Skin reflectance pulse oximetry: in vivo measurements from the forearm and calf. This study describes the results from a series of human experiments demonstrating the ability to measure arterial hemoglobin oxygen saturation (SaO2) from the forearm and calf using a reflectance pulse oximeter sensor. A special optical reflectance sensor that includes a heating element was interfaced to a temperature controller and a commercial Data-scope ACCUSAT pulse oximeter that was adapted for this study to perform as a reflectance pulse oximeter. The reflectance pulse oximeter sensor was evaluated in a group of 10 healthy adult volunteers during steady-state hypoxia. Hypoxia was induced by gradually lowering the inspired fraction of oxygen in the breathing gas mixture from 100 to 12%. Simultaneous SaO2 measurements obtained from the forearm and calf with two identical reflectance pulse oximeters were compared with SaO2 values measured by a finger sensor that was interfaced to a standard Datascope ACCUSAT transmittance pulse oximeter. The equations for the best-fitted linear regression lines between the percent reflectance, SpO2(r), and transmittance, SpO2(t), values in the range between 73 and 100% were SpO2(r) = -7.06 + 1.09 SpO2(t) for the forearm (n = 91, r = 0.95) and SpO2(r) = 7.78 + 0.93 SpO2(t) for the calf (n = 93, r = 0.88). The regression analysis of the forearm data revealed a mean +/- SD error of 2.47 +/- 1.66% (SaO2 = 90-100%), 2.35 +/- 2.45% (SaO2 = 80-89%), and 2.42 +/- 1.20% (SaO2 = 70-79%). The corresponding regression analysis of the calf data revealed a mean +/- SD error of 3.36 +/- 3.06% (SaO2 = 90-100%), 3.45 +/- 4.12% (SaO2 = 80-89%), and 2.97 +/- 2.75% (SaO2 = 70-79%). 