APRT and urolithiasis: For patients with urolithiasis or crystal nephropathy, the following diagnostic tests have been used to evaluate the APRT deficiency: (i) APRT gene analysis, (ii) measurement of APRT enzyme in erythrocytes or red cell lysates, (iii) characterization of 2,8-DHA crystals in the urinary sediment using infrared spectroscopy on biopsy, (iv) measurement of adenine levels in a 24-hour urine sample3.