Anion gap metabolic acidosis vs Non-anion gap metabolic acidosis
How to tell Anion gap metabolic acidosis and Non-anion gap metabolic acidosis apart when a board question makes them look alike.
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Anion gap elevation above normal is the primary branch point between these two metabolic acidoses.
Side by side
Low bicarbonate with an elevated anion gap indicates unmeasured acid accumulation.
Low bicarbonate with normal anion gap indicates bicarbonate loss or impaired renal acid excretion with chloride retention.
Anion gap acidosis occurs when new acids such as lactate, ketones, or toxic alcohol metabolites accumulate.
Non-anion gap acidosis occurs from GI bicarbonate loss or renal tubular acidification defects with hyperchloremia.
Corrected anion gap above 12 mEq/L plus low bicarbonate suggests AG acidosis; check serum lactate, beta-hydroxybutyrate, renal function, and osmolar gap when toxin ingestion is suspected.
Anion gap remains normal, chloride is often elevated, and urine anion gap helps distinguish GI bicarbonate loss (negative urine anion gap) from renal tubular acidosis (positive urine anion gap).
The trap
Failing to correct anion gap for hypoalbuminemia can mask true high-gap acidosis. Corrected gap and targeted acid source tests prevent dangerous underdiagnosis.
The repair move
Next best step: calculate anion gap and correct for albumin, then identify the acid source immediately with lactate, ketones, renal panel, and toxin assessment. Treat the cause first, such as rapid sepsis resuscitation and source control for lactic acidosis; reserve sodium bicarbonate infusion for severe acidemia, often when pH is at or below about 7.1 with hemodynamic instability.
Try a free question
CoreA 46-year-old man has bicarbonate 12 mEq/L, sodium 140 mEq/L, and chloride 102 mEq/L. BP 118/72 mm Hg, HR 96/min, temp 37.4 C. Serum lactate is 5.1 mmol/L. What is the most likely diagnosis?
Show answer and explanation
Answer: A. Anion gap metabolic acidosis. Elevated anion gap with high lactate identifies anion gap metabolic acidosis.
Why the other choices are tempting
- B. Non-gap acidosis would have near-normal anion gap with hyperchloremia pattern.
- C. Metabolic alkalosis would have elevated bicarbonate, not low bicarbonate.
- D. Respiratory alkalosis is defined by low PaCO2, not primary low bicarbonate with high gap.
- E. Chronic respiratory acidosis usually has compensatory high bicarbonate, not low bicarbonate.
Flashcards
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What formula is used to identify anion gap metabolic acidosis?
Anion gap equals sodium minus chloride minus bicarbonate, and elevation indicates unmeasured acids.
Which chloride trend is common in non-anion gap metabolic acidosis?
Hyperchloremia is common in non-anion gap metabolic acidosis.
Quick answers
What is the fastest way to tell Anion gap metabolic acidosis from Non-anion gap metabolic acidosis?
Anion gap elevation above normal is the primary branch point between these two metabolic acidoses.
What is the key clue for Anion gap metabolic acidosis?
Low bicarbonate with an elevated anion gap indicates unmeasured acid accumulation.
What is the key clue for Non-anion gap metabolic acidosis?
Low bicarbonate with normal anion gap indicates bicarbonate loss or impaired renal acid excretion with chloride retention.
What is the common trap with Anion gap metabolic acidosis vs Non-anion gap metabolic acidosis?
Failing to correct anion gap for hypoalbuminemia can mask true high-gap acidosis. Corrected gap and targeted acid source tests prevent dangerous underdiagnosis.