NEW YORK (Reuters Health) - A low ratio of amyloid beta protein 42 (Abeta42) to amyloid beta protein 40 (Abeta40) in plasma is associated with an increased risk of mild cognitive impairment (MCI) and Alzheimer's disease (AD), according to a report in the March issue of the Archives of Neurology.

"This blood test may be like a cholesterol test, predicting some of the risk of who will develop AD," Dr. Neill R. Graff-Radford from the Mayo Clinic in Jacksonville, Florida, told Reuters Health.

Dr. Graff-Radford and associates evaluated plasma amyloid beta protein and the Abeta42/Abeta40 ratio as possible biomarkers for AD and for the amnestic type of MCI that usually precedes AD.

Individually, Abeta42 and Abeta40 levels were not associated with the risk of AD or MCI, the authors report.

The Abeta42/Abeta40 ratio, however, was associated with conversion to MCI or AD. Patients with Abeta42/Abeta40 in the lowest quartile were more than 3 times as likely to progress to MCI or AD compared with patients in the highest quartile.

Similarly, the researchers note, among older subjects with the APOE 3/4 genotype, the incidence of MCI or AD after 5 years of follow-up was 32% among those with Abeta42/Abeta40 below the median, compared with 6% among patients with Abeta42/Abeta40 above the median.

"We believe this finding may be used in at least two ways," Dr. Graff-Radford said. "We may be able to identify persons at great risk of AD and design a cost-effective primary prevention trial by enriching the sample to be studied based on the blood test. Secondly, the blood test may be a screen of patients at risk for AD who go on to have a more expensive test, such as the PIB PET scan, which can image amyloid in the brain."

The research team plans to evaluate the test longitudinally in a much larger sample of 3000 subjects, Dr. Graff-Radford said. "We want to determine if there is an earlier time in the evolution of the plasma amyloid beta proteins that we can predict risk, and also to replicate our finding."