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.

If the patient had a pre-existing metabolic alkalosis with a bicarbonate of 30, then the patient would have a bicarbonate of 22 and an anion gap of 20. This would give ∆AG/∆Bicarb of 8/2 or 4.

If the patient had a pre-existing metabolic acidosis (non-anion gap) with a bicarbonate of 16, then the patient would have a bicarbonate of 8 and an anion gap of 20. This would give ∆AG/∆Bicarb of 8/16 or 0.5.

Concurrent metabolic alkalosis leads to ratios over 1 and preexisting metabolic acidosis (non-anion gap) yield a ratio below 1.

I had always been suspicious of this because the assumption of the one for one change in anion gap and bicarbonate. This didn’t seem to be very biologic. Turns out my suspicion was justified as numerous studies (Androgue, Elisaf) have shown that the ratio does not hold up.

In this paper by Paulson

*et al*they found:[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%.

That’s terrible. Why torture the brains of medical students with this type of worthlessness.

Good review here.

Dr. Topf:

Appreciate your remarks about an old controversary in nephrology.I wonder what do you make of the information listed below. I can ssure you, despite all your reservation about Delta gap, We nephrologist use ait all the time

Delta Gap corrected bicarb 0.7/10 rule

Delta Gap = AG -8 ( 8 is a normal AG)

Corrected Bicarb = Bicarb + delta gap ( Bicarb + Delta gap ( AG-Normal gap) coexisting disorders if < or > than between 24 – 26 ).

24-26 roughly = no other d/o

<24-26 = hyperchloremic acidosis or chronic resp alkalosis

>24-26 = metabolic alkalosis or chronic resp acidosis

Add delta gap back to CO2 = corrected bicarb ( Delta AG + CO2 from Chem-7)

if corrected bicarb = 24-26 then no other disturbance

if corrected bicarb < 24-26 then non-anion gap acidosis is superimposed (or chronic resp alkalosis)

if corrected bicarb >24-26 then met alkalosis is superimposed (or chronic resp acidosis)

if <8 = Non Anion Gap metabolic acidosis

2)-Chloride-Bicarbonate corection==7/10 Rule.

7/10 rule : Multiply Na excess by 0.7 and add to chloride ( Sodium above 140 X 0.7= )

1)-if hypochloremic = metabolic alkalosis or chronic resp acidosis ( Less than 100)

if hyperchloremic = metabolic acidosis or chronic resp alkalosis ( Greater than 100)

Imran,

Though "we nephrologists use it all the time" that doesn't mean that it is an evidence based practice or that it makes a difference in the clinical care of your patients. I appreciate your explanation of the concept I don't see anything in your explanation that assures me that it accurately predicts additional acid base disorders more than 44% of the time.