Delta –Delta or corrected HCO3- where is this other disorder hiding?

Often when we have a severe gap acidosis, we are forced to calculate the “delta/delta” and look for either a non gap acidosis or met alkalosis. How does this work?

Let’s work an example:

Ph 7.1, AG is 22 and serum Hco3 is 10 with a premise of AG metabolic acidosis.

Method one:

            Corrected bicarbonate = measured bicarbonate +( change in AG)

            Corrected bicarbonate = 10 + (22-12) = 20

If corrected bicarbonate is <22, a non gap acidosis is present as well
If corrected bicarbonate is >26, a met alk is also present.
If 22-26, a pure gap acidosis remains.

Method two: 
Think out loud,   “If the AG dropped increased from 12 to 22, there was a change of 10, hence bicarbonate should be roughly down by 10meq.  Normal bicarbonate is 24 and hence bicarbonate should be 14, but it’s 10, hence a non gap acidosis is present as well.”

Most online calculators use this method:

http://www.pharmacologyweekly.com/app/medical-calculators/anion-gap-calculator
http://errolozdalga.com/medicine/pages/AnionGap.DeltaDelta.cr.6.7.10.html

Problem with this method: Assumptions about all buffering occurring in the ECF and being totally by bicarbonate are not correct. Fifty to sixty percent of the buffering for a metabolic acidosis occurs intracellularly.

Method three: using the delta-delta ratio

delta ratio = (Increase in Anion Gap / Decrease in bicarbonate)

delta ratio = ( 10/14)=  <1 giving us the same diagnosis of combined gap and non gap acidosis.

See the below table

Delta Ratio	Assessment Guideline
0.4 - 0.8	Consider combined high AG & normal AG acidosis
1 to 2	Usual for uncomplicated high-AG acidosis Lactic acidosis: average value 1.6 DKA more likely to have a ratio closer to 1
> 2	Suggests a pre-existing elevated HCO3 level so consider: concurrent metabolic alkalosis

 As a general rule, in uncomplicated lactic acidosis, the rise in the AG should always exceed the fall in bicarbonate level.

The situation with a pure DKA  is a special case as the urinary loss of ketones decreases the anion gap and this returns the delta ratio downwards towards one. A further complication is that these patients are often fluid resuscitated with 'normal saline' solution which results in a increase in plasma chloride and a decrease in anion gap and development of a 'hyperchloraemic normal anion gap acidosis' superimposed on the ketoacidosis. The result is a further drop in the delta ratio.

Ref:

http://www.ncbi.nlm.nih.gov/pubmed/2240729?dopt=Abstract