A Spuriously High Phenytoin Level and an Abnormal Lipemic Index: When Aberrant Laboratory Results Lead to a Medical Diagnosis.

Abstract

An 89-year-old male was admitted for pneumonia and chronic obstructive pulmonary disease (COPD)3 exacerbation. Past medical history was significant for atrial fibrillation, congestive heart failure, COPD, hypertension, diabetes, epilepsy, and mild cognitive impairment. Renal and liver function were within normal limits, with plasma creatinine of 0.86 mg/dL (76 μmol/L) (reference interval 0.68–1.30 mg/dL [60–115 μmol/L]), aspartate aminotransferase of 11 IU/L (reference interval 10–38 U/L), and alanine aminotransferase of 13 IU/L (reference interval 10–80 U/L). Albumin was moderately decreased at 2.8 g/dL (28 g/L) (reference interval 3.4–5.0 g/dL [34–50 g/L]). The patient was on a number of medications, including phenytoin 200 mg in the morning and 160 mg at bedtime. Although he did not exhibit any seizures or other signs of neurological dysfunction during his hospital stay, a phenytoin level was requested to ensure adequate therapy. Total phenytoin was measured on the Siemens Dimension Vista® 1500 platform by particle-enhanced turbidimetric inhibition immunoassay (PETINIA) technology (1) and returned at 28 mg/L (111 μmol/L) (therapeutic range 10–20 mg/L [40–80 μmol/L]), along with error messages of “abnormal assay” and “abnormal reaction.” On-board dilution (4-fold) of the specimen yielded …