A blood leak detector is disclosed having a light source projecting a beam along an optical path, wherein the beam has a wavelength in a range of about 800 to 930 nm; a light detector receiving the beam through an aperture having a diameter in a range of 30 to 60 thousands of an inch; and a housing to receive a tubular liquid passage between the light source and light detector, the housing having a slot transverse to the optical path to receive the tubular liquid passage and the slot has a width narrower than the tubular liquid passage when uncompressed.