Anion Gap Other Than Metabolic Acidosis

Conventionally Serum Anion Gap is calculated as: 

                                                                  Serum AG  =  Na - (Cl + HCO3)

•            Na  is Primary Cation
•            CL+HCO3 are Primary Anion

Serum AG  =  All unmeasured anions – all unmeasured cations

What are Unmeasure Anions?

• Any ion in the serum other than Na, Cl, or HCO3

Causes of lower Serum AG

•Hypoalbumin

•Hyperkalemia

•Hypercalcemia

•Hypermagnesemia

•Lithium toxicity

•IgG multiple myeloma

Negative Serum AG:

•      Rarely seen.
•      Laboratory error ( this is not reproducible)
•      Overestimation of serum chloride:  marked hyperlipidemia, salicylate intoxication, bromide intoxication ( in past bromide used as sedative

Case discussion

A 41 year old male presents to the ER with the following lab results (hint: look for multiple simultaneous problems).

Na 140 mmol/L
K 4.0 mmol/L
Cl 110 mmol/L
pH 7.0
pCO2 35 mmHg
pO2 75 mmHg
HCO3 8 mmol/L

Answer:

The primary disturbance in this patient is an acidosis, as indicated by the acidemic pH in the blood (pH 7.0). The normal bicarbonate concentration is 24 mmol/L but this patient’s bicarbonate concentration is 8 mmol/L. This indicates a fall in bicarbonate of 16 mmol/L.

Thus, there is a primary metabolic acidosis.

The anion gap is 22.

Recall, anion gap is calculated by the formula:

AG = Na – HCO3 – Cl

AG = 140 – 8 – 110

AG = 22

The normal anion gap is about 12, so the anion gap is increased.

Thus, there is an anion gap metabolic acidosis.

The anion gap is increased by 10 but the bicarbonate has fallen by 16mmol/L. Therefore, there is also a fall in bicarbonate that is not accounted for by the H+ ions that accompanied the unmeasured anions in this case – this means there is also a non-anion gap metabolic acidosis.

The bicarbonate has decreased by 16.

We would expect that in a metabolic acidosis, there would be a 1 mmHg fall in pCO2 for every 1 mmol/L of bicarbonate.

Therefore, we would expect that the pCO2 would be 24 mmHg. Since it is 35mmHg, it is too high and this represents arespiratory acidosis.

Therefore, this is a case of a

1. Anion gap metabolic acidosis
2. Non-anion gap metabolic acidosis

1. Respiratory acidosis