Critical Care Medicine-Renal, Electrolyte and Acid Base Disorders>>>>>Acute Renal Failure
Question 8#

A 33-year-old male underwent a necrosectomy for necrotizing pancreatitis 4 days ago. For the past 2 days, he has been complaining of watery diarrhea. He has not noticed any blood in his stool. Laboratory parameters are as follows:

• Na 142 mmol/L
• Cl 118 mmol/L
• K 3.1 mmol/L
• CO₂ 15 mmol
• BUN 25 mg/dL
• Scr 1.03 mg/dL
• glucose 130 mg/dL
• pH 7.25
• PCO₂ 32 mm Hg
• PO₂ 110 mm Hg
• HCO₃ 15 mmol/L
• base excess—4

Which of the following findings in this patient would be MOST consistent with metabolic acidosis attributed to diarrhea?

A. Hypokalemia and positive value of urine anion gap (UAG)
B. Hyperchloremia and negative value of UAG
C. Hyperchloremia and positive value of UAG
D. Hyperkalemia and negative value of UAG

Correct Answer is B

Comment:

Correct Answer: B

This patient has hyperchloremic, nonanion gap metabolic acidosis due to excessive GI bicarbonate loss consequent to diarrhea. The response of the kidneys in this case is to reabsorb the chloride instead of bicarbonate, yielding no net change in the serum anion gap (AG).

Measurement of serum anion gap helps deduce the etiology for metabolic acidosis. Serum AG represents the difference in the measured cations (mainly sodium) and anions (chloride and bicarbonate). Serum AG is usually around 12 ± 4 mEq/L but can vary depending on the laboratory. Hyperchloremic metabolic acidosis could result from impaired renal acid excretion, bicarbonate loss, or administration of chloride-rich solutions during resuscitation.

UAG is the difference between measured urine cations and anions (UAG = Na + K − Cl) and helps differentiate renal causes of non–anion gap acidosis from the gastrointestinal causes.

Normally, the sum of the excreted urine sodium and urine potassium is greater than the amount of excreted urine chloride resulting in a positive UAG. With diarrhea and other nonrenal causes of hyperchloremic acidosis, the kidneys compensate by increasing the net acid excretion. The excretion of ammonium occurs in conjunction with chloride. In such situations, urine chloride exceeds the sum of urine sodium and potassium, resulting in a negative UAG. Urine AG is a useful tool to differentiate renal from GI causes in hyperchloremic metabolic acidosis. 

References:

1. Tolwani A, Saha M, Wille K. Metabolic acidosis and alkalosis. In: Vincent JL, Abraham E, Moore FA, Kochanek PM, Fink MP, eds. Chapter 104: Textbook of Critical Care. 7th ed. 726-742.
2. Gennari J, Weise W. Acid base disturbances in gastrointestinal disease. Clin J Am Soc Nephrol. 2008;3(6):1861-1868.