It could not only quick and easy, but it would be the first accurate test for diagnosing Alzheimer's disease, which can now only be diagnosed by careful psychiatric assessments and by examining the brain after death.

Tapan Khan and Daniel Alkon at the Blanchette Rockefeller Neurosciences Institute in Rockville, Maryland said their test distinguished Alzheimer's from other brain-damaging diseases such as Parkinson's.

Writing the Proceedings of the National Academy of Sciences, they said it might even be used to find Alzheimer's, the most common cause of dementia, early on, when drugs may do the most good.

"When it begins, Alzheimer's disease is often difficult to distinguish from other dementias or mild cognitive impairment," Alkon said in a statement.

"Potential treatments of Alzheimer's, however, are likely to have their greatest efficacy before the devastating and widespread impairment of brain function that inevitably develops after four or more years."

Alzheimer's disease is marked by inflammation, which in turn is caused by a variety of compounds in the body.

Alzheimer's specifically stimulates a change in an enzyme called MAP Kinase Erk 1/2, Alkon and Khan found.

They tested this on various tissue samples taken from people who had died of known causes, including people who had died with Alzheimer's.

When they tested skin cells with bradykinin, a common inflammatory signal, the Erk 1/2 response in Alzheimer's patients was different from that seen in tissues taken from other people.

That included patients with dementia caused by Parkinson's disease, multiple infarct dementia and Huntington's chorea.

More than 4.5 million people have Alzheimer's disease in the United States alone and 12 million worldwide. There is no cure.