The Acid-Base Detective Tool
The anion gap is a calculated number that helps identify WHY someone has metabolic acidosis. It's the difference between measured positive ions (sodium) and measured negative ions (chloride + bicarbonate). When unmeasured acids accumulate (like lactic acid or ketoacids), they push down bicarbonate while the anion gap rises. The mnemonic MUDPILES lists the causes: Methanol, Uremia, DKA, Propylene glycol, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, Salicylates.
What is Anion Gap?
Anion gap = Na - (Cl + HCO3). Normal: 8-12 mEq/L. Must correct for albumin: for each 1 g/dL albumin below 4.0, add 2.5 to the AG. High AG = unmeasured anion (acid) present. Used to classify metabolic acidosis as high-AG (MUDPILES) or normal-AG (HARDUPS: hyperchloremic).
↑ What High Anion Gap Means
Unmeasured acids are accumulating. The classic high-anion-gap acidosis causes: lactic acidosis (sepsis, shock), DKA, renal failure (uremia), and toxic ingestions (methanol, ethylene glycol, salicylates). This is a critical finding requiring urgent workup.
Common symptoms:
Rapid deep breathing (Kussmaul—blowing off CO2) · Confusion, lethargy · Symptoms of underlying cause (sepsis, DKA, poisoning) · Severe: cardiovascular collapse
↓ What Low Anion Gap Means
Low albumin (most common benign cause—albumin is an unmeasured anion), or lab error. Rarely: lithium toxicity, multiple myeloma.
Common symptoms:
Usually none from low AG itself
Why It Matters
When normal:
Classifies metabolic acidosis
Identifies potentially fatal conditions (DKA, lactic acidosis, toxic ingestion)
Calculated from routine BMP electrolytes
Can detect hidden acidosis even with normal bicarbonate (delta-delta)
Risks if abnormal:
High AG: potentially life-threatening acidosis
Missed high AG = missed diagnosis (check anion gap on every BMP!)
Must correct for albumin or AG will be falsely low
What Can Cause Abnormal Levels?
Lactic Acidosis
35% likelyTissue hypoperfusion (sepsis, shock, severe anemia, cardiac arrest). Most common high-AG acidosis in hospitalized patients.
Diabetic Ketoacidosis
25% likelyKetoacids (beta-hydroxybutyrate, acetoacetate) accumulate when insulin is deficient.
Renal Failure
Kidneys can't excrete organic acids. AG increases as GFR falls below ~20.
Toxic Ingestion
Methanol → formic acid. Ethylene glycol → glycolic/oxalic acid. Salicylate → salicylic acid.
Starvation Ketoacidosis
Prolonged fasting, alcoholic ketoacidosis.
Low Albumin (low AG)
Every 1 g/dL decrease in albumin lowers AG by ~2.5. Always correct for albumin.
What You Can Do
If high AG: check lactate, glucose/ketones, renal function, and consider toxic screen
Impact: Identifies the specific unmeasured acid causing the gap \u00B7 Timeline: URGENT
Always correct AG for albumin: AG_corrected = AG + 2.5 × (4 - albumin)
Impact: Prevents missing high-AG acidosis in hypoalbuminemic patients \u00B7 Timeline: Always
If lifestyle changes aren't enough:
Calculate delta-delta: Δ AG / Δ HCO3 to detect mixed acid-base disorders
Impact: Ratio >2: concurrent metabolic alkalosis. Ratio <1: concurrent non-AG acidosis. \u00B7 Timeline: With interpretation
Recommended retest: q2-4h during acute acidosis treatment; calculated with every BMP
Got your blood test report?
Upload your PDF and understand ALL your markers in 2 minutes. Plain language. Traffic light status. No medical jargon.
Analyze My Report — FreeFirst report is free. No credit card needed.