The world's first blood test diagnosing Alzheimer's from the United States has been given the green light to be sold in Europe.

The "PrecivityAD" blood test developed by American biotech firm C2N Diagnostics from St. Louis, Missouri, is a test for the detection of Alzheimer's disease pathology.

It measures two types of amyloid particles and various forms of a protein linked to Alzheimer's, which reveal if a person has a gene that raises risk for the disease.

The PrecivityAD test previously received a breakthrough designation from the U.S. Food and Drug Administration.

C2N points out PrecivityAD can't diagnose a case of Alzheimer's on its own. This noninvasive diagnostic, however, can help predict brain pathology in people having trouble with cognition. It can also be used as a tool in a wider clinical assessment.

The test isn't intended for general screening or for people without symptoms, It is meant for people 60 and older having thinking problems and are being evaluated for Alzheimer's, the most common form of dementia. Only doctors can order the test and results come within 10 days.

After taking the test, patients are given a score called an Amyloid Probability Score suggesting low, medium or high likelihood of having amyloid buildup in the brain based on test results combined in a formula that includes age and other conditions.

As explained by C2N, their blood test uses mass spectrometry to measure proteins in the blood that indicate the probability of amyloid deposits in the brain as measured by amyloid PET scans. These measurements derive from a single sample of blood.

Amyloid Probability Scores represent the estimated probability from 0 (low probability) to 100 (high probability) a patient is currently amyloid positive on amyloid PET imaging based on his Aß42/40 ratio, age and ApoE genotype. C2N said a positive amyloid PET scan is consistent with presence of amyloid plaques and an Alzheimer's disease diagnosis.

There are concerns related to the accuracy of the test results since C2N hasn't published any data on the test accuracy. The company, however, has published on the amyloid research leading to the test.

The test was launched late October in the U.S. and is sold in most states. It's not covered by insurance or Medicare. C2N charges $1,250 per test and offers discounts based on income.

More than 5 million people in the United States and millions more around the world have Alzheimer's. A positive Alzheimer's diagnosis means a person must have symptoms such as memory loss and evidence of a buildup of a protein called beta-amyloid in the brain.

The best way now to measure the protein is by using a PET brain scan that can cost upward of $5,000 usually not covered by insurance. This huge expense means most people opt not to get PET scans and don't know if their condition is Alzheimer's or something else.