delta delta vs corrected bicarb?

[johndoe3344]

Na 135
Cl 92
Bicarb 12

Delta delta = (31 - 12) / (24 - 12) = 1.58. Since delta delta is between 1 to 2, there is a pure high anion gap metabolic acidosis only. (superimposed metabolic alkalosis only for delta delta > 2)

Corrected bicarb method: 31 - 12 + 12 = 31. Since corrected bicarb is > 30, the patient has superimposed underlying metabolic alkalosis.

Why is the second one right and the first one wrong?

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[thehundredthone]

The threshold parameters for corrected bicarb and delta delta ratio are set to be slightly different. The delta ratio amplifies the degree of error with the formula as the measure bicarb decreases, since it is a ratio.

E.g. For a mixed acidosis + (metabolic) alkalosis, the delta ratio is usually said to be > 2, and the corrected bicarb > 30.

At a measured bicarb value of 18, the delta gap would need to be at least 12 to satisfy the delta ratio rule. For the corrected bicarbonate rule > 30, the delta gap would also need to be 12 or higher.

At the measured bicarb value of 12, the delta gap would have to be at least 24 to satisfy the delta ratio rule. On the other hand, for the corrected bicarb rule at > 30, the delta gap would only have to be 18 or higher.

Finally, at a measured bicarb value of 8, the delta gap would have to be at least 32 by the ratio method but only 22 by the corrected bicarbonate method.

As for why one and not the other, who knows? Our methods for these estimations are always slightly erroneous due to the assumptions we use, for e.g. we assume all buffering takes place in the ECF, and that HCO3 is the only buffering ion.

 

[theacks1]

Well said. To add to it, as a sub-i when my team looked at me when I said a DKA pt with a bicarb of 6 and AGap of 44 had a mixed acidosis/alkylosis I did some research. Apparently, the theoretical 1:1 change in AGap and bicarb is not the whole story. In Lactic acidosis, it can be up to a 2:1 change, in which the ratio would be better. On the other hand, in DKA, it is almost a perfect 1:1, so the corrected bicarb would make more sense. Ultimately, it is probably best to pick one and understand it isnt perfect.

For me, I like the corrected bicarb because it caught a combined DKA and K+ losing RTA that the ratio was missing who spent a week in DKA as the primary team couldn't figure out why the K+ would plummet as soon as they thought about insulin no matter how much they repleted. You practice based on what you've seen.