indicative of the presence of lymphosarcoma; and one of said at least two ranges is indicative of the presence of inflammatory bowel disease; and determining that said feline subject has a high probability of having lymphosarcoma if said index value is within said range that is indicative of lymphosarcoma; or has a high probability of having inflammatory bowel disease if said index value is within said range that is indicative of inflammatory bowel disease.