Patent ID: 8497471

Claim:
A method for determining thiopurine-S-methyl transferase (TPMT) activity in a patient prior to initiation of 6-MP administration, said method comprising: a. incubating a sample obtained from a patient for a specified period of time with an isotopically labeled substrate for TPMT under conditions suitable for the generation of an isotopically labeled 6-methylmercaptopurine (6-MMP) product by TPMT in the sample, thereby forming an incubated sample; b. ionizing said isotopically labeled 6-methylinercaptopurine (6-MMP) from said incubated sample under conditions suitable to produce one or more ions detectable by mass spectrometry; c. determining the amount of one or more ions by mass spectrometry; d. using the amount of said one or more ions to determine the amount of isotopically labeled 6-methylmercaptopurine (6-MMP) in said incubated sample; and e. using the amount of isotopically labeled 6-MMP determined in said incubated sample to determine TPMT activity in the patient.