Blood Gas of the Week #18
This week on BGOTW we are going to discuss using the delta ratio to evaluate a high anion gap metabolic acidosis (HAGMA). If the case below looks familiar, that’s because it’s the same patient we discussed in BGOTW #15. If you haven’t worked through that case yet, go here and take a few minutes to be sure you understand how we came to the diagnosis of HAGMA – it’s an important starting point for this week’s discussion. Then come back to this page to pick up the next part of the analysis.
Recap of the case:
We concluded that this was a high anion gap metabolic acidosis, and that the math for compensation didn’t quite work out. So, when we have a HAGMA the next question we should ask ourselves is ‘is this a pure HAGMA, or are there other processes being hidden by the high anion gap?’ So how do we go about figuring that out? There are a variety of additional acid-base analysis methods that we will begin using over the next several weeks. Some are arguably more valid than others – in my opinion, the important thing is to know that there are ways to look for mixed disorders, know how to use them, understand what their limitations are, and use them as guides when you are looking for mixed disorders – but don’t expect them to be perfect. This week we will talk about the Delta Gap and the Delta Ratio.
Delta Gap and Delta Ratio
The delta gap and delta ratio are used when we have a HAGMA and we want to try to determine if there are additional metabolic process occurring concurrently – a mixed disorder. The delta gap is just the difference between the change in the anion gap and the change in the bicarbonate. Here we run into our first challenge because humans have slightly different blood gas normals than veterinary species, and no one has determined the optimal ‘normal’ value to use for this calculation. For our purposes today we will just use the midpoint normal values. Let’s review the anion gap quickly:
The normal anion gap is about 10-20 depending on which reference you read and the species we are discussing. This patient has an anion gap of 31.8:
[138 + 5.6] – [110 + 1.8]
143.6 – 111.8
= 31.8 (we will round to 32)
Since the midpoint of the anion gap range is 15, the patient’s delta AG is 17:
32-15 = 17
The midpoint range of the bicarbonate for the cases we have been working with is about 20. So the delta bicarbonate is about 16 (again we will round to the nearest whole number to keep things simple):
20-1.8 = 18.2 (rounding to 18)
So the delta gap is about -1:
Delta AG – Delta Bicarbonate
17-18 = -1
Normal values for the delta gap have not been established for veterinary medicine. For the sake of discussion we will look at the human interpretation guidelines taken from this paper:
- -6 and lower: mixed high and normal anion gap acidosis
- -6 to 6: just a high anion gap metabolic acidosis
- 6 and higher:
What about the delta ratio? The delta ratio is the ration comparison (instead of the difference) between the delta AG and the delta bicarbonate.
Delta ratio = Delta AG / Delta Bicarbonate
17 / 18 = 0.94
Interpretation, again from the human medical side, is classically as follows:
- 0.4 or lower: normal anion gap metabolic acidosis
- 0.4-0.8: mixed normal and high anion gap metabolic acidosis
- 0.8-1.0: Purely a HAGMA
- 1.0-2.0: Still purely HAGMA (particularly if the anions are polyvalent?)
- >2.0: HAGMA with pre-existing metabolic alkalosis
So if these or similar values hold true for veterinary patients, this patient probably has a purely HAGMA. This would make sense given the history of ethylene glycol ingestion in this patient.
Limitations of the Delta Method
For veterinarians, the first and biggest limitation is this technique is not validated in veterinary species (any residents need a project?). Then we can move on to the other concerns.
- The theory behind the delta gap is that from a purely chemistry point of view is the AG and bicarbonate should change together, mole for mole, in opposite directions because a mole of acid should be buffered by a mole of bicarbonate. However, bicarbonate is not the only buffer system in the extracellular compartment.
- Delta method also assumes that all buffering occurs in the ECF, which is also not true. Depending on the substance being buffered some or most of the buffering might take place in the intracellular compartment
- There is a margin of error for every measurement on every machine, so the number on the print-out may not be the exact value that is present in the patient. The margin of error is hopefully small, but that error is going to be magnified quickly because AG and bicarbonate are both calculated values, not measured values. So, the error present in each of the values that is used to calculate the AG and the bicarbonate add up to a somewhat larger margin of error in those values (AG and bicarbonate). And every subsequent calculation magnifies the margin of error.
So, is it perfect – no. Should we use it – I do, and I think as long as we are aware of the limitations and consider the patient’s overall clinical picture in light of what we think we have determined from the delta method, it can be useful.
For more discussion of the pros, cons, and limitations of the delta method from the human perspective (where it is semi-validated?) check out these blogs and references:
Deranged Physiology (here)
Life in the Fast Lane (here)
References:
DiNubile MJ. The increment in the anion gap: overextension of a concept?. Lancet 1988 Oct 22; 2(8617):951-3 (PMID 2902392)
Reddy P. Clinical utility of anion gap in deciphering acid-base disorders. Int J Clin Pract. 2009 Oct;63(10):1516-25 (PMID 19769708)
Tsapenko MV. Modified delta gap equation for quick evaluation of mixed metabolic acid-base disorders. Oman Med J, 2013 Jan;28(1):73-4 (PMID 23386953)
Wrenn K. The delta (delta) gap: an approach to mixed acid base disorders. Ann Emerg Med 1990 Nov;19(11):1310-3 (PMID 2240729)
