How to read an anion gap
Other platforms explain it with text and video. In VMS you practice it on the patient.
The anion gap helps find the cause of a metabolic acidosis. It turns a basic chemistry panel into a short list of diagnoses.
Step by step
Confirm metabolic acidosis
Look for a low bicarbonate and, if you have a blood gas, a low pH. [1]
Calculate the gap
Anion gap = sodium minus (chloride plus bicarbonate). Many labs call about 4 to 12 mEq/L normal. [1]
Correct for albumin
Low albumin hides a high gap. Add about 2.5 to the gap for every 1 g/dL that albumin is below 4. [2]
Think of high-gap causes
A high gap means an added acid: ketoacidosis, lactic acidosis, kidney failure or poisons such as methanol, ethylene glycol or aspirin. [2]
Think of normal-gap causes
A normal gap with high chloride suggests bicarbonate loss, such as diarrhea or renal tubular acidosis, or large volumes of saline. [2]
Check the delta
Compare the rise in the gap with the fall in bicarbonate. A big mismatch suggests a second acid-base problem. [2]
Link to the patient
Match the result with glucose, ketones, lactate, kidney function and the history. [1]
Common errors
- Forgetting to correct for low albumin.
- Using a normal range from another lab.
- Stopping at a high gap without looking for a second disorder.
Typical adult ranges; use your lab's ranges [2]
| Test | Typical range | Note |
|---|---|---|
| Anion gap | about 4 to 12 mEq/L | Varies by lab method |
| Sodium | 135 to 145 mEq/L | Part of the formula |
| Chloride | 98 to 106 mEq/L | Part of the formula |
| Bicarbonate | 22 to 28 mEq/L | Low in metabolic acidosis |
| Albumin | 3.5 to 5.0 g/dL | Correct the gap if low |
Practice it in VMS
In VMS you do this on the patient, not on paper: every step goes on the checklist with a score, and in Teach mode Blocky tells you what comes next.
VMS cases that train this skill
Sources
Facts and steps follow AAMC Core Entrustable Professional Activities (Core EPAs); NCCPA PANCE Content Blueprint. VMS is not affiliated with these organizations. Updated: 2026-10-11.