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Different labs use different formulas and procedures to calculate the anion gap, so the reference range (or "normal" range) from one lab isn't directly interchangeable with the range from another. The reference range provided by the particular lab that performed the testing should always be used to interpret the results. [3]
The anion gap is calculated by subtracting the sum of the serum concentrations of major anions, chloride and bicarbonate, from the serum concentration of the major cation, sodium. (The serum potassium concentration may be added to the calculation, but this merely changes the normal reference range for what is considered a normal anion gap)
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.
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. [3]
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).
Although example reference ranges are given, these will vary depending on method of analysis used at the administering laboratory, as well as age, gender, ethnicity, and potentially unrelated health factors. Individual results should always be interpreted using the reference range provided by the laboratory that performed the test. [7]
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).
The standard definition of a reference range for a particular measurement is defined as the interval between which 95% of values of a reference population fall into, in such a way that 2.5% of the time a value will be less than the lower limit of this interval, and 2.5% of the time it will be larger than the upper limit of this interval, whatever the distribution of these values.