Last month, The US Food and Drug Administration approved a new blood test for assisting the diagnosis of Alzheimer’s disease. Produced by Roche, Elecsys pTau181 measures the concentration of a specific molecule—a phosphorylated form of the tau protein—in the blood. Tau is one of two proteins, the other being amyloid, that become malformed and accumulate in the brains of patients with certain types of dementia. It is believed that the buildup of these proteins interferes with the communication of brain cells, leading to these patients’ symptoms.

The test had already received authorization in July for marketing in Europe and is thus the first early screening system for Alzheimer’s for use in primary care settings approved in the planet’s two major pharmaceutical markets. It is an opener in what should soon become a crowded field, as there are several other tests in advanced stages of testing and approval.

How Do Such Tests Work?

Elecsys pTau181 looks in the blood plasma for a form of the tau protein that has a phosphate group attached, which is often found in elevated amounts in Alzheimer’s patients. This molecule is an indirect marker of the plaques of amyloid and neurofibrillary tangles of tau observed in the brains of patients with the disease.

Some other tests have also been approved, though not for early screening. These assess other biomarkers that relate to these two proteins. One test, called Lumipulse and made by the Japanese company Fujirebio, looks at the ratio between another form of phosphorylated tau (pTau217) and a key protein fragment that forms amyloid plaques (amyloid beta peptide 1-42).

The bottom line is that these tests offer clues to the probable presence of amyloidosis in the brain, which then needs to be diagnosed with greater accuracy using more invasive tests, such as a PET (positron emission tomography) scan and cerebrospinal fluid analysis by lumbar puncture, considered the clinical gold standard for diagnosing amyloid pathology in living patients. Even these, however, come with some degree of uncertainty; true diagnostic certainty can only be had with a...