How to determine if a patient is smoking from an Arterial Blood Gas (ABG)?

Identifying Smoking Status from Arterial Blood Gas Analysis

Elevated carboxyhemoglobin (COHb) levels of ≥3-4% in non-smokers and ≥10% in smokers can be used to determine if a patient is smoking based on arterial blood gas analysis. 1

Key ABG Findings in Smokers

Carboxyhemoglobin (COHb) Levels

• Non-smokers: COHb levels typically ≤3-4%
• Smokers: COHb levels generally 3-5%, but can be higher 1
• Rule of thumb: For each pack of cigarettes smoked per day, COHb rises approximately 2.5% 1
• Heavy smokers: May have COHb levels >10%, especially those with underlying lung pathology 1

Timing Considerations

• COHb levels in moderate/heavy smokers remain elevated for more than 6 hours after smoking
• Light smokers' COHb levels return to baseline after approximately one hour 2
• Pulse rate typically increases significantly 20 minutes after smoking 2

Measurement Methods

Laboratory Spectrophotometry

• Gold standard method using a CO-oximeter (spectrophotometer)
• Measures concentrations of various hemoglobin species by transilluminating blood with multiple wavelengths of light 1
• Either arterial or venous blood may be used, as COHb levels are similar in both 1

Pulse CO-Oximetry

• Non-invasive alternative to blood sampling
• Can detect smoking by moderate/heavy smokers if they smoked within 6 hours 2
• Can detect smoking by light smokers if they smoked within 20 minutes 2
• Less accurate than laboratory spectrophotometry but provides immediate results 1

Potential Pitfalls and Caveats

Pulse Oximetry Limitations

• Standard pulse oximeters using two wavelengths (660 and 990 nm) cannot differentiate between carboxyhemoglobin and oxyhemoglobin 1
• This can lead to falsely elevated SpO2 readings in smokers 3
• The difference between SpO2 and actual O2Hb is significantly greater in smokers (5.6 ± 3.1%) than non-smokers (2.1 ± 2.1%) 3

Blood Gas Machine Considerations

• Newer blood gas machines incorporate CO-oximeters and directly measure COHb
• Older machines may calculate oxygen saturation based only on PaO2 and pH, potentially missing COHb elevation 1
• Always check if your blood gas analyzer measures COHb directly

Other Causes of Elevated COHb

• Carbon monoxide poisoning: Much higher levels than smoking alone
• Environmental exposure: Urban pollution, occupational exposure
• Second-hand smoke exposure: May cause slight elevations in non-smokers

Clinical Algorithm for Determining Smoking Status from ABG

1. Check COHb level on ABG report

   • If ≤3%: Likely non-smoker
   • If 3-10%: Likely smoker (or recent CO exposure)
   • If >10%: Heavy smoker or possible CO poisoning 1

2. If COHb level is borderline (3-4%):

   • Consider timing of last possible cigarette
   • Check for clinical signs of smoking (nicotine staining, smell)
   • Ask about environmental CO exposure

3. If standard pulse oximetry shows normal SpO2 but patient has unexplained symptoms:

   • Consider hidden COHb elevation masking true oxygenation status
   • Request specific COHb measurement 1, 3

By following this approach, clinicians can accurately determine smoking status from arterial blood gas analysis, which is important for proper patient management and treatment planning.