Lab- Gaps
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anion gap...the discrepancy between the measured cations [Na+] and the
measured anions [Cl-] and [HCO3-] implying the presence of umeasured anions
which are contributing to a metabolic acidosis.

osmolal gap...the discrepancy between measured and calculated serum
osmolality implying the presence of small molecular weight substances
usually, but not invariably, alcohols.

saturation gap...the difference between measured and calculated (based upon
the pO2 and the oxyHb dissociation curve) oxygen saturation implying the
presence of a dyshemoglobin (such as metHb).

and now comes the DELTA GAP:

I made reference to this (though not by name) in a recent post on mixed
acidosis in patients with DKA who are rehydrated with NS solutions.  For
those of you who are unfamiliar with the concept I have found an EM
reference (1).

In brief, with a simple anion gap metabolic acidosis one would expect that
the fall in plasma bicarb (delta HCO3) would equal the rise in anion gap
(delta AG).  If there is a significant discrepancy (> 6) between these
'deltas', a delta gap exists and implies the presence of a mixed acid-base
disturbance.  If the fall in HCO3 is greater than the rise in the AG then
[Cl-] has replaced some of the HCO3 resulting in a superimposed
hyperchloremic metabolic acidosis.  On the other hand if the fall in HCO3 is
less than what one should expect based upon the extent of the AG then a
metabolic alkalosis is superimposed.

H. Louzon MD

Wrenn K
The delta (delta) gap: an approach to mixed acid-base disorders [see comments]
Ann Emerg Med, 19: 11, 1990 Nov, 1310-3

The anion gap (AG) is a helpful, yet underused, clinical tool. Not only does
the presence of a high AG suggest a certain differential, but knowledge of
the relationship between the rise in AG (delta AG) and the fall in
bicarbonate (delta HCO3) is important in understanding mixed acid-based
disorders. Simple arithmetic converts this relationship into a numerical
value, the delta gap (delta gap). The delta gap = delta AG - delta HCO3. If
the delta gap is significantly positive (greater than +6), a metabolic
alkalosis is usually present because the rise in AG is more than the fall in
HCO3. Conversely, if the delta gap is significantly negative (less than -6),
then a hyperchloremic acidosis is usually present because the rise in AG is
less than the fall in HCO3. Familarity with the relationship between the
changes in AG and HCO3 can be useful in unmasking occult metabolic disorders.