Abstract: A method and apparatus for characterizing the type of a blood sample are provided wherein an optical density spectrum of the sample is collected over a predetermined wavelength range. A reference optical density spectrum is collected over a predetermined wavelength range for a portion of the blood sample diluted in saline. Another portion of the blood sample is then mixed with an antibody corresponding to a known blood type (e.g., anti-A, anti-B, anti-D antibody). The optical density spectrum is then collected over a predetermined wavelength range for blood diluted with saline and each antibody in saline. The slopes are then calculated over a predetermined wavelength range for each spectrum. A numerical indicator of agglutination is then calculated by dividing the slope of each antibody-treated sample by the slope of the sample in saline. The resulting number is multiplied by 100.