A protective blood tube kept an Alzheimer's marker readable for five days

A BLOOD test for Alzheimer's disease is only as good as the sample that reaches the laboratory. Blood is cells floating in a pale yellow liquid. That liquid, called plasma, carries proteins that leak from the brain into the circulation. Two of them matter here. One is a form of the tau protein with a phosphate group attached at a particular spot, known as p-tau217, which rises in people whose brains are accumulating the protein clumps typical of the disease. The other is a small protein fragment called amyloid-beta 42, which falls in the blood of the same people as it collects in their brains.

In 2025 America's Food and Drug Administration cleared a test made by Fujirebio that runs on its Lumipulse machine and divides the first protein by the second. Because one goes up and the other goes down in disease, the ratio separates patients more sharply than either number alone. A test of this kind, which measures how much of a substance is in a sample, is called an assay. The catch is the handling. The FDA's decision summary warns that the specimens "degrade rapidly at room temperature" and tells laboratories to avoid storing or shipping them at that temperature. That rules out clinics that cannot spin and freeze blood within hours.

A preprint posted to medRxiv on November 2nd, and not yet reviewed by other scientists, tests a possible fix. You-Rim Lee, Thomas Karikari and three colleagues at the University of Pittsburgh School of Medicine drew three tubes of blood from each of 40 people with a median age of 76. A standard tube was spun and frozen at once as the reference. Two special tubes sat on a laboratory bench for five days at about 20°C, a test of room temperature handling. One, BD's P100, is coated with chemicals that block the enzymes that chop up proteins. The other, DNA Genotek's HEMAcollect PROTEIN, contains a liquid meant to stop blood cells from bursting and spilling their contents. Both are sold for research use only.

The first question was whether people stayed in the same order. If the person with the highest p-tau217 in fresh blood still had the highest after five days, and so on down the list, a shift in level can be corrected. In the P100 tube that order survived almost perfectly, and the median difference from fresh blood was about 12%. In the HEMAcollect tube the order also survived, but the median reading fell by 58%.

Amyloid-beta 42 fared badly in both. Its median level fell by 73% in the P100 tube and by 96% in the HEMAcollect tube, and the order of people was largely lost. That wrecked the ratio. A cutoff is the value above or below which a result counts as positive or negative. Using the FDA-cleared cutoffs, fresh blood called 15 of the 40 people positive. The five-day P100 samples called 35 positive and the HEMAcollect samples all 40. Only 18 of 40 people (P100) and 15 of 40 (HEMAcollect) landed in the same category as their fresh sample.

Judged on p-tau217 alone, the P100 tube did much better. It put 39 of 40 people in the same category as fresh blood. The HEMAcollect tube matched 28 of 40, and all 12 of its mismatches moved toward negative.

The study has clear limits. It involved only 40 people, all tested at one laboratory, on one machine, after one storage condition. The cutoffs used for p-tau217 alone were borrowed from another study, a study of 1,767 patients in Sweden, Spain and Italy published in Nature Medicine in 2025, and the authors call that comparison exploratory. Those cutoffs are not part of any cleared test. Fujirebio, which makes the Lumipulse assay used in the study, is listed among the funders of Dr Karikari's laboratory.

The findings fit earlier work. A study published in 2023 by Anita Sunde of Stavanger University Hospital in Norway and colleagues found that amyloid-beta levels declined when plasma was kept at about 18°C for longer than six hours. The Pittsburgh group argues that the stability of each marker in each tube has to be measured on its own before any such tube is used to diagnose patients.

For now the cleared two-protein test still needs blood that is processed quickly and kept cold. The P100 result suggests that a test based on p-tau217 alone might one day tolerate a few days in transit. Testing that would take more people, more laboratories, other storage times and cutoffs set for samples that have traveled.