Serum anion gap

Anion Gap

Anion gap (plasma or serum) is an entity which can be calculated from the electrolytes measured in laboratory.

Definition:

“Anion gap is defined as the sum of serum chloride and bicarbonate concentrations subtracted from the serum sodium concentration.”

Normal value:  can vary widely, reflecting both differences in the methods that are used to measure its constituents and substantial interindividual variability. The average serum anion gap in healthy individuals measured varies from 11 ± 2.5   to 15 ±2.5 m eq/l .

Indications: This entity is used mainly for the detection and analysis of acid-base disorders, assessment of quality control in the chemical laboratory, and detection of such disorders as multiple myeloma, bromide intoxication, and lithium intoxication.

Gamblegram:

serum cations=serum anions

The sum of both entities must always be equal. This equivalency is the foundation of the derivation of the serum anion gap.

A graphic display of the ionic environment of the serum (Gamble J, 1950)

Total cations in serum = total anions in serum =155 meq/l

As a routine procedure, only sodium, potassium, chloride, bicarbonate ions are measured. Therefore other remaining cations and anions are called unmeasured cations (UC) and unmeasured anions (UA) respectively.

UC include magnesium (3 meq/l) and calcium (5 meq/l) ions mainly.

UA include proteins (16 meq/l); phosphate (2 meq/l), sulphate (1 meq/l), other miscellaneous anions( 6 meq/l).

Therefore,

Na⁺ + K⁺ + UC = Cl^- + HCO₃^-+ UA……….(i)

By rearranging:

Na⁺ + K⁺  - (Cl^- + HCO₃^-) = UA-UC..........(ii)

Under normal conditions, in a healthy individual, UA> UC (UA=25 meq/l; UC=8 meq/l); therefore, there is anion gap.

The concentration of potassium in the blood usually is relatively small compared with that of sodium, chloride, and bicarbonate;

therefore, many a times when calculating the anion gap potassium ion is omitted as

UA-UC = Na⁺ - (Cl^- + HCO₃^-) …………(iii)

Measured cations (MC)= 147 meq/l

Measured anions (MA) = 130 meq/l

MC-MA = UA-UC

147-130= 25-8

17=17

taken from an indepth review by Kraut and Madias (Clin J Am Soc Nephrol 2: 162–174, 2007).

Deviations in the normal values of serum anion gap would be discussed in some other post.