osmolal gap, defined as the difference between the measured and calculated serum osmolality Normal osmolal gap is considered less than 10 Calculated serum osmolality is determined by the following formula: Calculated Sosm = (2 × serum[Na]) + (glucose [mg/dL])/18 + (blood urea nitrogen [mg/dL])/2.8 In addition to serum electrolytes and serum albumin, initial laboratory studies should include serum glucose, BUN, creatinine, serum osmolality, and urinalysis with urine pH A higher than expected urine pH in the setting of acidosis may suggest renal tubular acidosis (RTA) Further studies are based upon clinical suspicion TABLE 100.11 CAUSES OF METABOLIC ACIDOSIS WITH RESPECT TO THE ANION GAP Elevated anion gap Lactic acidosis Hypoperfusion Inborn errors of carbohydrate metabolism Mitochondrial disorders Diabetic ketoacidosis Inborn errors of metabolism Organic acidemias Fatty acid oxidation defects Ingestions Methanol, ethanol, ethylene glycol, salicylates Renal failure Normal anion gap Renal tubular acidosis Diarrhea Enteric fistulae Ureterosigmoidostomy Early renal failure