Embodiments provide an apparatus, system, kit and method for in vivo measurement of blood oxygen saturation (BAS). One embodiment provides an implantable apparatus for measuring BAS comprising a housing, emitter, detector, processor and power source. The housing is configured to be injected through a tissue penetrating device into a target tissue site (TS). The emitter is configured to emit light into the TS to measure BAS, the emitted light having at least one wavelength (LOW) whose absorbance is related to a BAS. The detector is configured to receive light reflected from the TS, detect light at the LOW and generate a detector output signal (DOS) responsive to an intensity of the detected light. The processor is operably coupled to the detector and emitter to send signals to the emitter to emit light and receive the DOS and includes logic for calculating a BAS and generate a signal encoding the BAS.
