The steps of acid-base diagnosis are as follows. (1)
Draw arterial blood gases (ABG) to measure the PaO2, PaCO2, and pH. The ABG will also contain a bicarbonate concentration [HCO3-] which is calculated not measured.
At the sametime draw a venous electrolyte profile which consists of sodium [Na+], potassium [K+], chloride [Cl-], and bicaronate [HCO3-]. All of these values are measured.
Compare the calculated [HCO3] from the ABG and the measured [HCO3] from the electrolyte profile. The two values should be within two mEq/L of each other. If they are not some sort of error has been made.
Calculate the anion gap (AG). The anion gap is from the electrolyte profile and is the serum sodium concentration minus the sum of the chloride concetration plus the bicarbonate concentration
AG = Na+ – (Cl- + HCO3-) The normal anion gap is 9 plus or minus 3 mEq/L.
Consider the four common causes of high-AG acidosis: ketoacidosis, lactic acidosis, renal failure, and toxins* (eg., saliculate, paraldehyde, ethylene glycol, methanol, toluene, polyglutamic acid.
*Suspect toxins as a cause of high anion gap acidosis when laboratory measured plasma osmolarity exceeds the calculated plasma osmolarity (Posm = 2Na+ + Glu/18 + BUN/2.8)**.
**The concentration units for sodium (Na+) are mEq/L and for glucose (Glu) and for blood urea nitrogen (BUN) are mg/dL.
Consider two causes of a normal anion gap acidosis: bicarbonate loss from the GI tract (diarrhea), and renal tubular acidosis.
Estimate the body’s compensatory response to the acid-base disorder. See upcoming article “Compensatory Responses for Simple Acid-Base Abnormalities.
Compare the anion gap with the bicarbonate concentration [HCO3]. A patient may have more than one type of acid base disorder. By comparing the anion gap and the bicarbonate concentration. For example the presence of an anion gap with a normal ( or even increased) bicarbonate concentration can signal the presence of a metabolic acidosis along with either a chronic respiratory acidosis or metabolic alkalosis.
Compare the change in the chloride concentration [Cl-] with the change in sodium concentration [Na+].
(1) Textbook of Critical Care 6th ed, 2011. JL Vincent et al. pp. 824-827.