P50 Haemoglobin — FRCA Primary MCQ
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Correct answer: C — 3.5 kPa
The correct answer is C. P50 is the oxygen partial pressure at which haemoglobin is 50% saturated and therefore measures haemoglobin–oxygen affinity. Under standard conditions—pH 7.40, PCO2 5.3 kPa, 37°C and normal 2,3-bisphosphoglycerate—the P50 of normal adult haemoglobin is approximately 3.5 kPa (about 26.6 mmHg). A lower P50 indicates increased oxygen affinity; thus 2.7 kPa is closer to the left-shifted value associated with fetal haemoglobin, while 1.7 kPa is markedly too low. A value of 5.3 kPa approximates normal mixed venous PO2 rather than P50. At 8.0 kPa, normal HbA is substantially more than 50% saturated.
Reference: Gatterer H et al. A new approach to haemoglobin oxygen affinity research at high altitude: Determination of haemoglobin oxygen dissociation curves and 2,3-bisphosphoglycerate in an experimental human crossover hypoxic chamber study. 2025. https://pubmed.ncbi.nlm.nih.gov/40314729/