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Sarah Wheeler, Li Liu, Octavia M Peck Palmer, Puzzling Phosphorus: Unlikely Results, Clinical Chemistry, Volume 59, Issue 9, 1 September 2013, Pages 1415–1416, https://doi.org/10.1373/clinchem.2013.203224
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Case Description
Discordant plasma phosphorus concentrations (Table 1) were obtained with 2 clinical instruments (concentration range, 4.5–23.8 mg/dL; reference interval, 2.5–4.6 mg/dL) for a 50-year-old woman. Her medical history was positive for idiopathic pulmonary arterial hypertension, chronic kidney disease (stage 1), severe right heart failure, ascites, and thrombocytopenia. During hospitalization, the patient received enteral feeding, cefuroxime, docusate, dronabinol, gabapentin, metolazone, nafcillin, and morphine.
Plasma phosphorous results.a
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Answers
Hyperphosphatemia can occur in renal failure, diabetes, endocrinopathies, hypoparathyroidism, increased dietary/pharmacologic intake of phosphates or vitamin D, and immunoproliferative diseases such as multiple myeloma, in which paraproteins interfere with the colorimetric method (1, 2). Plasma phosphorus is quantified through reaction with ammonium molybdate to form a colored phosphomolybdate complex (3). The manufacturer of the DxC 800 System (Beckman Coulter) lists nafcillin as an interferant; however, the interference mechanism is unknown (4). Unlike other known antibiotic interferants, nafcillin is colorless and lacks protein and lipid components. Investigation revealed that the patient's blood sample had been collected through the same intravenous line that had administered nafcillin. In-house in vitro nafcillin-spiking studies revealed substantial interference only with the DxC 800 instrument.
