Anion Gap, Plasma

Diagnostic Use

The anion gap is a calculated parameter that gives information about the number of unmeasured anions in the plasma.

Anion gap = Na⁺ + K⁺ - Cl^- - HCO₃^-

The principle of electroneutrality states that the number of positively charged ions (cations) and the number of negatively charged ions (anions) in plasma must be equal. Measured cations include sodium and potassium, whereas measured anions include chloride and bicarbonate.

Under normal circumstances, subtracting the total number of measured anions from the total number of measured cations results in a "gap" of approximately 10-20mmol/L (accounted for by anions such as albumin, sulphates and phosphates).

The anion gap increases in metabolic acidosis occuring due to an increase in the amount of organic acid present, whereas if there is a metabolic acidosis due to loss of bicarbonate, the anion gap would be expected to remain in the normal range (the lost bicarbonate is replaced by retained chloride). Because of this principle, we can measure anion gap in patients with a metabolic acidosis, to assist in determining the underlying cause.

Department

Core Biochemistry

Delphic Registration Code

AGAP

Constituent Tests

• Sodium, Plasma
• Potassium, Plasma
• Chloride, Plasma
• Bicarbonate, Plasma

Synonyms

AG Calculation
AGap

Turnaround Time

3 hours

Test Code

7521

Specimen Type

• Serum
• Plasma

Specimen Requirements

Tube/Container Type:

Preferred

• Lithium Heparin - Non-gel (Green Cap)

Acceptable

• Serum (Red Cap)
• SST II (Gold Cap)
• Lithium Heparin - Gel (Green Cap)

Preferred Sample Volume (ml)

4.5

No of Samples Required

1

Paediatric Specimen Requirements

Tube/Container Type:

Preferred

• Lithium Heparin - Non-gel (Green Cap)

Acceptable

• Serum (Red Cap)
• SST II (Gold Cap)
• Lithium Heparin - Gel (Green Cap)

Preferred Sample Volume (ml)

0.6

Additional Specimen Information

Unsuitable samples: containing air or > 4 hours old not capped

Paediatric Specimen - Heparin microtainer 600 µL - kept tightly capped

Aliquot Instructions

Sample Type

• Serum
• Plasma

Minimum Volume (ml)

0.2

Transport to Lab

Ambient (8 - 24 degrees Celsius)

Additional Referral/Aliquot Information

Specimen should be kept capped to maintain anaerobic conditions

Interpretation

Anion gap is most useful when assessing a metabolic acidosis.

The more common causes of a raised anion gap include renal failure, ketoacidosis, lactic acidosis, toxic alcohol ingestion and pyroglutamic acidosis due to paracetamol toxicity. Acronyms such as MUDPILES and GOLDMARK are often used to remember a full list of causes of a high anion gap metabolic acidosis.

Causes of a normal anion gap metabolic acidosis include diarrhoea, adrenal insufficiency and renal tubular acidosis.

A low anion gap may be observed in patients with hypoalbuminaemia. An extremely low/negative anion gap (or extremely high anion gap) may suggest a specimen integrity problem. Consider recollecting a fresh specimen, or discussing such cases with a chemical pathologist who will be able to advise further.

Reference Intervals

10 – 20 mmol/L.

See interpretation for more detail

Test Method

Calculated test:

Anion gap = Na⁺ + K⁺ - Cl^- - HCO₃^-

Non-Funded Test Cost

$0.72 (Exclusive of GST)

Billing Code

7521

Contact Information

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