The oxygen-hemoglobin dissociation curve describes this relationship. It is sigmoidal: SpO₂ 97% ≈ PaO₂ 95 mmHg, SpO₂ 90% ≈ PaO₂ 60 mmHg, SpO₂ 75% ≈ PaO₂ 40 mmHg. The steep portion of the curve means small PaO₂ changes below 60 mmHg cause large SpO₂ drops.

At PaO₂ ~60 mmHg, SpO₂ is approximately 90%. Below this point, the dissociation curve becomes steep, meaning even small further decreases in PaO₂ cause rapid oxygen desaturation. This is why SpO₂ 90% is often used as a clinical threshold for supplemental oxygen.

The actual curve can shift with temperature (rightward shift with fever), pH (Bohr effect), 2,3-DPG levels, carbon monoxide exposure, methemoglobinemia, and certain hemoglobin variants. Pulse oximetry itself can be inaccurate in poor perfusion, dark nail polish, anemia, and dark skin pigmentation.

ABG is preferred when: precise PaO₂ is needed, CO poisoning is suspected (SpO₂ is falsely normal), acid-base status is needed, ventilator management requires accurate data, or pulse oximetry readings are unreliable.