Abstract: Carbon Monoxide (CO) is a colorless, tasteless, and odorless gas which causes asphyxiation by combining with a strong affinity to blood hemoglobin. Unless a patient is quickly identified as having CO poisoning, the patient could face severe medical consequences including death.This patent describes a reagentless optical method for measuring carboxyhemoglobin (HbCO) in a whole undiluted blood sample that can be obtained from a fingerstick. A battery-operated triple wavelength device measures light transmission at frequencies of 548 nm, 810 nm and 950 nm through a 40 .mu.L sample of undiluted whole blood placed in standard disposable glass capillary tube. Having a portable point-of-care device, for uses by paramedics during an emergency situation, in the emergency room, or in a physician's office, can provide accurate immediate confirmation of CO poisoning.