Critical Care Medicine-Renal, Electrolyte and Acid Base Disorders>>>>>Acid Base Disorders
Question 1#

A 33-year-old woman presents to the primary care physician for a month-long history of palpations and diarrhea. She has no known medical history. She denies taking any prescribed medication and has not traveled anywhere recently; however, she reports being exposed to “lots of sick people” as part of her job as a nurse. On physical examination, she appears anxious, but the rest of her examination results, including vital signs, are within normal limits. Her BMI is 21 kg/m2 . Laboratory studies are as follows:

• Sodium 139 mEq/L (mmol/L)
• Potassium 3.6 mEq/L (mmol/L)
• Chloride 117 mEq/L (mmol/L)
• Bicarbonate 16 mEq/L (mmol/L)
• Glucose 135 mg/dL
• BUN 12 mg/dL
• Creatinine 0.7 mg/dL
• Albumin 4.2 g/dL

Which of the following is the MOST likely cause for her laboratory abnormalities?

A. Laxative abuse
B. Bulimia nervosa
C. Factious disorder
D. Exogenous insulin use
E. Diuretic use

Correct Answer is A

Comment:

Correct Answer: A

This patient has a normal anion gap metabolic acidosis which can be seen in patients abusing laxatives. Stool contains a significant amount of bicarbonate along with potassium and sodium. With increased amounts of diarrhea, bicarbonate is lost, leading to a metabolic acidosis. Other causes of normal anion gap metabolic acidosis include renal causes, such as RTA, which emphasizes the importance of measurement of the urine anion gap (U_Na + U_K − U_Cl ). The urine anion will aid in estimating the kidneys’ ability to excrete acid. A positive urine anion gap is suggestive of renal causes of a normal anion gap metabolic acidosis, whereas a negative urine anion gap points toward a gastrointestinal source, such as diarrhea.

The first step with this kind of presentation is to determine the type of acid-base and electrolyte disturbance. Once it is determined that it is a normal anion gap metabolic acidosis, the second step is to differentiate between renal causes versus extrarenal causes by calculating the urine anion gap; a positive urine anion gap points toward renal causes whereas a negative urine anion gap is mostly of gastrointestinal etiology. 

Reference:

1. Goldstein MB, Bear R, Richardson RMA, Marsden PA, Marsden ML, Haleperin ML. The urine anion gap: a clinically useful index of ammonium excretion. Am J Med Sci. 1986;292(4):198-202.