Pseudo-anion gap acidosis

A 74-year-old Caucasian male was referred to our clinic for low serum bicarbonate levels and a high anion gap (HAG). Past medical history included hypertension, hypothyroidism, iron deficiency anaemia secondary to gastrointestinal bleeding and hyperlipidaemia. He was on Bicitra 30 mL orally three times daily, levothyroxine 75 mcg daily, atorvastatin 10 mg daily, ferrous sulphate 324 mg daily and chlorthalidone 12.5 mg daily. He denied smoking, alcoholism or illegal drug abuse. Physical exam was unremarkable. His laboratory values obtained over the 18 months after our evaluation are outlined in Table 1. Arterial blood gas showed a pH of 7.37,PCO2 of 35 mmHg andPO2 of 87 mmHg when the serumHCO3 level was 12 mEq/L. Serum lactate levels on several occasions were <2.2 mmol/L. Serum ketones were negative and the betahydroxybutyrate level was mildly elevated at 3.60 mg/dL (normal 0–3.0 mg/dL). Salicylate level was <1 mg/dL. D-Lactate levels were immeasurable. Thiamine levels were normal. A serum and urine protein electrophoresis did not reveal any paraproteinaemia. Urinalysis revealed a specific gravity of 1.020, pH 6.0 with negative protein and blood by dipstick. Urine toxicology screens for ethanol, methanol, ethylene glycol and isopropanol were negative. His urine organic acid profile was normal and no 5-oxoproline was detected. He was started on various forms of bicarbonate supplementation and atorvastatin was stopped, but neither bicarbonate levels nor anion gap changed significantly (Table 1). In the absence of any apparent explanation for a decrease in serum bicarbonate with an HAG despite extensive work-up, we questioned the validity of the laboratory testing.