Correspondence

A case is presented of a 41-year-old man with a history of recurrent renal stones over 10 years. Analysis of the stone showed that, although it gave a positive reaction with the non-speci c phosphotungstic acid test, uricase failed to identify any urate present. Analysis in a reference laboratory con rmed its composition as dihydroxyadenine. Patients who are homozygous for the rare autosomal-recessive adenine phosphoribosyltransferase de ciency, excrete large amounts of 2,8-dihydroxyadenine, which has poor solubility at normal urinary pH. Treatment with the xanthine oxidase inhibitor allopurinol induces a total cessation of stone formation. Increased awareness of the condition and knowledge of the limitations of some methods of laboratory analysis for renal stones should help to identify this type of stone and prevent renal damage. Ann Clin Biochem 2004; 41: 160–161 Case history A 41-year-old man presented to his general practi-tioner with renal colic. He had su¡ered similar episodes over the previous 10 years and passed many stones. There was no positive family history. An intra-venous urogram showed gross ureteric obstruction. The stone was radiolucent, suggesting it was composed of urate. The stone was removed uretero-scopically. A 24-h urine collection showed a urinary urate concentration of 4.3 mmol per 24 h (reference range 2.3-4.5). Serum urate (303 mmol/L) was within the reference range (5420 mmol/L). Laboratory analysis of the stone gave a positive test for urate with the non-speci¢c reduction of phospho-tungstic acid, but analysis with uricase failed to detect urate. The stone was sent for analysis by infrared spectroscopy and was shown to be composed mainly (92%) of 2,8-dihydroxyadenine (2,8-DHA).1 Analysis of his red blood cells showed complete absence of the enzyme adenine phosphoribosyltransferase (APRT).2 His father and two siblings all showed reduced levels of APRT (seeTable1). The patient was treated with 300 mg of allo-purinol daily. His urinary 2,8-DHA decreased from 0.106 mmol/L at presentation to 0.063 mmol/L. He has been stone-free for 5 years