ICU Metabolic · Figge Albumin Correction · MUDPILES Differential

Anion Gap Calculator

Calculates observed and albumin-corrected anion gap (Figge equation), delta ratio for mixed disorders, and MUDPILES differential for high anion gap metabolic acidosis (HAGMA).

NM Clinically reviewed by Dr. Nikhil Mahajan, PT, MPT  ·  Jan 15, 2025
Observed AG Na⁺ − (Cl⁻ + HCO₃⁻)
Albumin-Corrected AG (Figge) AG + 2.5 × (4.0 − Albumin)
Delta Ratio (AG − 12) ÷ (24 − HCO₃)
Normal Range 8–12 mEq/L

Basic Metabolic Panel — Required

mEq/L
Normal: 136–145
mEq/L
Normal: 98–106
mEq/L
Normal: 22–29

Albumin — For Figge Correction Recommended in ICU

g/dL
Normal: 3.5–5.0 · Default 4.0 if blank
Observed AG — mEq/L
→
Corrected AG — mEq/L
Correction Added — mEq/L
Status —

Delta Ratio Interpretation — Mixed Acid-Base Disorders

Delta Ratio Interpretation Clinical Meaning
< 0.4 Pure NAGMA Hyperchloremic normal AG acidosis — no HAGMA component
0.4–0.8 Mixed HAGMA + NAGMA Both high AG and normal AG acidosis present simultaneously
1.0–2.0 Pure HAGMA Isolated high anion gap metabolic acidosis
> 2.0 HAGMA + Metabolic Alkalosis High AG acidosis masked by concurrent metabolic alkalosis

Normal Anion Gap Metabolic Acidosis (NAGMA) — Differential

Diarrhea (most common cause of hyperchloremic metabolic acidosis)
Renal Tubular Acidosis (RTA) — Types I, II, IV
Normal saline infusion (dilutional hyperchloremic acidosis)
Ileostomy / ureteral diversion losses
Adrenal insufficiency (hypoaldosteronism)
Carbonic anhydrase inhibitors (acetazolamide)
Toluene inhalation (initially hyperchloremic)

Anion Gap — Clinical Reference for ICU and Emergency Medicine

The anion gap (AG) is one of the most useful calculations in critical care medicine. It measures the difference between the routinely measured cations (Na⁺) and anions (Cl⁻ + HCO₃⁻), representing the concentration of unmeasured anions in the serum — principally albumin, phosphate, sulfate, and organic acids. A normal AG (8–12 mEq/L) indicates these unmeasured anions are within expected range. An elevated AG indicates the accumulation of unmeasured anions — usually pathological fixed acids — pointing toward a high anion gap metabolic acidosis (HAGMA).

The Figge Albumin Correction — Why It's Essential in the ICU

Albumin is the dominant unmeasured anion, contributing approximately 2–2.5 mEq/L per g/dL of albumin. Critically ill patients are frequently hypoalbuminemic due to malnutrition, hepatic dysfunction, fluid resuscitation, or inflammatory protein loss. A patient with albumin 2.0 g/dL has a falsely depressed AG — an observed AG of 10 (apparently "normal") may represent a true corrected AG of 15 (HAGMA), completely changing the clinical workup. The Figge equation (1998): Corrected AG = Observed AG + 2.5 × (4.0 − serum albumin) — should be applied to every ICU patient routinely.

MUDPILES — Systematic HAGMA Differential

MUDPILES covers the major causes of high anion gap metabolic acidosis:

  • M — Methanol: Formic acid accumulation; visual symptoms; osmol gap elevated
  • U — Uremia: Retained organic acids in chronic kidney disease / ESRD
  • D — Diabetic Ketoacidosis: Most common HAGMA in ED/icu; glucose typically > 250 mg/dL
  • P — Propylene Glycol: IV lorazepam, diazepam diluent; osmol gap elevated
  • I — Isoniazid / Iron: Isoniazid seizures; iron overdose causes lactic acidosis
  • L — Lactic Acidosis: Most common ICU cause; Type A (hypoperfusion) or Type B (metformin, liver failure)
  • E — Ethylene Glycol: Antifreeze; calcium oxalate crystals in urine; renal failure
  • S — Salicylates: Aspirin OD; mixed HAGMA + respiratory alkalosis; tinnitus

Delta Ratio — Detecting Mixed Disorders

When a HAGMA is present, the delta ratio determines whether a concurrent metabolic alkalosis or normal anion gap acidosis is also present. In a pure HAGMA, for every 1 mEq/L rise in AG above 12, bicarbonate should fall by approximately 1 mEq/L — giving a delta ratio of 1.0–2.0. A delta ratio below 1.0 suggests an additional normal AG acidosis (HCO₃ is lower than expected for the AG elevation alone — extra HCO₃ is being consumed by a NAGMA process). A delta ratio above 2.0 suggests an additional metabolic alkalosis (HCO₃ is higher than expected — a pre-existing alkalosis was partially protecting against HCO₃ loss).

NM
Dr. Nikhil Mahajan, PT, MPT Clinical Reviewer · January 15, 2025 · View credentials

Frequently Asked Questions

Is lactic acidosis always a high anion gap?
Yes — lactic acidosis causes HAGMA because lactate is an unmeasured organic anion. Type A lactic acidosis (tissue hypoperfusion in sepsis, shock, cardiac arrest) is the most common ICU cause of HAGMA. Type B lactic acidosis occurs without tissue hypoxia — causes include metformin toxicity, liver failure, thiamine deficiency, malignancy, and nucleoside reverse transcriptase inhibitors (NRTIs). Always check serum lactate when HAGMA is identified without an obvious cause.
What is the osmol gap and when should I calculate it?
The osmol gap = Measured Osmolality − Calculated Osmolality, where Calculated Osmolality = 2×Na + Glucose/18 + BUN/2.8. Normal osmol gap is less than 10 mOsm/kg. An elevated osmol gap (> 10) in the context of HAGMA strongly suggests toxic alcohol ingestion — methanol or ethylene glycol — because these alcohols are osmotically active but not included in the calculated osmolality. Calculate the osmol gap whenever methanol or ethylene glycol poisoning is suspected.
Can DKA present with a normal anion gap?
Yes — this is called "euglycemic DKA" or "hyperchloremic DKA." Early DKA treated aggressively with IV saline can develop a hyperchloremic normal anion gap acidosis as the ketones are excreted renally and replaced by chloride from the saline infusion. The ketones are gone but the acidosis persists — now NAGMA rather than HAGMA. This can lead to clinicians stopping insulin too early. Always check urine/serum ketones alongside the anion gap in DKA management.
What is the relationship between anion gap and bicarbonate in pure HAGMA?
In a pure HAGMA, for every 1 mEq/L increase in the anion gap above the normal value of 12, bicarbonate should decrease by approximately 1 mEq/L from the normal value of 24. This 1:1 relationship forms the basis of the delta ratio (expected range 1.0–2.0 for pure HAGMA). If bicarbonate falls more than expected for the AG rise (delta ratio < 1), a concurrent NAGMA is present. If bicarbonate falls less than expected (delta ratio > 2), a concurrent metabolic alkalosis is present.