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Adrian C. Sewell, PhD, Department of Pediatrics, University Children’s Hospital, Theodor-Stern-Kai 7, D-60590 Frankfurt am Main (Germany) Dear Sir, In the course of studying patients with suspected renal calculi, 24-hour urine collections for the determination of calcium and phosphate are mandatory [1]. Recently such an analysis was performed on 3 consecutive 24-hour urines from a patient who had suffered from renal colic for the past 4 years. After admission he underwent a transverse ureterolithotomy for a small left ureteric calculus. On storing the acidified collections at 4 o C, a heavy, crystalline deposit was noted in the first sample. Analysis of this deposit was negative for all the normal constituents of renal stones. Microscopic examination showed crystals unlike those seen in urinary sediments (fig. 1). Heating a small amount of the dried deposit resulted in the release of a purple vapor which on condensing formed black needles giving a positive starch reaction. A survey of the patient’s notes revealed that he underwent intravenous urography (IVU) during the first 24hour collection period. During the IVU the patient received 50 ml of Urografin 325 (18% w/v meglumine diatrizoate and 40% w/v sodium diatrizoate containing 325 mg iodine/ml; Schering Pharmaceuticals). This contrast medium crystallized in a shape similar to that found in the patient’s urine (fig. 1). Thinlayer chromatography of dilute Urografin and a solution of the patient’s urinary crystals in n-butanol/l M ammonia/ethanol (5:2:1) showed the same migration pattern. Crystal formation was investigated with respect to both temperature and pH. Ten random urines were analyzed for pH, protein, calcium, phosphate, creatinine, urea and uric acid. One-milliliter aliquots of each urine were spiked with 25 μl of Urografin (representing an IVU using 50 ml of contrast medium and a 24-hour urinary output of 2,000 ml). One set of aliquots was incubated at 25 °C, the other at 4°C. No crystal-luria was noted in those samples incubated ‚·3⁄4E Fig. 1. Light microscopy of crystals present in the patient’s urine (a) and that of a solution of Urografin pH 2.0 (b). ×75. KAHGER E-Mail [email protected] Fax+ 41 61 306 12 34 © 1996 S. KargerAG, Basel 0028-2766/96/0723-0487$ 10.00/0 at 25 o C, whereas at 4 o C a few crystals were noted in some but not all urines. This was repeated after acidification with 25 μl of 2 M hydrochloric acid to bring the pH to about 3.0. Crystalluria was present in all urine samples both at 25 and 4°C, the latter forming crystals within 6 h of incubation. No correlation was found between the degree of crystalluria and any of the measured