Unidentified anion gap metabolic acidosis.

A 35-month-old female with nonketotic hyperglycinemia (NKH) presented to the Emergency department with severe hypoglycemia, fever, and several episodes of seizures. Due to worsening respiratory status, additional seizures and anion gap worsening metabolic acidosis the patient was transferred to the pediatric intensive care unit. The useful mnemonics for causes of high anion gap metabolic acidosis are the classic MUDPILES (representing Methanol, Uraemia, Diabetes, Paraldehyde, Iron (and Isoniazid), Lactate, Ethylene glycol, and Salicylate) and the more recently proposed GOLD MARK (Glycols [ethylene and propylene], Oxoproline, l-lactate, d-lactate, Methanol, Aspirin, Renal failure, and Ketoacidosis) as causes of the anion gap metabolic acidosis were all ruled out. Relatively stable concentrations of salicylate (approximately 10 mg/dL, 0.7 mmol/L) were noted, despite no evidence the patient received aspirin Therefore further laboratory testing was performed. A Basic-Acid-Neutral (BAN) gas chromatography mass-spectroscopy (GC-MS) Drug screen of urine was undertaken. A large benzoic acid peak was identified by spectral match, which supported the clinical history that the patient was taking sodium benzoate powder 1175 mg as a dietary supplement three times a day. However, salicylate was not identified. This patient had benzoic acid concentration in excess of 2000 μg/mL. Given that benzoic acid is a weak acid, with a pK of approximately 4 it is almost completely ionized at pH 7. Therefore, the large amount of benzoic acid was not only thought to be contributing to the patient's anion gap metabolic acidosis, but the source of the interference in the salicylate assay.