CASE Patient A was a 68-year-old woman with a history of lymphoplasmacytic lymphoma who presented after several weeks of mucosal bleeding. On admission, her cancer involved 95% of bone marrow cells. She was pancytopenic and febrile (38.3 [degrees]C). Leukocyte count was 0.39 x [10.sup.3]/[micro]L (neutrophils, 0.03 x [10.sup.3]/ AL), hematocrit 25%, platelet count 21 x [10.sup.3]/[micro]L, relative serum viscosity 2.3 (reference interval, 1.4-1.8), blood urea nitrogen 6.4 mmol/L (18 mg/dL), creatinine 62 [micro]mol/L (0.7 mg/dQ, and estimated glomerular filtration rate 60 mL/min/1.73 [m.sup.2]. She had an IgM[KAPPA] monoclonal component of 42.8 g/L. This patient was started on vancomycin 1 g intravenously (IV) every 12 h, ceftazidime 2 g IV every 8 h, and a course of chemotherapy. On day 3 after the beginning of antibiotic treatment, a trough specimen was collected for measurement of vancomycin. The concentration, measured with a Beckman Coulter Synchron competitive turbidimetric immunoassay, was 0.1 mg/L. The result, which was incompatible with ongoing vancomycin therapy, signaled a problem to the technologist. No analytical issues were evident upon review of calibration, controls, and results for other chemistry tests performed on the same specimen. In an attempt to resolve an apparent falsely low result, a 1:1 mix of the specimen was made with the Beckman liquid comprehensive control serum (level 3, 30.4 mg/L) and demonstrated near complete recovery (result of mix after adjusting for dilution, 28.6 mg/L). However, a 1:1 mix with pooled patient serum containing vancomycin (11 mg/L) led to only 15% recovery (result of mix after adjustment for dilution, 1.7 mg/L). The specimen was subsequently sent to another laboratory, where a vancomycin concentration of 9.8 mg/L was measured by use of a competitive enzyme-finked immunoassay (Emit, Olympus).