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Alzheimer's disease now can be precisely diagnosed only after death. But government scientists say an experimental test using blood and skin cells may one day detect the fatal brain disorder in its earliest stages.

Laboratory experiments with the test show that the skin and blood cells of Alzheimer's disease patients have fundamental differences that can be detected long before the disease symptoms appear, said Dr. Jay H. Robbins of the National Cancer Institute.Robbins, senior author of a report in the Proceedings of the National Academy of Sciences, said an early-stage test eventually could help researchers develop drugs for Alzheimer's, differentiate it from other diseases and help people at high risk of Alzheimer's to make important decisions.

But he said still more work is needed before the test will be ready for general use.

"This test could be useful in identifying the disease absolutely in patients who are diagnosed as probable Alzheimer's disease," said Robbins. "Many people who have blood relatives with the disease might want to know. The decision, for instance, could be im-portant for family planning."

Early diagnosis of Alzheimer's could give a boost to researchers now struggling to develop drugs for the disorder, said Dr. Zaven Khachatuarian, director of the Alzheimer's Association Ronald and Nancy Reagan Research Institute.

Khachatuarian said that drugs ranging from hormones to anti-inflamatories to calcium channel blockers are being studied, and all of these could be effectively tested only on people who are confirmed to have Alzheimer's.

"Our ability to detect the disease early will be very important in developing these therapies," he said.

Early detection will be even more important once drugs are developed, said Khachatuarian, because "Alzheimer's may start as early as 40 years before symptoms show up, and this means that to slow down the progression of this disease, we need to be able to detect it and treat it early."

The Alzheimer's Association estimates that at least 4 million Americans, including former President Reagan, suffer from Alz-heimer's. Its symptoms include a gradual loss of memory and eventual failure of all other body functions that are directed by the brain, resulting in death.

In an experiment to prove the accuracy of the test, Robbins and his colleagues analyzed unlabeled cells from 95 patients, some of whom had died of what had been shown in autopsies to have been Alzheimer's disease.

The researchers were able to correctly identify cells from 27 Alzheimer's patients and to separate out cells from patients suffering from other brain disorders, such as Parkinson's disease and Huntington's disease. There were negative results from 28 of 31 normal cells from healthy donors.

Included in the specimens were cells from patients in rare families whose members have a 50 percent chance of developing Alzheimer's.

Robbins said the test, using unlabeled, frozen cells, correctly identified specimens from three Alz-heimer family members who gave the cells years before they developed symptoms of the disease.

"If we want to (test Alzheimer's drugs), it is important to have tests that will identify people 10 to 15 years before they have symptoms," he said.

The test identifies specimens from Alzheimer's patients by detecting cells that have lost the ability to repair certain kinds of DNA damage.