A method for diagnosing lymphoma in cats while ruling out Inflammatory Bowel Disease comprising the steps of: providing a quantity of blood obtained from a cat; obtaining a portion of said quantity of blood containing thymidine kinase; measuring in said portion the activity of thymidine kinase; and determining a high probability for said cat to have lymphoma if said amount of thymidine kinase is present at an activity level of at least 15.5 units per liter; wherein TK activity is measured by a method comprising: (a) reacting the blood portion with 3'- azido-3'-deoxythymidine in the presence of a phosphate donor selected from adenosine triposphate and cytidine triphosphate and a buffer system, thus producing 3'-azido-3'-deoxythymidine mono phosphate (AZTMP), the buffer system including 10-100 mM HEPES or Tris with pH ranging from 6.8-8.0, 1-30 mM DTE, 0.2-8 mM ATP and MgCl 2 at a concentration of at least two times the concentration of ATP with a pH ranging from 6.8-8.0; and (b) performing a competitive immunoassay for determining the 3'-azido-3'-deoxythymidine mono phosphate (AZTMP), the immunoassay comprising: b.1. a first incubation of the sample with paramagnetic particles coated with anti-AZTMP polyclonal antibody, in such a way that AZTMP binds to the solid phase; and b.2. a second incubation of the sample with an AZTMP analogue conjugated to an isoluminol derivative, the tracer conjugate competing for binding with the AZTMP in the solution b.3. the unbound material is removed with a wash cycle; and b.4. the light signal is measured by a photomultiplier. providing a quantity of blood obtained from a cat; obtaining a portion of said quantity of blood containing thymidine kinase; measuring in said portion the activity of thymidine kinase; and determining a high probability for said cat to have lymphoma if said amount of thymidine kinase is present at an activity level of at least 15.5 units per liter; wherein TK activity is measured by a method comprising: (a) reacting the blood portion with 3'- azido-3'-deoxythymidine in the presence of a phosphate donor selected from adenosine triposphate and cytidine triphosphate and a buffer system, thus producing 3'-azido-3'-deoxythymidine mono phosphate (AZTMP), the buffer system including 10-100 mM HEPES or Tris with pH ranging from 6.8-8.0, 1-30 mM DTE, 0.2-8 mM ATP and MgCl 2 at a concentration of at least two times the concentration of ATP with a pH ranging from 6.8-8.0; and (b) performing a competitive immunoassay for determining the 3'-azido-3'-deoxythymidine mono phosphate (AZTMP), the immunoassay comprising: b.1. a first incubation of the sample with paramagnetic particles coated with anti-AZTMP polyclonal antibody, in such a way that AZTMP binds to the solid phase; and b.2. a second incubation of the sample with an AZTMP analogue conjugated to an isoluminol derivative, the tracer conjugate competing for binding with the AZTMP in the solution b.3. the unbound material is removed with a wash cycle; and b.4. the light signal is measured by a photomultiplier. (a) reacting the blood portion with 3'- azido-3'-deoxythymidine in the presence of a phosphate donor selected from adenosine triposphate and cytidine triphosphate and a buffer system, thus producing 3'-azido-3'-deoxythymidine mono phosphate (AZTMP), the buffer system including 10-100 mM HEPES or Tris with pH ranging from 6.8-8.0, 1-30 mM DTE, 0.2-8 mM ATP and MgCl 2 at a concentration of at least two times the concentration of ATP with a pH ranging from 6.8-8.0; and (b) performing a competitive immunoassay for determining the 3'-azido-3'-deoxythymidine mono phosphate (AZTMP), the immunoassay comprising: b.1. a first incubation of the sample with paramagnetic particles coated with anti-AZTMP polyclonal antibody, in such a way that AZTMP binds to the solid phase; and b.2. a second incubation of the sample with an AZTMP analogue conjugated to an isoluminol derivative, the tracer conjugate competing for binding with the AZTMP in the solution b.3. the unbound material is removed with a wash cycle; and b.4. the light signal is measured by a photomultiplier. b.1. a first incubation of the sample with paramagnetic particles coated with anti-AZTMP polyclonal antibody, in such a way that AZTMP binds to the solid phase; and b.2. a second incubation of the sample with an AZTMP analogue conjugated to an isoluminol derivative, the tracer conjugate competing for binding with the AZTMP in the solution b.3. the unbound material is removed with a wash cycle; and b.4. the light signal is measured by a photomultiplier.