Required section · Section 3 of 6

Compensation, gap, and method limits

Acute respiratory acidosis raises HCO3 about 1 mEq/L per 10 mmHg PaCO2 rise, while chronic respiratory acidosis raises it roughly 3 to 4 over days. Acute and chronic respiratory alkalosis likewise differ because renal compensation takes days. These are physiologic ranges, not formulas that restore pH to normal.

For metabolic acidosis, Winter's formula estimates expected PaCO2 = 1.5 × HCO3− + 8 ± 2 mmHg. It gives an expected range, not proof of compensation or diagnosis. A value outside the range prompts specimen, method, trend, and clinical review for a second process or evolving timing.

Use the laboratory anion-gap interval rather than the commonly taught 8 to 12 mmol/L interval alone. Published albumin corrections use 2.5 times either 4.0 minus albumin or 4.5 minus albumin, so local policy decides the convention. Declare interval and albumin convention before interpreting the gap.

Four panels show respiratory and metabolic acidosis and alkalosis, with respiratory acidosis highlighted. — Figure 1Primary and compensating directions for four acid-base patterns.

Knowledge checks

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Knowledge check 1

What is the potassium-excluded anion gap for Na 138, Cl 96, and HCO3− 27 mmol/L using Na − (Cl + HCO3−)?

Choose one option.

15 mmol/L

42 mmol/L

111 mmol/L

Knowledge check 2

Which facts affect anion-gap interpretation?

Choose at least 2 options.

Local analyzer interval

Albumin and local correction convention

Potassium inclusion policy

Age alone

Section status

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