Oxygen saturation

Oxygen saturation is the percentage of hemoglobin binding sites in the blood occupied by oxygen. Because hemoglobin is the blood's oxygen carrier, saturation is a direct indicator of how well the lungs are oxygenating the blood and how much oxygen is available for delivery to tissues.

Clinically, saturation is most often measured as SpO2 with a pulse oximeter — a sensor clipped to a fingertip or earlobe that passes light through the tissue and calculates saturation from how oxygenated versus deoxygenated hemoglobin absorb it. A normal SpO2 is roughly 95-100%; readings below about 90% indicate hypoxemia and usually warrant intervention. Arterial blood gas analysis measures saturation (SaO2) directly and adds the partial pressure of oxygen (PaO2), which relates to saturation through the S-shaped oxygen-hemoglobin dissociation curve.

Pulse oximetry is quick and painless, which is why oxygen saturation is monitored alongside the traditional vital signs. But readings have limits: poor peripheral circulation, cold extremities, nail polish, and motion can distort them, and severe anemia is a different trap — SpO2 can read normal while the blood's oxygen-carrying capacity is sharply reduced, and carbon monoxide poisoning produces falsely normal readings because the device cannot distinguish carboxyhemoglobin from oxyhemoglobin.

The CCMA exam covers oxygen saturation within vital signs — normal ranges, proper pulse oximeter technique, and factors that skew readings — while USMLE Step 1 connects saturation to hemoglobin physiology and the dissociation curve. (In environmental engineering contexts like the FE Civil exam, oxygen saturation instead refers to dissolved oxygen levels in water.)