Case 5

A 28 year old female with history of Sjogren’s syndrome presents to her PCP for a check up visit. In review of systems, she reports a 2 day episode of watery diarrhea 2 days ago. She is otherwise doing well. Physical examination is unremarkable. She is noted to have the following serum chemistry:

Na 138, K 4.2, Cl 108, HCO3- 14

Because of her history, the physician decides to check her urine electrolytes.

Urine chemistry: K 31, Na 100, Cl 105

What is the acid base disorder? What is the likely cause?

Answer

1. History: Note that in this scenario, we are not given an arterial blood gas. Therefore we must deduce the diagnosis primarily from the history and the limited workup available.

The patients presents to an outpatient visit very asymptomatic. Besides the history of diarrhea, there is nothing else in the history to suggest an active acid base disorder. However, her serum electrolytes indicate a low HCO3- concentration which suggests acidosis.We can safely rule out a chronic respiratory alkalosis as basis of the low HCO3- since hyperventilation would be evident on exam. In the absence of other data, we have to assume that the patient has a metabolic acidosis.

We are not given serum electrolytes and therefore we cannot calculate the anion gap, but based on the history, we can assume that the patient does not have lactic acidosis, ketoacidosis, uremia and has not ingested any toxins.

Assuming the patient has normal anion gap acidosis, our differential becomes diarrhea vs RTA. We have to consider RTA in this patient, because of the history of Sjogren's.

2. Urine Anion Gap. We are given urine electrolytes and thus to distinguish between RTA and diarrhea, we can calculate the urine anion gap , otherwise known as the Urinary Net Charge. Remember that the UAG is an indirect measure of ammonium excretion, which should be very high in the presence of acidosis if renal function is not impaired.

UAG = Na + K - Cl
UAG = 100 + 31 - 105 = 26.

A positive UAG suggest RTA because in the setting of diarrhea, ammonium chloride concentration in the urine would be high and the UAG would be negative. A postive value suggests that the kidney is unable to adequately excrete ammonium, leading to a reduction in net acid excretion and thus metabolic acidosis.

Assessment: Metabolic acidosis likely secondary to renal tubular acidosis.

Note that further workup is needed in order to distinguish the different types of RTA. Sjogren's is most commonly seen in type I RTA and is associated with hypokalemia and a urine pH that that does not fall beyond 5.3, even in the setting of increased acid load. Checking the urine pH after administration of NH4Cl would establish the diagnosis.