Henderson–Hasselbalch pH check
Calculate pH from HCO₃⁻ and pCO₂ to check a blood gas for internal consistency
Sources last checked: 2026-10-13
Henderson–Hasselbalch pH check
Calculate pH from HCO₃⁻ and pCO₂ to check a blood gas for internal consistency
Most analysers calculate HCO₃⁻ from measured pH and pCO₂ with this same equation, so a mismatch usually points to a transcription error or to HCO₃⁻ taken from a separate venous sample. Compare with the measured pH; a difference of more than about 0.03 deserves a second look.
Interpretation
| < 7.35 — acidaemia | Calculated pH below the normal range of 7.35–7.45. |
|---|---|
| 7.35–7.45 — normal range | Calculated pH within the normal range. |
| > 7.45 — alkalaemia | Calculated pH above the normal range of 7.35–7.45. |
What is the Henderson–Hasselbalch pH check?
The Henderson–Hasselbalch equation links pH to the ratio of bicarbonate to dissolved CO₂, using a pK of 6.1 and a CO₂ solubility of 0.03 mmol/L per mmHg. Because pH, pCO₂ and HCO₃⁻ are bound together by it, any blood gas whose three values do not fit the equation contains an error somewhere.
When to use it
- Checking a blood gas for internal consistency before interpreting it
- When HCO₃⁻ comes from a venous chemistry panel and pH/pCO₂ from an arterial sample
- Teaching why pH depends on the HCO₃⁻/pCO₂ ratio rather than either value alone
When not to use it
- As a substitute for measured pH in clinical decisions
- Extreme temperature or very unusual plasma composition, where the pK of 6.1 is less exact
How it is calculated
pH = 6.1 + log₁₀ ( HCO₃⁻ / (0.03 × pCO₂) ) HCO₃⁻ in mmol/L, pCO₂ in mmHg; 0.03 = CO₂ solubility (mmol/L per mmHg)
Clinical pearls and limitations
- Most analysers calculate HCO₃⁻ from measured pH and pCO₂ with this same equation, so a mismatch usually points to a transcription error or to HCO₃⁻ taken from a separate venous sample. Compare with the measured pH; a difference of more than about 0.03 deserves a second look.
- Analysers calculate HCO₃⁻ from pH and pCO₂, so their own output always fits — a mismatch usually comes from transcription or mixed samples.
- The simpler Henderson form, H⁺ (nmol/L) = 24 × pCO₂ (mmHg) / HCO₃⁻, is handy for mental checks: H⁺ 40 nmol/L ≈ pH 7.40.
- For pCO₂ in kPa, multiply by 7.50 to get mmHg first.
Frequently asked questions
What is the Henderson–Hasselbalch equation for blood?
pH = 6.1 + log₁₀(HCO₃⁻ / (0.03 × pCO₂)), with HCO₃⁻ in mmol/L and pCO₂ in mmHg.
Why does my calculated pH differ from the measured pH?
Usually because the HCO₃⁻ came from a different sample or time, or a value was copied wrongly. Differences beyond about 0.03 deserve a check.
What is 0.03 in the equation?
It is the solubility coefficient of CO₂ in plasma (mmol/L per mmHg), which converts pCO₂ into dissolved CO₂ concentration.
Evidence and guidelines
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