Step (1):

Decide if the PH is alkalotic (PH > 7.45) or acidotic (PH < 7.35), if normal PH (7.35-7.45) move to step 3.

Step (2):

Decide what explains the alkalotic or the acidic PH, is it explained by serum HCO3 or PCO2?

Step (3):

Calculate the anion gap (Na – [Cl + HCO3]).

Step (4):

If AG is elevated, calculate the ΔΔ to check if there is an associated normal anion gap metabolic acidosis or metabolic alkalosis:

ΔΔ Calculation:

ΔΔ = (Calculated anion gap – Normal anion gap) + Serum HCO3.

•  ΔΔ = 22-26 → No associated metabolic alkalosis or Normal anion gap (NAG) acidosis.
•  ΔΔ < 22→ Associated NAG acidosis.
•  ΔΔ < 26 → Associated metabolic alkalosis.

Example (1):

PH: 7.5, PCO2: 20, HCO3: 15, Na: 145, Cl: 100.

Step (1): The PH is alkalotic.

Step (2): The PH is explained by the low PCO2→ primary respiratory alkalosis.

Step (3): AG = 145 – (15 + 100) = 30 → High anion gap metabolic acidosis.

Step (4):  ΔΔ = [Calculated AG (30) – normal AG (12)] + serum HCO3 (15) = 33 → Metabolic alkalosis ( 33 > 26).

Final acid-base disorder: Primary respiratory alkalosis + High anion gap metabolic acidosis + metabolic alkalosis.

This patient was an alcoholic with pneumonia (Respiratory alkalosis), alcoholic ketosis (HAG acidosis), and vomiting (Metabolic alkalosis).

Example (2):

PH: 7.4, PCO2: 40, HCO3: 24, Na: 145, Cl: 100.

Step (1): The PH is normal (skip to step 3).

Step (2): Skip

Step (3): AG = 145 – (24 + 100) = 21 → High anion gap metabolic acidosis.

Step (4):  ΔΔ = [Calculated AG (21) – normal AG (12)] + serum HCO3 (24) = 33 → Metabolic alkalosis ( 33 > 26).

Final acid-base disorder: High anion gap metabolic acidosis + metabolic alkalosis.

This patient has advanced CKD and presented with vomiting.

Example (3):

PH: 7.5, PCO2: 20, HCO3: 15, Na: 140, Cl: 103.

Step (1): The PH is alkalotic.

Step (2): The PH is explained by the low PCO2→ primary respiratory alkalosis.

Step (3): AG = 140 – (15 + 103) = 22 → High anion gap metabolic acidosis.

Step (4):  ΔΔ = [Calculated AG (22) – normal AG (12)] + serum HCO3 (15) = 25 → no associated metabolic alkalosis or NAG acidosis ( ΔΔ = 22-26).

Miscellaneous:

• Calculate the anion gap regardless of PH or serum HCO3 values.
• High anion gap metabolic acidosis may coexist with metabolic alkalosis.
• Normal anion gap metabolic acidosis can’t coexist with metabolic alkalosis.