Routine labs can tell you a great deal about your patient’s chronic diarrhea:
- Potassium: Potassium should, of course, be low in patients with chronic diarrhea. If it is high and the patient has normal kidney function, then your patient may have Addison’s disease, or primary adrenal insufficiency.
- Microcytic anemia: microcytic anemia suggests anemia of iron deficiency and therefore chronic blood loss. But you also need to know that celiac disease causes diarrhea with iron deficiency anemia. In fact, a patient with a history of “irritable bowel syndrome” who is found to have unexplained microcytic anemia doesn’t actually have irritable bowel syndrome. He or she has celiac disease – at least until proven otherwise.
- Macrocytic anemia: This can be either from liver disease or from B12 or folate malabsorption. To distinguish the two, get a peripheral blood smear. Megaloblastic anemia (with hypersegmented neutrophils) points to B12 or folate deficiency rather specifically.
- Liver chemistry studies: deranged liver function studies may suggest malabsorption secondary to cholestasis or poor synthetic function of bile salts. In addition, both thyroid disease and celiac disease can cause abnormal liver chemistry studies for unclear reasons (to me!).
- Normal anion gap metabolic acidosis: this suggest small bowel diarrhea. Why? Small bowel diarrhea implies malabsorption, and malabsorption means that the patient is failing to absorb bicarbonate (among other molecules) in his or her gut. You can confirm that the normal anion gap metabolic acidosis is coming from the gut, and not from the kidney, by checking the urinary anion gap (Na + K – Cl), which should be negative.
- Chloride sensitive (low urine chloride) metabolic alkalosis: A patient who has unexplained large bowel diarrhea may need a colonoscopy to look for a villus adenoma. Unfortunately, this acid-base trick loses its value in patients with concomitant vomiting because vomiting also causes a chloride sensitive metabolic alkalosis. Either way, these patients tend to be dry and the “cure” for their acid-base disturbance is normal saline.
- TSH: Hyperthyroidism causes hyperdefecation, that is, an increase in stool frequency without an increase in stool mass. Hypothyroidism can cause watery stools if liquid stools manage to escape around an area of impaction.
References
- Kurtz, Ira, MD, Acid-Base Case Studies (2004, reviewed here)
- Sabatine, Marc S., MD Pocket Medicine (2011)
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