A device, system and method for the detection of bleeding inside the GI tract include irradiating the bodily fluids at different narrow band wavelengths as the device moves along the GI tract, detecting over time the absorbance or transmittance signals or measuring absorbance or transmittance spectra of the in- vivo fluids that pass through the sensing head of the device, processing the detected data, determining blood concentration over time and displaying blood concentration over time. The diagnostic device includes a housing with a gap that remains in contact with bodily fluids and through which the fluids may pass, and a sensing head comprising at least three LEDs on one side of the gap, each LED irradiating the bodily fluids at a different narrow band wavelength, and a light detector positioned at the opposite side of the gap and facing the LEDs, for detecting light which passes through the in-vivo bodily fluids.
