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:
Which of the following is the MOST likely cause for her laboratory abnormalities?A. Laxative abuse
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 (UNa + UK − UCl ). 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.