How to read an anion gap
Other platforms explain it with text and video. In Making Synapses 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 Making Synapses
In Making Synapses 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.
Making Synapses cases that train this skill
Sources
Facts and steps follow AAMC Core Entrustable Professional Activities (Core EPAs); NCCPA PANCE Content Blueprint. Making Synapses is not affiliated with these organizations. Updated: 2026-10-11.