A portable, hand-holdable Blanching Response Tester apparatus (BRT) pressable against the skin of a human patient to provide an indication of a non-blanchable erythema indicative of an incipient pressure sore includes a housing having in a front end wall thereof an optically transmissive window and within the housing a broad-band light source electrically energizable to emit light including energy in the near infrared (0.8.mu. to 1.5.mu.) outwards through the window. A first, leading photodetector spaced laterally apart from the light source and a second, trailing photodetector spaced equidistant from the light source in an opposite direction have fields of view which include regions of the skin ahead of an behind a central area of the skin illuminated by the light source. Electronic signal processing circuitry within the BRT housing includes a pair of differential amplifiers having opposite pairs of inputs connected to the pair of photodetectors, one amplifier driving a first, "NO-GO" red, light-emitting diode mounted in an upper wall panel of the housing, and the other amplifier driving a second, "GO" green LED. When the BRT window is pressed against the skin and the light source energized, if light reflected from the skin by leading photodetector over a questionable area of the skin exceeds light reflected from a known healthy sample area of skin beneath the trailing photodetector the red, NO-GO LED is illuminated if the difference in light values exceeds a first threshold values signifying a pressure sore, and if light received by the trailing photodetector exceeds that received by the leading photodetector by a second threshold value, the green, GO LED is illuminated.