0521779407-14 CUNY1086/Karliner 0 521 77940 7 June 4, 2007 21:16
974 Metabolic Acidosis
measured serum osmolality – calculated osmolality=osmolar
gap, normal value <10 mOsm/kg
urine calcium oxalate crystals is consistent with ethylene glycol
poisoning
➣measure plasma creatinine, BUN, glucose, serum ketones, sali-
cylate level, ethylene glycol, serum L-lactate, consider D-lactate
level if clinically indicated
➣urine dipstick for ketones may underestimate degree of ketosis
due to marked increase in the beta-OH butyrate/acetoacetate
ratio
■normal plasma anion gap
➣calculate urine anion gap (UAG): UAG=urine (Na+K−Cl)
negative: extrarenal origin, gastrointestinal loss of HCO3
positive: renal origin, RTA’s
assess proximal tubular function
➣abnormal (glycosuria, phosphaturia, aminoaciduria): type I
proximal RTA
➣if normal, measure plasma K levels
if normal or low K, measure urine pH
plasma K <3.5 mEq/L, urine pH >5.5, type I hypokalemic
distal RTA, screen for nephrocalcinosis on KUB of abdomen
plasma K 3.5–5.0 mEq/L, urine pH <5.5, RTA of renal insuf-
ficiency
if increased plasma K: type IV hyperkalemic distal RTA, mea-
sure urine pH
if pH<5.5, suggests low mineralocorticoid secretion
if pH >5.5, suggests collecting duct abnormalitydifferential diagnosis
■increased plasma anion gap
➣increased osmolar gap: methanol, ethylene glycol poisoning,
alcoholic ketoacidosis
➣normal osmolar gap: diabetic ketoacidosis, L-lactic acidosis, D-
lactic acidosis, alcoholic ketoacidosis (alcohol no longer pre-
sent), uremic acidosis (GFR <15 ml/min), salicylate poisoning
■normal plasma anion gap
➣negative urine anion gap: diarrhea, external loss of pancreatic or
biliary secretions, ureterosigmoidoscopy
➣positive urine anion gap
type II proximal RTA: multiple myeloma, cystinosis, chronic
mercury and lead poisoning, ifosfamide