Delta Ratio

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. The anion gap is calculated first and results in either a high anion gap metabolic acidosis or a normal anion gap acidosis. Occasionally there will be a low anion gap but this is usually an on 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: AG – 12 / 24 - HCO3^[1]

and reflects either an increase in the anion gap versus a decrease in the bicarbonate.^[2]

A high anion gap metabolic acidosis usually occurs because of an increase in anions. So in the equation AG = Na⁺ - Cl¯ - HCO3¯ - A¯ 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.

A normal anion gap acidosis [ NAGMA] has more to do with a change in Cl¯ or HCO3¯. So the AG doesn’t change; but to maintain electrical equilibrium if Cl¯ goes up, HCO3⁺ must come down. Hence hyperchloremia always causes a metabolic acidosis as HCO3¯ must fall. And alternatively if the HCO3¯ rises, the Cl¯ must fall. For a list of the common causes of this change in bicarbonate or chloride, see normal anion gap acidosis.

From a Delta Ratio point of view;

If there is a pure HAGMA [ a high anion gap metabolic acidosis ], the problem is due to an unmeasured anion. The Anion Gap rises more than the HCO3¯ falls, since the anions are unable to diffuse out of the bloodstream, while bicarbonate and hydrogen ions diffuse with ease [ as H₂CO₃ ]. So usually the delta ratio will give a result of 1-2.^[3][4]

If the result of the ratio is greater than 2 in a high anion gap metabolic acidosis, it is usually because the bicarbonate level was higher than usual to start with. This is commonly found in people with chronic respiratory acidosis from chronic lung disease such as COPD, who can't breathe off their carbon dioxide due to their poor lung function, or due to a concurrent metabolic alkalosis.^[5] Mathematically this is reflected in a high anion gap, but because the bicarbonate was high to begin with, it will appear to fall only a small amount. So the numerator is large, the denominator is small, and the result is > 2. This means a combined high anion gap metabolic acidosis and either respiratory acidosis or metabolic alkalosis [ causing the high bicarbonate ].

If the anion gap is normal, and all of the change has occurred in the chloride / bicarb proportions, then the numerator will be low and the denominator will be high. so a normal anion gap acidosis produces an delta ratio < 0.4. The classic is a hyperchloremic metabolic acidosis from over enthusiastic saline administration.

If the ratio is somewhere between low [ <0.4 ] and high [ 1-2 ] then it is usually due to a combination of high anion gap metabolic acidosis and normal anion gap acidosis.^[6] So a person with cholera may have a normal anion gap acidosis due to the diarrhea, and become dehydrated and develop a lactic acidosis from shock, and so develop a high anion gap metabolic acidosis.

References