Serum Anion Gap

Why Use

Acidosis with an increased anion gap has a distinct differential diagnosis compared to non-gap acidosis.

When to Use

Use to determine if a patient’s metabolic acidosis also has an elevated anion gap.

Formula

Pearls / Pitfalls

A “normal” anion gap value assumes that unmeasured proteins with charges (e.g., immunoglobulins with positive charges, albumin with a negative charge) are within their respective reference ranges. Patients with low albumin or elevated immunoglobulins may have a lower baseline anion gap, which can obscure the recognition of an increased anion gap in the setting of acidosis.

Management

Management of an anion gap acidosis focuses on correcting the underlying cause and varies based on the specific etiology. Anion Gap Metabolic Acidosis : MUDPILERS Methanol Uremia Diabetic ketoacidosis/alcoholic ketoacidosis Paraldehyde Isoniazid Lactic acidosis Ethanol/ethylene glycol Rhabdomyolysis/renal failure Salicylates Non-Anion Gap Acidosis : HARDUPS Hyperalimentation Acetazolamide Renal tubular acidosis Diarrhea Uretero-pelvic shunt Post-hypocapnia Spironolactone Acute Respiratory Acidosis: CNS depression (drugs/CVA) Airway obstruction Pneumonia Pulmonary edema Hemo/pneumothorax Myopathy Chronic Respiratory Acidosis: COPD Restrictive lung disease Any hypoventilation state Metabolic Alkalosis : CLEVER PD Contraction Licorice Endocrine causes (Conn’s, Cushings, or Bartter’s Syndrome) Vomiting Excess alkali Refeeding alkalosis Post-hypercapnia Diuretics Respiratory Alkalosis : CHAMPS (think speed up breathing) Central Nervous System (CNS) disease Hypoxia Anxiety Mechanical ventilators Progesterone Salicylates/sepsis

Advice

In cases of hypoalbuminemia or hyperalbuminemia, the corrected anion gap result may be more accurate. If no albumin is entered, or if albumin is within the normal range, no correction is necessary.

More Information

Interpretation: The delta ratio is the ratio of the amount of additional anion in a body to the amount of additional H + . The anion’s volume of distribution and its excretion affect this ratio. Organic acids with a greater distribution may produce lower anion gaps compared to inorganic acids, which may be confined to the extracellular compartment. Delta ratio Suggests… <0.4 Pure normal anion gap acidosis 0.4-0.8 Mixed high and normal anion gap acidosis 0.8-2.0 Pure anion gap acidosis >2 High anion gap acidosis with pre-existing metabolic alkalosis