The delta anion gap/delta HCO3– Ratio in patients with a high anion gap metabolic acidosis
The delta AG/delta HCO3 ratio is the ratio of the increase in AG above baseline to the decrease in HCO3 below the baseline. The delta-delta helps us determine if there are additional metabolic acid-based disorders present. The delta-delta is also called the delta ratio.
Delta-delta (ΔΔ) = ΔAG / ΔHCO3–
Where:
ΔAG (i.e.Delta AG) = the excess anion gap; and ΔHCO3– = the bicarbonate deficit
– ΔAG = (calculated AG – expected AG).
– ΔHCO3– =24 (i.e. normal bicarb level) – HCO3–(i.e. actual bicarb level). i.e. ΔHCO3– = (24 – HCO3–)
Delta-delta (ΔΔ) = (calculated AG – expected AG) / (24 – HCO3–)
Calculated anion gap (AG) = Na+ – (Cl– + HCO3–)
Expected anion gap = [Albumin] x 2.5. E.g. If the albumin is 4, the expected anion gap would be 10. The normal AG =12
Some author’s say the ΔAG = AG(actual anion gap)-12(normal anion gap).T This doesn’t seem to take into account the situations where the AG is affected by hypoalbuminemia and hyperalbuminemia. My inclination at this point is to use the expected anion gap with albumin correction but a literature search needs to be done to confirm that.
Using the Delta Ratio (Delta-Delta)
- A delta-delta of 1 to 2 indicates a pure AG metabolic acidosis. The value is usually between 1 and 1.6
- A delta-delta < 1 indicates an AG metabolic acidosis and a simultaneous non-anion gap metabolic acidosis.
- A delta-delta > 2 indicates an AG metabolic acidosis and a simultaneous metabolic alkalosis.
The delta-delta is an easy way to remember is to compare the delta AG (Calculated AG – Expected AG) to the delta HCO3–(24 – measured HCO3).
If the HCO3–has decreased more than expected from the anion gap, it means some other process is taking out additional bicarb but not affecting the AG. This means there is a concurrent non-anion gap acidosis that is contributing to the lower bicarb. In the delta-delta equation, the denominator (24 – HCO3–) is larger than the numerator making the ratio <1.
If the HCO3– has decreased less than expected, it means some other process is raising the HCO3– level. This means there is a concurrent metabolic alkalosis raising the bicarb. In the delta-delta equation, the denominator (24 – HCO3–) is significantly smaller than the numerator making the ratio > 2
Sample patient
The delta-delta is (30 – expected AG of 10) / (24 – HCO3–of 21) = 20/3 = 6.7. This ratio is > 2, indicating a concurrent metabolic alkalosis. This could be due to vomiting and/or contraction alkalosis.
Albumin Corrected AG, delta gap, and delta-delta
Albumin-corrected anion gap = anion gap + [ 2.5 × (4 – albumin) ]
Albumin-corrected delta gap = albumin-corrected anion gap – 12
Albumin-corrected delta ratio = albumin-corrected delta gap / (24 – bicarbonate)
What does the delta anion gap mean?
The delta anion gap is the excess anion gap. ΔAG = (calculated AG – expected AG). But what does that mean?
For example, if our calculated anion gap is 20, our Delta AG = 20 -12 = 8. Intensivist Dr. Roger Seheult says it means that there are 8 units of negatively charged conjugate bases in the system more than there should. That means the bicarb that was lost took up hydrogen from an acid leaving behind 8 units of the acid’s conjugate base: HCO3– + H+A– ↔ H2CO3 + A– ↔ CO2 +H2O + A– The delta anion gap tells us how many bicarb molecules we should have lost. In this case, we should have lost 8. That is the delta gap = the number of bicarb we should have lost should have caused our bicarb to drop from 24 (normal) to 16 (which is 24 minus the 8 that were lost). Another way to view this is that if we take our current bicarb level of 16 and add to it the delta anion gap of 8, we should get 24 (normal bicarb). Bicarb + delta anion gap ≈ 24. But if we do this addition and get something much less than 24, like say 17, it tells us that there is something else that is happening that is causing us to lose bicarb. But if our addition gives us something like 30, it tells us that there is something else that is happening that is causing us to gain bicarb. So calculating the delta anion gap is so important to calculate because it can tell us if there are multiple processes going on or not. It will be able to tell you if you have another non-anion gap metabolic acidosis going on, or if you also have metabolic alkalosis going on on top of your AG metabolic acidosis.
Dr. Roger Seheult’s Approach
Dr. Seheult recommends:
Step1: Calculate the anion gap(AG).
Step2: Calculate the delta anion gap.
Step3: Add the delta anion gap to the current HCO3–. If your sum is < 22, you have an additional non-anion gap metabolic acidosis occurring. If the sum is >26, you have an additional metabolic alkalosis occurring.
The Delta Gap (is different from delta-delta or delta ratio)
Delta gap. Delta gap is also different from the delta anion gap.
Read
https://en.wikipedia.org/wiki/Delta_Ratio
https://www.uptodate.com/contents/the-delta-anion-gap-delta-hco3-ratio-in-patients-with-a-high-anion-gap-metabolic-acidosis
T. Tsapenko MV. Modified delta gap equation for quick evaluation of mixed metabolic Acid-base disorders. Oman Med J. 2013;28(1):73–74. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3562975/