To the Editor:
With the advances in medicine and the development of newer therapeutic agents, the laboratorian needs to be constantly aware of the potential analytical interferences from these medications. Here we demonstrate a potential analytical interference with a case report of a patient who received the recombinant uric acid oxidase Rasburicase (Sanofi-Synthelabo) and the sample handling requirements for uric acid analysis. Rasburicase is a potent uricolytic agent that catalyzes the enzymatic oxidation of uric acid to allantoin, a water soluble metabolite that is readily excreted by the kidney (1). It is used in the treatment and prevention of hyperuricemia, predominantly in patients with hematologic malignancies (1)(2)(3)(4)(5). The half-life of Rasburicase is estimated to be between 16 and 22 h (2).
A 28-year-old man presented with ascites to the emergency department at Sir Charles Gairdner Hospital, a tertiary teaching hospital in Western Australia. The patient was subsequently diagnosed with B-cell non-Hodgkin lymphoma 7 days postadmission. The day before commencement of the patient’s scheduled chemotherapy (day 8 of admission), he developed tumor lysis syndrome. The patient had severe metabolic acidosis (9 mmol/L plasma bicarbonate; reference interval, 22–32 mmol/L), a plasma uric acid concentration of 1.59 mmol/L [270 mg/L; reference interval, 0.20–0.42 mmol/L (34–70 mg/L)], and a creatinine concentration of 97 μmol/L [11.0 mg/L; reference interval, 60–120 μmol/L (6.8–13.6 mg/L)]. Despite prompt resolution of the patient’s metabolic acidosis with fluid hydration, he was transferred to the intensive care unit on day 12 of his admission because of respiratory failure and worsening renal failure. The patient’s laboratory results confirmed ongoing tumor lysis, with plasma concentrations of 304 μmol/L (34.4 mg/L) creatinine, 1.13 mmol/L (190 mg/L) uric acid, 1740 U/L (reference interval, 125–250 U/L) lactate dehydrogenase, and 5.55 mmol/L (reference interval 0.80–1.40 mmol/L) phosphate; total calcium corrected to 40 g/L albumin was 1.62 mmol/L (reference interval, 2.25–2.60 mmol/L). Given the patient’s substantial hyperuricemia, despite adequate fluid hydration and continuous veno-venous hemodialysis, he was given Rasburicase (9 mg intravenously; 0.15 mg/kg body weight) on the day of his intensive care unit admission (day 12) and, again, the following day (day 13). The patient’s plasma uric acid concentration decreased dramatically to 0.52 mmol/L (88 mg/L) before his second infusion of Rasburicase on day 13 and became undetectable [<0.05 mmol/L (<8 mg/L)] on day 14 of his admission. The patient’s renal function improved, and he was discharged on day 26 with a plasma creatinine concentration of 83 μmol/L (9.4 mg/L).
In the laboratory, we were surprised to obtain an undetectable uric acid concentration on day 14 of this patient’s admission. Plasma uric acid analysis is performed by an enzymatic colorimetric assay (Hitachi 917, Roche Diagnostics) with a limit of detection of 0.05 mmol/L (8 mg/L). The between-run CVs of the assay are 1.1% and 1.2%, at mean concentrations of 0.23 mmol/L (34 mg/L) and 0.54 mmol/L (92 mg/L), respectively. Analytical interference was excluded by checking the sample for hemolysis, lipemia, and icteric indexes. The sample was reassayed to check for short sampling. The patient was not prescribed α-methyldopa, desferoxamine, or calcium dobesilate because, at therapeutic concentrations, these medications may cause artefactually low uric acid concentrations, as documented in the manufacturer’s product insert. The patient did not receive ascorbic acid because high concentrations of this compound can interfere with peroxidase in the oxidative reaction of chromogenic reagents. We were unaware that the patient had received Rasburicase because the blood samples were sent to the laboratory as part of routine biochemistry testing. When the sample collected on day 13 was reanalyzed on day 14, the uric acid concentration was undetectable, instead of the expected result of 0.52 mmol/L (88 mg/L). This prompted us to evaluate the magnitude of in vitro uricolytic activity of Rasburicase, and we undertook two experiments. We retrieved a sample from day 8 of admission [pre-Rasburicase plasma uric acid concentration, 1.59 mmol/L (270 mg/L)] and another from day 14 of admission [post-Rasburicase plasma uric acid concentration, <0.05 mmol/L (<8 mg/L)]. These two samples had been stored at 4 °C. In the first experiment, we mixed the day 8 and day 14 samples in equal proportions (0.5 mL:0.5 mL). This sample was kept at room temperature (∼25 °C), and uric acid analysis was performed at 30-min intervals for 3 h. As seen in Fig. 1⇓ . (RT1), there was a rapid decrease in uric acid concentrations within 3 h, confirming ongoing in vitro enzymatic degradation of uric acid.
In the second experiment, we allowed one aliquot of the mixed sample (as above) to stand at room temperature and kept another at 4 °C. This experiment was carried out on day 26, but the two samples were taken on days 8 and 14, respectively. In vitro enzymatic uric acid degradation in the mixed samples continued at room temperature (RT2). The uric acid concentrations remained relatively stable in the sample maintained at 4 °C. The change from baseline to 6 h was 0.05 mmol/L (8 mg/L) and was not statistically significant. This finding demonstrates that the in vitro uricolytic activity of Rasburicase is minimized in a sample maintained at 4 °C as reported previously (1)(3).
We conclude that, in monitoring uric acid concentrations in patients receiving recombinant uric acid oxidase therapy, blood samples should be kept on ice immediately after collection, during specimen transport, and before analysis. As long as compliance of sample handling is achieved at 4 °C, the results of uric acid measurements should be valid.
- © 2003 The American Association for Clinical Chemistry