Biochemistry · Topic
Acid–base compensation rules !
Formulas that tell whether the lungs or kidneys compensated as expected or a second disorder is hiding.
Updated 2026-11-01
Key facts
- Metabolic acidosis: expected pCO₂ = 1.5 × HCO₃⁻ + 8 ± 2 (Winter). Shortcut: the pCO₂ roughly matches the last two digits of the pH.
- Metabolic alkalosis: pCO₂ rises about 0.7 mmHg for each 1 mmol/L rise in HCO₃⁻ (hypoventilation is limited by hypoxaemia).
- Respiratory acidosis: for each 10 mmHg of pCO₂, HCO₃⁻ rises 1 (acute) or 3.5–4 (chronic).
- Respiratory alkalosis: for each 10 mmHg fall in pCO₂, HCO₃⁻ falls 2 (acute) or 4–5 (chronic).
- Compensation never overshoots: a pH on the opposite side means a mixed disorder.
In detail
- Changes are counted from the laboratory's normal values, not from zero; the formulas give an expected value with a margin.
- If the measured pCO₂ in a metabolic acidosis is higher than expected, a respiratory acidosis coexists (the patient is tiring or the respiratory centre is depressed).
- If it is lower than expected, a respiratory alkalosis coexists (sepsis, salicylates, cirrhosis, pregnancy).
- In respiratory disorders, an HCO₃⁻ above the value expected for the chronic form means an added metabolic alkalosis (for example diuretics in COPD).
- Respiratory compensation starts in minutes and is complete in 12–24 hours; renal compensation needs 3–5 days, which is why the acute/chronic split exists only for respiratory disorders.
- Also called
- Expected compensation, Winter's formula
Clinical notes
- A patient in ketoacidosis with a 'normal' pCO₂ is in fact hypoventilating for the degree of acidosis and may be close to respiratory failure.
- In COPD with chronic CO₂ retention a high HCO₃⁻ is expected; the pH shows whether there is an acute-on-chronic decompensation.
- Pregnancy produces a compensated chronic respiratory alkalosis as a normal state.
Related conditions
- Respiratory acidosis
- Respiratory alkalosis
- Metabolic alkalosis
- High anion gap metabolic acidosis
- Chronic obstructive pulmonary disease (COPD)
Related
Test yourself
Think of the answer, then tap the question to see it.
What pCO₂ is expected in a metabolic acidosis with HCO₃⁻ of 12 mmol/L?
By Winter, 1.5 × 12 + 8 = 26 ± 2 mmHg; a measured pCO₂ of 40 would mean an added respiratory acidosis.
How much does HCO₃⁻ rise in chronic respiratory acidosis?
About 3.5–4 mmol/L for each 10 mmHg rise in pCO₂ (only 1 in the acute form).
Why is respiratory compensation of metabolic alkalosis limited?
Because hypoventilating lowers arterial oxygen, and hypoxaemia drives breathing again.
What does an alkaline pH mean in a patient with metabolic acidosis?
Compensation cannot explain it: there is a mixed disorder, such as an added primary respiratory alkalosis.
Educational information, not medical advice. For any symptom, see a health professional.
Sources: Arterial Blood Gas (ABG) Test: MedlinePlus Medical Test (MedlinePlus, US National Library of Medicine, 2026); Blood gases: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Respiratory acidosis: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Respiratory alkalosis: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).
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Representative model, for educational purposes only.