One of the concepts that is regularly taught in the evaluation of acid-base status is determining if there are multiple acid base disorders by evaluating the ratio of the delta anion gap/delta bicarbonate.
I teach this concept as determining what the bicarbonae would be in the absence of or prior to the anion gap.
The concept comes from the idea that for every mEq of bicarbonate that is consumed by the strong acid (other anion) the anion gap should rise by one. So if the bicarb is 16, a delta of 8, we would expect an anion gap of 20, a normal anion gap of 12 plus the delta bicarbonate of 8. This is a ∆AG/∆Bicarb of one.
[Some authors] suggested that mixed disturbances should be considered if the ratio is less than 0.8 or greater than 1.2. Paulson, applying this rule to a group of normal control subjects and patients with simple metabolic acidosis, noted that the formula erroneously categorized 56% [specificity of 44%] of this group as mixed disturbances. Use of the 95% confidence interval of ±8 mEq/L increased the specificity to 97% but with a poor sensitivity of only 27%.