A method for detecting circulating levels of beta amyloid peptides in an individual by administration of one or more inhibitors of an extracellular proteolytic pathway in blood and detection of the beta amyloid peptides in the blood or a fraction thereof. An example of an inhibitor is an anticoagulant. Levels of desired beta amyloid peptides such as ?ß1-40 and ?ß1-42 can be determined in the blood samples collected from the individuals.