Delta Ratio

The Delta Ratio is a formula that can be used to assess elevated anion gap metabolic acidosis and to evaluate whether a mixed acid base disorder (metabolic acidosis) is present. The anion gap (AG) is calculated first and if an anion gap is present, results in either a high anion gap metabolic acidosis (HAGMA) or a normal anion gap acidosis (NAGMA). A low anion gap is usually an oddity of measurement, rather than a clinical concern [see anion gap for an explanation of low anion gap metabolic acidosis].

The equation for calculating the Delta Ratio is:

and reflects either an increase in the anion gap or a decrease in the bicarbonate concentration ([HCO3¯]).

The ratio gives one of four results:

Results 2 and 4 are the ones which have mixed acid-base disorders.

Results 1. and 4. are oddities, mathematically speaking:

Results 2-4 all involve HAGMAs. A high anion gap metabolic acidosis usually occurs because of an increase in anions. So in the equation:

it is the [A¯] that is the cause. For a list of the common anions responsible, see high anion gap metabolic acidosis. KULT is probably the easiest of the mnemonics to use [ Ketones, Ureamia, Lactate, Toxins ]. Toxins are an uncommon cause of high anion gap metabolic acidosis - a list of the commonest toxins is ACE GIFTs [ ibid]. Metformin as a pure toxicological cause is vanishingly rare.

Mathematically this is reflected in a high anion gap, but because the bicarbonate was high to start, it will appear to fall only a small amount. 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.

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