satoris-logo.jpgSatoris, a Redwood City, Calif., biotech developing a diagnostic test for Alzheimer’s disease, is close to raising $5 million in a second funding round, VentureWire reports (subscription required). The funds will allow the company to make its test available to outside researchers.

Satoris made a big splash in mid-October, when researchers identified 18 proteins they said could be used to identify people with Alzheimer’s disease via a simple blood test. What’s more, the test also appeared to detect incipient Alzheimer’s in people with mild cognitive impairment, meaning that it could theoretically be used to predict who is most likely to eventually develop the often devastating disease.

An Alzheimer’s blood test would be a major development, since there’s currently no good physical way of diagnosing, much less predicting, the condition. The disease is typically identified — often imprecisely — via cognitive testing. In fact, the only certain way to diagnose Alzheimer’s is via brain autopsy, which isn’t a particularly attractive option to most people.

Satoris says successful development of its test should help accelerate the development of new Alzheimer’s treatments. Which would be great, because at the moment, early diagnosis of the degenerative condition wouldn’t actually do anyone much good, even though the disease can cause brain damage well in advance of actual symptoms. Current treatments for the disease do little more than delay the onset of memory loss and other cognitive symptoms.

What’s more, the Satoris test may still be of limited usefulness simply because it is likely to produce huge numbers of mistaken diagnoses, at least in its current form. In the Nature Medicine paper that heralded the test’s discovery, the research team found that the 18 proteins correctly identified 90 percent of the blood samples from Alzheimer’s patients and 88 percent of the non-Alzheimer’s samples.

That sounds pretty good until you imagine trying to use that test as a widespread screening tool. An estimated five percent of Americans aged 65 to 74 have Alzheimer’s, according to this NIH fact sheet. Now imagine testing all of the roughly 20 million people in that age group with the Satoris test. It will correctly detect Alzheimer’s in 900,000 of the one million people that actually have it, which is great. At the same time, though, it will incorrectly diagnose more than twice as many people — 2.3 million — who don’t actually have the disease. That doesn’t even get into its possible predictive uses, where the potential errors — and their consequences — are even worse. (Hat tip to Derek Lowe, who recently crunched similar numbers over at In the Pipeline.)

All of which goes to underscore why diagnostic tests are actually a lot trickier than people frequently realize. While many of us would be more than willing to undergo surgery or take a drug that had a 90 percent chance of curing us, that sort of success rate is pathetically low in the diagnostic area, since false-positive and false-negative results can have such outsized consequences. No doubt insurers will be asking some hard questions if Satoris tries to bring the test to market without refining it a great deal further.

Which it looks like it intends to do. Satoris previously raised $1 million from individuals in 2004, and will be looking for another $15 million next year to launch a “home brew” version of the test — a tactic in which Satoris would test blood samples mailed to it by doctors, potentially allowing it to sidestep FDA regulation. The company’s current investors, however, appear unconcerned; Life Science Angels and Brain Trust Accelerator Fund are expected to participate in the current round, according to VentureWire.