What are the normal arterial blood gas (ABG) values for a healthy adult?

Normal Arterial Blood Gas (ABG) Values

For healthy adults at sea level, normal ABG values are: pH 7.35-7.45, PaCO2 4.6-6.1 kPa (34-46 mmHg), PaO2 10.6-14.4 kPa (80-108 mmHg) depending on age, and oxygen saturation (SaO2) 94-98%, with values declining progressively with advancing age. 1

Core ABG Parameters

pH (Acidity)

• Normal range: 7.35-7.45 units 1
• Values <7.35 indicate acidosis; values >7.45 indicate alkalosis 1

PaCO2 (Partial Pressure of Carbon Dioxide)

• Normal range: 4.6-6.1 kPa (34-46 mmHg) 1
• This reference range is based on healthy adult men aged 18-38 years and remains consistent with modern clinical practice 1
• Values >6.1 kPa (>45 mmHg) are considered abnormal, though values up to 6.7 kPa may warrant consideration 1
• PaCO2 is tightly regulated and does not significantly change with age 1

PaO2 (Partial Pressure of Oxygen)

Age-dependent normal ranges 1:

• 18-24 years: 13.4 kPa (100.5 mmHg); range 12.0-14.8 kPa (90-111 mmHg)
• 25-34 years: 13.4 kPa (100.5 mmHg); range 12.1-14.7 kPa (91-110 mmHg)
• 35-44 years: 13.2 kPa (99 mmHg); range 11.1-15.2 kPa (83-114 mmHg)
• 45-54 years: 13.0 kPa (97.5 mmHg); range 10.9-15.1 kPa (82-113 mmHg)
• 55-64 years: 12.1 kPa (91 mmHg); range 10.9-13.3 kPa (82-100 mmHg)
• ≥65 years: 11.9 kPa (89 mmHg); range 9.0-14.8 kPa (68-111 mmHg)

Critical point: PaO2 declines approximately 0.3 kPa (2.3 mmHg) per decade of life 1, 2

SaO2/SpO2 (Oxygen Saturation)

Age-dependent normal ranges 1:

• 18-24 years: Mean 96.9% (range 96.1-97.7%)
• 25-34 years: Mean 96.7% (range 95.3-98.1%)
• 35-44 years: Mean 96.7% (range 95.5-97.9%)
• 45-54 years: Mean 96.5% (range 94.4-98.5%)
• 55-64 years: Mean 95.1% (range 94.5-97.3%)
• ≥65 years: Mean 95.5% (range 92.7-98.3%)

For elderly patients (>70 years): Mean SaO2 is 95.3% (±1.4%) for men and 94.8% (±1.7%) for women, with 2SD ranges of 92.5-98.1% and 91.5-98.2% respectively 1

Target saturation range for clinical practice: 94-98% achieves normal or near-normal values for most UK adults and avoids clinically significant hypoxaemia 1

Important Clinical Considerations

Age-Related Changes

• Oxygen saturation and PaO2 decline with age, but this is a normal physiological finding 1
• PaCO2 remains stable across age groups in men; in women, PaCO2 may increase by approximately 0.075 mmHg per year 3
• The alveolar-arterial oxygen gradient (P(A-a)O2) increases by approximately 0.12 mmHg per year with aging 3

Sex Differences

• Women may have slightly lower PaO2 values than men in older age groups 2
• Reference values for PaO2 and SaO2 in elderly persons are sex-specific but age-independent beyond 70 years 2

Position Effects

• Recumbent (lying down) values are typically lower than seated values 1
• Mean SaO2 for recumbent healthy adults >70 years is approximately 95.0%, which is below the normal range for seated young adults 1

Clinical Thresholds for Hypoxaemia

• Various definitions exist: SaO2 <94%, <92%, <90%, or PaO2 <8 kPa (60 mmHg) 1
• No known risk of hypoxic tissue injury exists above 90% saturation 1
• Critical care guidelines set 90% as the minimum threshold below which SaO2 should not fall 1

Altitude Considerations

• These values apply to sea level populations 1, 4
• At 1,400 meters altitude, PaO2 values are lower and require altitude-specific reference equations 4
• At 2,240 meters, mean PaO2 is approximately 71 mmHg with mean SaO2 of 94% 3

Common Pitfalls

Avoid assuming normal oximetry excludes pathology: Normal SpO2 does not rule out abnormal pH, PaCO2, or anaemia—blood gas measurements are still required when these may affect outcomes 1

Do not apply young adult reference ranges to elderly patients: Using inappropriate age-specific ranges may lead to unnecessary interventions or missed pathology 1

Remember position matters: Compare values obtained in similar positions (seated vs. recumbent) 1

Account for body habitus: In women, higher BMI correlates with decreased SaO2 (0.14% per kg/m²) and PaO2 (0.52 mmHg per kg/m²) 3