1. ANIONIC GAP
Defination and types of anionic gap

2. Metabolic Acidosis -- Anion Gap
If you determine that your patient has a primary metabolic acidosis (low pH with low HCO3), the next step is to calculate the anion gap because the anion gap helps you determine the etiology of the metabolic acidosis.

3. The anion gap is the difference between the measured serum cations (positively charged particles) and the measured serum anions (negatively charged particles). The commonly measured cation is sodium and the measured anions include chloride and bicarbonate.
Anion gap = [Na+] - ([Cl-] + [HCO3-])

4. The normal anion gap is 8-12
The normal anion gap is An anion gap of greater than 12 is "increased".  The differential diagnosis for an elevated anion gap metabolic acidosis (simply called "anion gap acidosis") differs from the differential diagnosis for an non-elevated anion gap metabolic acidosis (simply called "non-anion gap acidosis"). 

5. Anion gap metabolic acidosis
"MUDPILES" Methanol intoxication Uremia Diabetic or alcoholic ketoacidosis Paraldehyde Isoniazid or Iron overdose Lactic acid Ethylene glycol intoxication Salicylate intoxication

6. Non-anion gap metabolic acidosis
Renal tubular acidosis
Diarrhea
Gastrointestinal fistula
Post-hyperventilation
Post-anion gap acidosis