Acid-Base Disturbance in Patients with Fever, Abdominal Pain, and Watery Diarrhea
Normal anion gap metabolic acidosis (Option C) is the expected finding in a patient presenting with fever, abdominal pain, and watery diarrhea.
Pathophysiology of Diarrhea-Associated Acidosis
Watery diarrhea leads to significant bicarbonate loss from the gastrointestinal tract, resulting in metabolic acidosis. Unlike other causes of metabolic acidosis such as lactic acidosis or ketoacidosis, diarrhea typically causes a normal anion gap acidosis due to:
- Direct loss of bicarbonate from intestinal secretions
- Compensatory increase in chloride reabsorption (hyperchloremia)
- Maintenance of the electrical neutrality principle
The clinical presentation described (fever, abdominal pain, and watery diarrhea) strongly suggests an infectious diarrheal illness, which classically presents with normal anion gap metabolic acidosis 1.
Clinical Correlation
The Infectious Diseases Society of America (IDSA) guidelines highlight that patients with infectious diarrhea commonly present with:
- Fever
- Abdominal pain
- Watery stools
- Signs of volume depletion 2
These symptoms correlate with various pathogens including Salmonella, Shigella, Campylobacter, and viral causes, all of which can lead to significant fluid and electrolyte losses 2.
Laboratory Findings in Diarrheal Illness
In patients with significant diarrhea, laboratory evaluation typically reveals:
- Decreased serum bicarbonate
- Increased serum chloride (hyperchloremia)
- Normal anion gap (typically 8-12 mEq/L)
- Signs of dehydration (elevated BUN/creatinine ratio, hemoconcentration)
The anion gap calculation (Na⁺ - [Cl⁻ + HCO₃⁻]) remains normal because the decrease in bicarbonate is balanced by a proportional increase in chloride 1.
Differential Considerations
The other options are less likely in this clinical scenario:
Increased anion gap (Option A): Typically seen in lactic acidosis (shock, sepsis), ketoacidosis (diabetes, starvation), or toxin ingestion. While sepsis can develop in severe diarrheal illness, the primary acid-base disturbance in uncomplicated diarrhea remains a normal anion gap acidosis 3.
Decreased anion gap (Option B): Rare finding associated with multiple myeloma, bromide intoxication, or severe hypoalbuminemia. Not typically associated with diarrheal illness 4.
Negative anion gap (Option D): Extremely rare laboratory finding, usually representing a laboratory error or severe hypermagnesemia, hypercalcemia, or lithium toxicity. Not associated with diarrheal illness 4.
Management Implications
Understanding that normal anion gap metabolic acidosis is the expected finding helps guide management:
- Fluid resuscitation with balanced electrolyte solutions
- Oral rehydration solution (ORS) for mild to moderate dehydration 2
- Correction of electrolyte abnormalities
- Treatment of underlying infectious cause if identified
The IDSA recommends reduced ORS as first-line therapy for mild to moderate dehydration in patients with acute diarrhea from any cause 2.
Conclusion
In a patient presenting with fever, abdominal pain, and watery diarrhea, the expected acid-base disturbance is a normal anion gap metabolic acidosis (Option C) due to bicarbonate loss from the gastrointestinal tract and compensatory hyperchloremia.