IgM monoclonal protein presenting as pseudohypercreatininaemia

Sir, A 63-year-old male was referred for a serum creatinine that was noted to be elevated and increasing. Routine blood tests at a community-based laboratory had revealed a urea of 7.1 mmol/l and creatinine 238 μmol/l (normal 71–133) performed on dry slide technology (DT 60, Johnson & Johnson Clinical Diagnostics). Two days later, his creatinine was 255 μmol/l on the same instrument. Renal sonogram and urinalysis were normal. A 24 h urine sample showed 0.08 g/day protein, and a creatinine clearance of 45 ml/min, based on a serum creatinine of 187 μmol/l (Vitrous 250, Johnson & Johnson Clinical Diagnostics). A repeat creatinine 3 months later on the same technology showed his creatinine to be 305 μmol/l. He was referred to us for further evaluation. Past medical history and review of systems was unremarkable; he was on no regular medications, and denied the use of tobacco or alcohol. Repeat creatinine was 99 μmol/l (using the Hitachi 717 (Roche Diagnostics) employing a Jaffe reaction). Other labs included a normal haemoglobin, platelet count, electrolytes, calcium, phosphorus and urea. We subsequently measured creatinine in repeated and duplicate samples on the DT 60, the Vitrous 250, the Jaffe reaction and another enzymatic method ( Table 1 ). His total protein was slightly elevated at 86 g/l, and so a serum protein electrophoresis was ordered, revealing a monoclonal IgM κ at a concentration of 28.6 g/l. A presumed diagnosis of pseudohypercreatininemia secondary to a monoclonal protein was made. A radiopharmaceutical glomerular filtration rate was measured to be 93 ml/min, using Tc-99m DTPA. A computed tomography scan and bone marrow biopsy revealed no evidence of Waldenstrom's macroglobulinaemia or multiple myeloma. Creatinine values using different methods Creatinine values using different methods Three cases of elevations in serum creatinine secondary to a monoclonal IgM have been previously described in patients known to have Waldenstrom's macroglobulinaemia. These falsely elevated results occurred with an enzymatic assay (Roche Diagnostics) and were confirmed to be normal using high performance liquid chromatography (HPLC) [ 1 ]. Fortunately, this appears to be an uncommon problem, although possibly under-recognized. The mechanism remains unknown, although its occurrence with a number of different assays implies that there may be more than one. The previous report in the literature referred to an aqueous test method [ 1 ]. Our patients’ serum was tested with similar methods without any resulting interference. As clinicians who see patients frequently for elevated serum creatinine values, nephrologists must be aware of the limitations of this test. Pseudohypercreatininaemia secondary to a monoclonal protein should be considered in patients with an isolated creatinine elevation, particularly when associated with a normal urea, or an elevated total protein. Measuring the serum creatinine using a completely different method may prevent other unnecessary tests or procedures. Conflict of interest statement . None declared.

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