The laboratory finding reveals that 57 year-old patient is suffering from metabolic acidosis. What kind of metabolic acidosis is he suffering from?
ABG: 7.21/32/98   
100% O2 Sat on Room Air
Electrolytes: Na 145 mEq/L, K 4.5 mEq/L, Cl 105 mEq/L, HCO3 25 mEq/L  
a.            Anion-gap metabolic acidosis
b.            Non-Anion-gap metabolic acidosis
c.             Cation gap metabolic acidosis
d.            Non-Cation-gap metabolic acidosis

Answer(a):  If the patient is suffering from metabolic acidosis (low pH with low HCO3), the next step is to calculate the anion gap because the anion gap helps determining the etiology of the metabolic acidosis.
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-])
The normal anion gap value is between 8 and 12. 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").
So, in the above example:
Anion gap = [Na+] - ([Cl-] + [HCO3-])
Anion gap = 145 - (105 + 25)
Anion gap = 15
The calculated anion gap = 15(above the normal gap of 8-12), therefore there is an anion gap acidosis.