Anion Gap Calculator

Detective Work in the Blood: The Anion Gap

In the emergency room or ICU, doctors often act like detectives. When a patient presents with confusion, rapid breathing, or poisoning, one of the first clues they look for is Metabolic Acidosis—a condition where the body's pH drops too low. The Anion Gap (AG) is the mathematical key to classifying this acidosis and identifying the culprit, whether it's diabetes, kidney failure, or antifreeze ingestion.

The Concept: Electroneutrality

The human body strives to verify that total positive charges (Cations) equal total negative charges (Anions).
However, standard blood panels only measure the major ions:
- Sodium ($Na^+$): Proposed major cation.
- Chloride ($Cl^-$) & Bicarbonate ($HCO_3^-$): Major anions.
There are many "unmeasured" anions (like proteins, phosphates, and organic acids) that make up the difference. This "gap" between the measured positive and negative ions represents those unmeasured anions.

The Formula

The standard equation used in most labs involves Sodium, Chloride, and Bicarbonate (often reported as Total CO2):
$$Anion Gap = [Na^+] - ([Cl^-] + [HCO_3^-])$$
Example:
- Na = 140 mEq/L
- Cl = 104 mEq/L
- HCO3 = 24 mEq/L
$AG = 140 - (104 + 24) = 140 - 128 = 12$.
This is a normal result.

High Anion Gap Metabolic Acidosis (HAGMA)

If the gap is high (>12-16 mEq/L), it means there are extra unmeasured acids in the blood "eating up" the bicarbonate buffer.
Doctors remember the causes with the mnemonic MUDPILES:
- M: Methanol (wood alcohol poisoning).
- U: Uremia (Kidney failure).
- D: DKA (Diabetic Ketoacidosis).
- P: Paraldehyde (rare).
- I: Infection / Iron / Isoniazid.
- L: Lactic Acidosis (Sepsis, shock).
- E: Ethylene Glycol (Antifreeze).
- S: Salicylates (Aspirin overdose).

Normal Anion Gap Acidosis

If the patient is acidic but the gap is normal, it means Chloride has increased to replace the lost Bicarbonate (Hyperchloremic Acidosis).
Common causes:
- Severe Diarrhea (loss of bicarb).
- Renal Tubular Acidosis (RTA).

Important Note on Albumin

Albumin is a major unmeasured anion. If a patient has low albumin (hypoalbuminemia), their "normal" gap effectively drops. For every 1 g/dL drop in albumin, the expected Anion Gap drops by 2.5.