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Blood gases
Arterial blood pH
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The negative logarithm of hydrogen ion activity in arterial blood. A pH of 7.35 to 7.45 corresponds to about 35 to 45 nmol/L of H+, and the direction of the deviation defines acidaemia or alkalaemia.
Reference interval
| Conventional (pH units) | SI (pH units) | |
|---|---|---|
| Adults | 7.35–7.45 pH units | 7.35–7.45 pH units |
Specimen: Arterial whole blood (heparinised syringe) · Conventional × 1 = SI
Causes of high values
Alkalaemia: metabolic alkalosis (vomiting, diuretics, mineralocorticoid excess) or respiratory alkalosis (hyperventilation from anxiety, pain, hypoxia, pulmonary embolism, sepsis, pregnancy, high altitude).
Causes of low values
Acidaemia: metabolic acidosis (diabetic ketoacidosis, lactic acidosis, renal failure, diarrhoea, poisoning) or respiratory acidosis (hypoventilation, COPD exacerbation, neuromuscular weakness, central depression). A pH below 7.2 is generally considered severe.
Pre-analytical and method notes
A normal pH does not exclude a mixed disorder, because opposing processes can offset each other. Temperature-corrected values, air bubbles and delayed analysis alter the result; interpret the pH with PaCO2 and bicarbonate. Reference intervals vary by laboratory, method and population; use the interval printed on the report.
Related calculators
References
- Rifai N, Horvath AR, Wittwer CT. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. Elsevier, 6th edition (2018) — Tier 3 (textbook)
- Loscalzo J, et al.. Harrison's Principles of Internal Medicine. McGraw-Hill, 21st edition (2022) — Tier 3 (textbook)
- Hall JE, Hall ME. Guyton and Hall Textbook of Medical Physiology. Elsevier, 14th edition (2020) — Tier 3 (textbook)
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