Why does an asymptomatic patient have a PaO₂ of 33 mm Hg yet a pulse oximetry saturation (SpO₂) of 98 % on room air, and how should this discrepancy be managed?

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Last updated: October 1, 2026 • View editorial policy

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A PaO₂ of 33 mm Hg with SpO₂ 98% indicates a critical measurement error that requires immediate investigation—this discrepancy is physiologically impossible and suggests either a pre-analytical blood gas error, venous sampling, or equipment malfunction.

Understanding the Discrepancy

This combination cannot exist in a living patient. A PaO₂ of 33 mm Hg would correspond to an SpO₂ of approximately 60-65% based on the oxyhemoglobin dissociation curve 1. The fact that pulse oximetry shows 98% saturation creates three possible explanations:

Most Likely Causes (in order of probability):

  1. Venous blood sample analyzed instead of arterial blood

    • Venous PO₂ typically ranges 35-40 mm Hg
    • This would explain the low PO₂ while arterial saturation remains normal
    • Check the blood gas sample source and collection technique

  2. Pre-analytical error in blood gas processing

    • Air bubbles in the sample
    • Prolonged delay before analysis
    • Sample contamination
    • Equipment malfunction in the blood gas analyzer

  3. Pulse oximetry interference (less likely given normal reading)

    • Standard two-wavelength pulse oximeters cannot differentiate carboxyhemoglobin from oxyhemoglobin 2
    • However, this would cause falsely elevated SpO₂, not explain the low PaO₂
    • Certain hemoglobinopathies (e.g., hemoglobin Köln) can create SpO₂-SaO₂ discrepancies of approximately 10% 3

Immediate Management Steps

Do not treat based on the PaO₂ value alone. Follow this algorithm:

  1. Assess the patient clinically 1

    • Measure respiratory rate and heart rate (tachypnea and tachycardia are more sensitive than cyanosis for detecting hypoxemia)
    • Check mental status (confusion/agitation may indicate hypoxemia)
    • If patient appears well with normal vital signs, suspect measurement error

  2. Repeat arterial blood gas immediately

    • Ensure proper arterial sampling technique
    • Analyze sample within 5 minutes
    • Compare PaO₂ with simultaneous SpO₂ reading

  3. If repeat ABG confirms low PaO₂ with normal SpO₂:

    • Obtain co-oximetry to measure actual SaO₂ (not calculated) and check for dyshemoglobinemias
    • Consider genetic testing for unstable hemoglobinopathies 3
    • Rule out carbon monoxide poisoning (COHb can falsely elevate SpO₂ readings) 2

Clinical Decision Making

Since the patient is asymptomatic with SpO₂ 98% on room air, trust the clinical picture and pulse oximetry over the aberrant PaO₂ value 1. The BTS guidelines emphasize that oxygen therapy decisions should be based on SpO₂ targets:

  • For patients without hypercapnic risk: Target SpO₂ 94-98% 1
  • This patient meets target range—no oxygen therapy indicated

Critical Pitfall to Avoid

Do not initiate oxygen therapy based solely on a discrepant PaO₂ value when SpO₂ is normal and the patient is asymptomatic. Recent evidence shows that 23.6% of ICU patients experience SpO₂-SaO₂ discrepancies, but occult hypoxemia (truly low PaO₂ with falsely normal SpO₂) occurs in only 4.7% of cases 4. The magnitude of discrepancy in your case (65% difference in expected saturation) far exceeds what occurs even with occult hypoxemia.

When to Suspect True Occult Hypoxemia

Occult hypoxemia is more likely in patients with 4, 5:

  • Black race (aOR 1.35 for discrepancy)
  • Higher BMI
  • Lower pH (acidosis shifts the curve rightward, increasing dissolved oxygen at same saturation) 6
  • Elevated creatinine
  • Higher comorbidity burden

However, even in these populations, a 65-point discrepancy between measured and expected values indicates measurement error, not physiologic variation.

Bottom Line

Repeat the arterial blood gas with meticulous attention to sampling technique. If the patient remains asymptomatic with SpO₂ 98%, the original PaO₂ of 33 mm Hg represents a technical error rather than true hypoxemia requiring intervention. Clinical assessment and pulse oximetry should guide management until a reliable arterial sample confirms oxygenation status 1.

References

2
Guideline

practice recommendations in the diagnosis, management, and prevention of carbon monoxide poisoning.

American Journal of Respiratory and Critical Care Medicine, 2012

5
Research

Pulse Oximetry: 2023 Year in Review.

Respiratory care, 2024

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This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.