⚖️ Metabolic Acidosis: Understanding AG, HAGMA, and NAGMA
Metabolic acidosis is a common acid-base disturbance encountered in clinical settings and a high-yield topic for exams like the USMLE, NCLEX, and Step 1. It refers to a decrease in serum bicarbonate (HCO₃⁻) and a drop in pH, which results in systemic acidosis. A key tool for differentiating causes of metabolic acidosis is the anion gap (AG).
🧠 What Is the Anion Gap?
The anion gap is calculated using the formula:
AG = Na⁺ - (Cl⁻ + HCO₃⁻)
It reflects unmeasured anions in the serum and helps distinguish between two types of metabolic acidosis:
HAGMA: High Anion Gap Metabolic Acidosis
NAGMA: Normal Anion Gap Metabolic Acidosis
📊 Table: Metabolic Acidosis Breakdown
| Category | Description | Common Causes |
|---|---|---|
| AG (Normal) | ~12 ± 4 mEq/L | Used to assess acidosis types |
| HAGMA | ↑ AG due to accumulation of unmeasured acids | MUDPILES: Methanol, Uremia, DKA, Propylene glycol, Iron/INH, Lactic acidosis, Ethylene glycol, Salicylates |
| NAGMA | Normal AG, ↓ HCO₃⁻ replaced by ↑ Cl⁻ | HARDASS: Hyperalimentation, Addison’s, RTA, Diarrhea, Acetazolamide, Spironolactone, Saline infusion |
🔬 HAGMA vs. NAGMA
HAGMA (High AG): Unmeasured acids like lactate, ketones, and toxins cause the drop in bicarbonate. The body compensates without increasing chloride.
NAGMA (Normal AG): Loss of bicarbonate is matched by a gain in chloride, maintaining electrical neutrality—hence, no rise in AG.
🧠 Clinical Relevance for Exams
You’ll frequently see metabolic acidosis on exams presented with:
Lab values (pH, AG, HCO₃⁻)
Case scenarios (e.g., alcoholic with confusion → HAGMA)
Renal pathology (e.g., Type I RTA → NAGMA)
Quick tip: If Cl⁻ is elevated and AG is normal, think NAGMA. If Cl⁻ is normal and AG is elevated, think HAGMA.
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✅ Summary
Understanding metabolic acidosis and how to apply the anion gap helps you quickly diagnose the underlying cause and distinguish between HAGMA and NAGMA. It’s not just an exam topic—it’s clinical reasoning in action.
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