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Metabolic acidosis has three main root causes: increased acid production, loss of bicarbonate, and a reduced ability of the kidneys to excrete excess acids. [5] Metabolic acidosis can lead to acidemia, which is defined as arterial blood pH that is lower than 7.35. [6]
The magnitude of this difference (i.e., "gap") in the serum is calculated to identify metabolic acidosis. If the gap is greater than normal, then high anion gap metabolic acidosis is diagnosed. The term "anion gap" usually implies "serum anion gap", but the urine anion gap is also a clinically useful measure. [4] [5] [6] [7]
High anion gap metabolic acidosis is typically caused by acid produced by the body. More rarely, it may be caused by ingesting methanol or overdosing on aspirin . [ 1 ] [ 2 ] The delta ratio is a formula that can be used to assess elevated anion gap metabolic acidosis and to evaluate whether mixed acid base disorder (metabolic acidosis) is present.
When this happens the numerator is large, the denominator is small, and the result is a delta ratio which is high (>2). This means a combined high anion gap metabolic acidosis and a pre-existing either respiratory acidosis or metabolic alkalosis (causing the high bicarbonate) – i.e. a mixed acid–base metabolic acidosis. [citation needed]
Urine NH 4 + is difficult to measure directly, but its excretion is usually accompanied by the anion chloride. A negative urine anion gap can be used as evidence of increased NH 4 + excretion. In a metabolic acidosis without a serum anion gap: A positive urine anion gap suggests a low urinary NH 4 + (e.g. renal tubular acidosis).
Hyperparathyroidism – can cause hyperchloremia and increase renal bicarbonate loss, which may result in a normal anion gap metabolic acidosis. Patients with hyperparathyroidism may have a lower than normal pH, slightly decreased PaCO2 due to respiratory compensation, a decreased bicarbonate level, and a normal anion gap.
The serum anion gap is useful for determining whether a base deficit is caused by addition of acid or loss of bicarbonate. Base deficit with elevated anion gap indicates addition of acid (e.g., ketoacidosis). Base deficit with normal anion gap indicates loss of bicarbonate (e.g., diarrhea).
Hyperchloremic acidosis is a form of metabolic acidosis associated with a normal anion gap, a decrease in plasma bicarbonate concentration, and an increase in plasma chloride concentration [1] (see anion gap for a fuller explanation).