Diagnosing Alzheimer's is a guessing game—and many doctors are guessing wrong, according to early results from a new study presented Wednesday in London. The Washington Post reports doctors tested 4,000 Medicare patients with mild cognitive impairment or dementia and discovered many of them definitively do not have Alzheimer's.

Using PET scans, the study revealed only 54.3 percent of mild cognitive impairment patients and 70.5 percent of dementia patients had brains containing amyloid plaques. These plaques can be a sign of Alzheimer's.

The patients who had them may or may not have Alzheimer's; the remaining percentage definitely don't have the disease. "To get that right diagnosis, that's really important," one woman, whose Alzheimer's diagnosis was ruled out by a PET scan, tells the AP.

The findings show many people are potentially taking unnecessary Alzheimer's medication, and doctors may want to change their treatment to something cheaper or more effective. Unfortunately, the only ways to test for amyloid plaques while a patient is alive are expensive PET scans, which aren't usually covered by insurance, or invasive spinal taps.

Or are they? Another study presented at the same London conference revealed a new blood test that could potentially reveal the plaques, New Scientist reports.

The tests are easy and cheap enough that doctors could administer them to patients during regular checkups. Since plaques start showing up 15 to 20 years before Alzheimer's symptoms, patients could use the results to change certain lifestyle habits to decrease their risk.

(This startup buys young blood, injects it into older people.)

This article originally appeared on Newser: Many People Being Treated for Alzheimer's May Not Have It