What are the differential diagnoses and recommended treatment for a patient with a non‑anion‑gap metabolic acidosis (pH 7.17, bicarbonate 8 mmol/L, normal lactate, mild hyponatremia, normal potassium) secondary to a high‑output ostomy (~1.4 L/24 h)?

Non-Anion Gap Metabolic Acidosis with High Output Ostomy

This patient has severe non-anion gap metabolic acidosis from massive bicarbonate losses through the high-output ostomy (1400 mL/24h), requiring immediate intravenous normal saline resuscitation, strict oral hypotonic fluid restriction to <500 mL/day, glucose-saline replacement solution with sodium ≥90 mmol/L, and loperamide 2-8 mg before meals. 1, 2

Differential Diagnosis

The primary diagnosis is bicarbonate loss through high-output ostomy, but you must systematically exclude other causes:

Primary Cause

• High-output ostomy bicarbonate losses: Jejunostomy/ileostomy effluent contains approximately 90 mmol/L sodium and significant bicarbonate, leading to non-anion gap acidosis when output exceeds 1200 mL/24h 3, 4

Critical Exclusions Before Treatment

• Intra-abdominal sepsis or abscess 1, 2
• Bowel obstruction 2
• Enteritis or recurrent inflammatory bowel disease in remaining bowel 1, 2
• Sudden discontinuation of anti-motility medications 1
• Renal tubular acidosis (less likely given clinical context) 4

Immediate Treatment Algorithm

Step 1: Acute Resuscitation (First 24-48 Hours)

Intravenous fluid management:

• Administer intravenous normal saline 2-4 L/day initially 3, 2
• Keep patient nil by mouth to demonstrate output is driven by oral intake 3, 1, 2
• Avoid excessive IV fluids which cause edema due to high circulating aldosterone levels 3, 2
• Gradually withdraw IV saline over 2-3 days while reintroducing restricted oral intake 3

Step 2: Oral Fluid Management (Ongoing)

Critical fluid restrictions:

• Restrict all hypotonic fluids (water, tea, coffee, fruit juices, alcohol) to <500 mL daily 3, 1, 2
• Avoid hypertonic fluids (fruit juices, Coca-Cola, commercial feeds with sorbitol/glucose) which paradoxically increase stomal losses 3, 5

Replacement solution:

• Provide glucose-saline solution with sodium concentration ≥90 mmol/L to sip throughout the day 3, 1, 2
• Modified WHO cholera solution (without potassium): 60 mmol/L sodium chloride + 30 mmol/L sodium bicarbonate + 110 mmol/L glucose in 1 liter water 3, 2
• Alternative solution: 120 mmol/L sodium chloride + 44 mmol/L glucose in 1 liter water 3, 2
• Patient should sip ≥1 liter daily; may chill or flavor with small amounts of fruit juice for palatability 3

Step 3: Anti-Motility Medications

• Loperamide 2-8 mg before each meal to reduce motility and stoma output 1, 2
• Add codeine phosphate if loperamide alone is insufficient 1, 2
• For secretory output >3 L/24h, add H2 antagonists or proton pump inhibitors 2

Step 4: Electrolyte Correction

Addressing the hyponatremia (Na 133):

• Correct through sodium-containing replacement solutions and IV saline as above 3, 2
• Add salt to diet to limit of palatability 3, 1
• Consider sodium chloride capsules 500 mg (up to 14/24h) if patient tolerates 3, 2

Potassium management (K 4.7 is acceptable):

• Do NOT supplement potassium directly 3
• Hypokalaemia in high-output ostomy is secondary to sodium depletion with hyperaldosteronism 3, 2
• Correct by addressing sodium/water depletion first 3, 1, 2
• Check and correct magnesium before considering any potassium supplementation 3, 1, 2

Monitoring Parameters

Target goals:

• Daily urine volume ≥800 mL 3, 1, 2, 5
• Urine sodium concentration >20 mmol/L 3, 1, 2, 5
• Maintain hydration and stable body weight 3, 1, 5
• Monitor serum electrolytes, particularly sodium, potassium, and magnesium 1, 2

Critical Pitfalls to Avoid

• Never encourage drinking hypotonic fluids to quench thirst – this paradoxically increases stomal sodium losses and worsens acidosis 3, 1, 2, 5
• Do not give excessive IV fluids – causes edema due to elevated aldosterone 3, 2
• Do not supplement potassium before correcting sodium depletion and magnesium 3, 1, 2
• Do not use commercial rehydration solutions designed for diarrhea – these have insufficient sodium concentration (<90 mmol/L) 3

Bicarbonate Therapy Consideration

For this severe acidosis (pH 7.17, HCO3 8), bicarbonate administration is controversial 6, 7:

• The sodium bicarbonate in the WHO replacement solution provides some alkali 3, 2
• Definitive treatment is stopping bicarbonate losses through the interventions above, not simply administering bicarbonate 6, 7
• Focus on correcting the underlying cause rather than aggressive pH normalization 6, 8