The presence of dyshemoglobins will cause a calculated %SO2 result to be falsely (elevated, decreased) and a pulse oximeter %Spo2 value to be falsely (elevated, decreased).

• A. Elevated, elevated
• B. Decreased, decreased
• C. Elevated, decreased
• D. Decreased, elevated

Answer: (D)

Dyshemoglobins alter how oxygen saturation is measured by two different methods, and they do so in opposite directions. The amount of functional hemoglobin that can actually bind and carry oxygen is reduced when dyshemoglobins are present, so a calculated saturation based on arterial oxygen tension (PaO2) and standard assumptions about hemoglobin function will fall short of the true activity—this makes the calculated %SO2 appear decreased. At the same time, pulse oximetry relies on light absorption at two wavelengths and assumes only oxyhemoglobin and deoxyhemoglobin. Carboxyhemoglobin, for example, absorbs light in a way that mimics oxyhemoglobin, causing the device to overestimate how much hemoglobin is oxygenated. The result is a pulse oximeter reading that appears falsely elevated despite impaired oxygen transport. In practice, this means you can see a low calculated saturation while the SpO2 reading is higher than reality, underscoring why co-oximetry is needed to accurately quantify different Hb species.