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how to check for validity of abg

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arterial blood gas sample validity air bubbles delay analysis heparin guidelines

To check whether an ABG is internally valid, use the Henderson-Hasselbalch equation:
[ \textbf{pH = 6.1 + log}\left(\frac{HCO_3^-}{0.03 \times PaCO_2}\right) ]

Quick bedside method

  1. Take the reported PaCO₂ and HCO₃⁻.
  2. Calculate the expected pH using the formula.
  3. Compare it with the machine-reported pH.
If the difference is more than 0.03 pH units, suspect an error such as transcription error, mixed samples, or analyzer/sample issue.

Example

ABG: pH 7.40, PaCO₂ 40 mmHg, HCO₃⁻ 24 mmol/L
[ pH = 6.1 + \log\left(\frac{24}{0.03 \times 40}\right) ]
[ = 6.1 + \log(20) = 6.1 + 1.30 = 7.40 ]
So this ABG is valid.

Alternative calculation

Calculate hydrogen ion concentration:
[ [H^+] = \frac{24 \times PaCO_2}{HCO_3^-} ]
Normal ([H^+]) is about 40 nmol/L, corresponding to pH 7.40.

Also confirm the specimen is reliable

An ABG may be mathematically valid but clinically misleading if:
  • It was not arterial or was contaminated with venous blood.
  • There are air bubbles, which can raise PaO₂ and lower PaCO₂.
  • There was a delay in analysis, especially without correct handling.
  • There was excess liquid heparin, which can dilute the sample.
  • The patient’s oxygen therapy or ventilator settings changed just before sampling.
Blood gas samples should be promptly analyzed, well mixed, free of air bubbles, and collected in an appropriately heparinized syringe. Blood-gas preanalytical guidance describes common handling errors.
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