What could cause a pulse oximetry of 100%?

• A. Anemia
• B. Lung disease
• C. Dehydration
• D. Carbon monoxide or anxiety attack

Answer: (D)

Pulse oximetry measures how much of the hemoglobin in your blood is carrying oxygen, but it can be fooled by certain forms of hemoglobin. Carbon monoxide binds with hemoglobin to form carboxyhemoglobin, and standard two-wavelength pulse oximeters can’t distinguish carboxyhemoglobin from oxyhemoglobin. Because of that, the device often shows a normal or very high reading—even near 100%—even when the patient isn’t actually well-oxygenated tissue. This is why carbon monoxide poisoning can present with a pulse oximetry reading of 100%.

Other factors don’t inherently raise SpO2 in the same way. Anemia and dehydration reduce the blood’s oxygen content rather than its saturation percentage, so SpO2 may stay normal even though overall oxygen delivery is reduced. Lung disease typically lowers SpO2 by impairing gas exchange. Anxiety alone doesn’t change the fraction of hemoglobin bound with oxygen, though a patient who is anxious and not hypoxic can still have a normal reading.