Serum Anion Gap

OVERVIEW

This page is dedicated to discussing the topic of the serum anion gap. At its core the anion gap aims to characterize the relative amount of “unaccounted” anions (i.e. anions that are not chloride or bicarbonate) that are present in the serum of the patient.

FORMULA

Base Formula: Sodium – (Chloride + Bicarb). Must take albumin levels into account as albumin is an anion in the serum. The more albumin a person has the more of an anion gap is acceptable.

BASE CONCEPT: WHAT CAN ELEVATE THE GAP?

In thinking about how the anion gap can be elevated, it is important to consider the formula outlined above. In the simplest sense, an increase in anions that are NOT chloride or bicarbonate will cause an elevated anion gap in patients. Similarly, a decrease in EITHER chloride or bicarbonate will have a similar effect on the gap.

From a cation point of view, an increase in sodium (that is not met by a proportional increase in either chloride or bicarbonate) will also result in an elevated gap.

The specific suspected cause of these ion changes, as well as their implications, varies depending upon other contextual factors, such as the acid/base status of the patient.

UTILITY IN THE CONTEXT OF METABOLIC ACIDOSIS

In the context of metabolic acidosis, there are a few things that can cause an elevated anion gap. Most all of these involve some sort of acid production such as…

  • Lactic acidosis 
  • Ketoacidosis
  • Kidney disease: retention of hydrogen ions, and anions suck as sulfate, phosphate, and urate
  • Toxic alcohol ingestion:
  • Aspirin poisoning 
  • Acetaminophen 
  • D-lactic acidosis: caused by bacterial fermentation of ingest but unabsorbed carbs. Seen in short bowel syndrome/jejunoileal bypass.
UTILITY OUTSIDE OF METABOLIC ACIDOSIS

Outside of the context of metabolic acidosis, an elevated anion gap can be caused by: 

  • Serum alkalosis: albumin will lose H+ ions making albumin have more of an anion effect.
  • Hyperalbuminemia: major anion
  • Hyperhposphatmeia 
  • Anionic paraprotein: IgA monoclonal immunoglobimemia

 

 

Page Updated: 07/17/2017