Approach to the Patient
Diagnostics
• CBC, Chem 7, LFTs; consider urine electrolytes, ABG, & serum osmoles
Note: HCO3 from ABG is calculated & should be w/i 2 mmol/L of Chem 7 total CO2
Step-wise Approach
• Step 1: Is there an acidemia or alkalemia?
Acidemia: pH < 7.36; Alkalemia: pH > 7.44
• Steps 2 & 3: Is the primary disturbance metabolic or respiratory? Is there compensation?
• Step 4a: Is there an anion gap?
Anion gap acidosis: (Na – (Cl + bicarb)) > 14 (see chart)
Note: Needs to be corrected for albumin; a fall in serum albumin 1 g/dL from the nl value (4.2 g/dL) decreases the anion gap by 2.5 meq/L. Corrected AG = AG + (2.5 × [4.2 – albumin]).
• Step 4b: If an anion gap is present, is there an osmolar gap?
Osmolar gap: Measured serum Osm – Calculated Osm >10 mOsm/L, where
Calculated Osm = (2 × [Naμ+μ]) + glucose/18 + BUN/2.8 + Ethanol/4.6
• Step 4c: If no anion gap is present, what is UAG?
Urinary anion gap: Na + K – Cl
Note: The UAG can help differentiate GI & renal causes of non-AG (or hyperchloremic) metabolic acidosis, as base can be lost from the gut or kidney (negative UAG: GI loss [ie, diarrhea, small bowel fistula, ileostomy]; positive UAG: Renal loss, particularly RTA types I & IV)
• Step 5: What is the delta ratio, also known as the “delta/delta”?
(AG – nl AG)/(nl HCO3 – HCO3), or simply (AG – 12)/(24 – HCO3)
• If delta/delta > +6, suggests concomitant metabolic alkalosis, or prior compensated respiratory acidosis
• If delta/delta = 0, suggests uncomplicated AG metabolic acidosis
• If delta/delta > –6, suggests concomitant hyperchloremic non-AG metabolic acidosis
Treatment and Disposition
• Both will largely depend on severity & underlying etiology of the disorder
• Limited role for bicarbonate in the absence of hemodynamic collapse