How this instrument works
The Henderson-Hasselbalch equation relates blood pH to the ratio of bicarbonate (HCO3⁻), a base, to dissolved carbon dioxide — which is derived from PCO2 using CO2's solubility coefficient in blood, 0.0308 mEq/L per mmHg. The same equation applies regardless of whether a sample is arterial or venous; venous blood follows the same acid-base chemistry, it's simply drawn from a different part of the circulation.
This calculator takes the HCO3 and PCO2 values from a venous blood gas panel and solves for pH: 6.1 + log10(HCO3 ÷ (0.0308 × PCO2)). It's a way to check the internal consistency of a blood gas report or see what pH a given HCO3/PCO2 pair implies — it doesn't measure blood directly.
Venous blood gas values normally differ somewhat from arterial values drawn from the same person at the same time — venous PCO2 typically runs a few mmHg higher, and venous pH runs slightly lower, reflecting tissue CO2 pickup on the venous side of circulation. This calculator only performs the pH arithmetic on whichever HCO3/PCO2 pair is entered; it doesn't convert between venous and arterial values or apply any correction for that difference.
- Enter bicarbonate (HCO3) in mEq/L from the venous blood gas panel.
- Enter PCO2 in mmHg from the same panel.
- Read the calculated pH.
Worked example — three venous panels
HCO3 24 mEq/L and PCO2 45 mmHg — typical venous values, with PCO2 a bit higher than a typical arterial sample — computes to a pH of about 7.338 (6.1 + log10(24 ÷ (0.0308 × 45))).
A lower bicarbonate of 20 mEq/L with a higher PCO2 of 50 mmHg, a pattern consistent with a mixed acid-base picture, computes to a lower pH of about 7.214.
A higher bicarbonate of 28 mEq/L with a lower PCO2 of 40 mmHg computes to a higher pH of about 7.457.
Questions
Does the same equation apply to venous and arterial blood?
Yes. The Henderson-Hasselbalch equation describes the same acid-base chemistry regardless of where the sample is drawn from. What differs between venous and arterial samples is the typical HCO3 and PCO2 values themselves, not the equation relating them to pH.
How much lower is venous pH than arterial pH normally?
In practice, a bit lower — a systematic review and meta-analysis of adult studies (Byrne et al., Respirology, 2014) found arterial pH typically about 0.03 units higher than venous pH (95% CI 0.029-0.038), consistent with the commonly cited 0.02-0.04 range. That's a clinical correlation between simultaneously drawn samples, separate from this calculator's own HCO3/PCO2 inputs.
What is the 0.0308 figure in the formula?
It's CO2's solubility coefficient in blood at body temperature (37°C), in mEq/L per mmHg. Multiplying it by PCO2 converts the partial pressure of CO2 into the concentration of dissolved CO2 in blood, which is the denominator the Henderson-Hasselbalch equation uses alongside bicarbonate.
Who is this calculator for?
Clinicians and other trained users reading a blood gas panel — it's a lab-interpretation tool that performs the Henderson-Hasselbalch arithmetic on values already measured by a blood gas analyzer. It isn't intended for patients to interpret their own results without clinical guidance.
Can venous blood gas testing replace arterial blood gas testing?
For pH specifically, venous and arterial values tend to agree reasonably well — the Byrne et al. (2014) systematic review found peripheral venous blood gas analysis compares favorably with arterial for pH estimation, though PCO2 and PO2 agree less closely. Whether venous sampling is adequate depends on the clinical question being asked; that decision sits with the treating clinician.
Does this calculator diagnose acid-base disorders like acidosis or alkalosis?
No. It computes pH from two entered values; it doesn't assess whether an abnormal result reflects a respiratory or metabolic process, whether compensation is occurring, or how the result fits with the anion gap and the rest of the clinical picture. A full acid-base interpretation requires that broader context.
What if I don't have HCO3 and PCO2 values to enter?
The calculator can't compute a result without both. It's a tool for interpreting existing lab values from a blood gas panel, not a way to estimate blood pH from symptoms or other indirect information.
References
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.