The Bird's Eye-View, Two Components:
Metabolic When normal metabolism is impaired - acid forms, e.g., poor blood supply stops oxidative metabolism and lactic acid forms. This acid is not respiratory so, by definition, it is "metabolic acid." If severe, the patient may be in shock and require treatment, possibly by neutralizing this excess acid with bicarbonate, possibly by allowing time for excretion/metabolism.
That's it! The whole of acid-base balance in six sentences. As you explore this site, keep this bird's eye-view in mind. We will also have to deal with low levels of metabolic and respiratory acid (alkalosis) - but this initial overview helps to keep the subject in focus.
Check if pH is less than 7.4, which means acidosis
Check if PCO2 is less than <40mmgh, if it is below 40 than you likely have a metabolic acidosis (if it is greater than 40mmgh you probably have respiratory acidosis)
Check the anion gap (Sodium - (Chloride+Bicarb) . If that value is between 8-12, then it is a non-anion gap acidosis. If the gap is >12 then there is a anion gap. Some people use 16 instead of 12 as the cut-off point. So if sodium was 140mEq, chloride was 100 and bicarb was 30, then the anion gap would be 10. 10 falls in the rage of 8-12, therefore you have a non-anion gap acidosis.
Diarrhea, renal tubular acidosis and hyperchloemia can cause non-anion gapped acidosis. Those are the big 3 I've seen.
MUDPILES (methanol, uremia, DKA....) cause anion gapped acidosis