The Delta Ratio (∆/∆)

The delta ratio is sometimes used in the assessment of elevated anion gap metabolic acidosis to determine if a mixed acid base disorder is present.

         Delta ratio = ∆ Anion gap/∆ [HCO3-] or ↑anion gap/ [HCO3-] 

  Delta DeltaMeasured anion gap – Normal anion gap
   Delta del    Normal [HCO3-] – Measured [HCO3-]
         
Delta Delta = (AG – 12)
Delta delaaa(24 - [HCO3-])

In order to understand this, let us re-examine the concept of the anion gap.

If one molecule of metabolic acid (HA) is added to the ECF and dissociates, the one H+ released will react with one molecule of HCO3- to produce CO2 and H2O. This is the process of buffering. The net effect will be an increase in unmeasured anions by the one acid anion A- (ie anion gap increases by one) and a decrease in the bicarbonate by one meq.

Now, if all the acid dissociated in the ECF and all the buffering was by bicarbonate, then the increase in the AG should be equal to the decrease in bicarbonate so the ratio between these two changes (which we call the delta ratio) should be equal to one.

As described previously, more than 50% of excess acid is buffered intracellularly and by bone, not by HCO3- . In contrast, most of the excess anions remain in the ECF, because anions cannot easily cross the lipid bilayer of the cell membrane.  As a result, the elevation in the anion gap usually exceeds the fall in the plasma [HCO3- ].  In lactic acidosis, for example, the ∆/∆ ratio averages 1.6:1.


On the other hand, although the same principle applies to ketoacidosis, the ratio is usually close to 1:1 in this disorder because the loss of ketoacids anions (ketones) lowers the anion gap and tends to balance the effect of intracellular buffering. Anion loss in the urine is much less prominent in lactic acidosis because the associated state of marked tissue hypoperfusion usually results in little or no urine output.

A delta-delta value below 1:1 indicates a greater fall in [HCO3-] than one would expect given the increase in the anion gap. This can be explained by a mixed metabolic acidosis, i.e a combined elevated anion gap acidosis and a normal anion gap acidosis, as might occur when lactic acidosis is superimposed on severe diarrhea. In this situation, the additional fall in HCO3- is due to further buffering of an acid that does not contribute to the anion gap. (i.e addition of HCl to the body as a result of diarrhea)

A value above 2:1 indicates a lesser fall in [HCO3-] than one would expect given the change in the anion gap. This can be explained by another process that increases the [HCO3-],i.e. a concurrent metabolic alkalosis. Another situation to consider is a pre-existing high HCO3- level as would be seen in chronic respiratory acidosis.

 

 

Delta ratio

 

Assessment Guidelines

 
   < 0.4

 

 Hyperchloremic normal anion gap acidosis

 

    < 1

 

High AG & normal AG acidosis

 

  1 to 2

 

Pure Anion Gap Acidosis
Lactic acidosis: average value 1.6
DKA more likely to have a ratio closer to 1 due to urine ketone loss

 

 

    > 2

 

High AG acidosis and a concurrent metabolic alkalosis

or a pre-existing compensated respiratory acidosis

See case 3 and 7 for examples.

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