One of the major diagnostic and therapeutic disease goals for the healthcare industry is Alzheimer's disease (see: Early Detection of Alzheimer's Disease: Mutations of Three Genes Studied; Exercise and a Mediterranean Diet Lower the Risk of Alzheimer's Disease). It looks like we are closing in (sort of) on a useful imaging test (see: U.S. gives nod to Eli Lilly's brain plaque test). Below is an excerpt from an article about it:
U.S. regulators gave the nod to an imaging test from Eli Lilly and Co. that can for the first time help doctors detect brain plaque tied to Alzheimer's disease, the company said. The U.S. Food and Drug Administration approved the radioactive dye, called Amyvid, to help doctors rule out whether patients have Alzheimer's, the most common form of dementia....The dye binds to clumps of a toxic protein called beta amyloid that accumulates in the brains of patients with Alzheimer's. Doctors can then see the plaque light up on a positron emission tomography, or PET, scan. Patients with Alzheimer's always have some brain plaque, so its absence in the test would tell doctors to look for other causes of mental decline, such as depression or medications....But Lilly...said the test should not be used to diagnose Alzheimer's, since brain plaque can also be tied to other neurologic conditions and may occur naturally in older people with normal mental states. An FDA advisory panel recommended against approving the dye last year, saying doctors might have trouble interpreting scans of the plaque, and the FDA rejected Amyvid last March. Since then, Eli Lilly said it has worked to identify better ways of training doctors to use the test. Dr. Daniel Skovronsky, CEO of Avid, said one in five patients who are diagnosed with Alzheimer's turn out not to have the disease after an autopsy. "The approval of Amyvid offers physicians a tool that, in conjunction with other diagnostic evaluations, can provide information to help physicians evaluate their patients," ....Lilly, as well as Pfizer Inc, are the farthest along in developing experimental medicines to treat Alzheimer's. Lilly expects to release final data for its contender, solanezumab, as soon as this summer.
All of this is a little confusing so let me briefly summarize. Brain plaque can be identified by this new test but this finding is not pathognomonic for Alzheimer's. The test is based on a dye that binds to beta amyloid which is a component of these plaques. "Amyloid beta is a peptide of 36–43 amino acids that is processed from the amyloid precursor protein. While it is most commonly known in association with Alzheimer's disease, it does not exist specifically to cause disease. Evidence has been found that Aβ has multiple non-disease activities," according to the Wikipedia.
So, when a physician works up a patient for possible Alzheimer's, a negative test using this new imaging procedure tells him or her to possibly seek other causes for the observed mental changes such as depression or the result of medication. The procedure is thus helpful during such a workup. There also appear to be a number of Alzheimer-like syndromes that can cause mental changes (see: Misdiagnosis common in early-onset Alzheimer's):
It’s unclear whether all ...[of the] disorders that might look like Alzheimer's are truly separate disorders, or belong to one family. [A researcher in the field] is unconvinced that an autopsy could truly distinguish Alzheimer's from all other dementias and neurological disorders involved in the misdiagnoses. It's hard to diagnose Alzheimer's clinically, but we're not at the point where we can definitively define it in autopsy either, she said.
A key sentence in the excerpt above is the last one: Lilly expects to release final data for its contender, solanezumab, as soon as this summer. Lilly seems to be pursuing a companion diagnostic strategy (see: Some Interesting Insights into Companion Diagnostics; Companion Diagnostics Gains Wider Acceptance in FDA Despite Challenges). A physician needs to confirm a diagnosis like Alzheimer's before treating it with a drug such as the one Lilly is developing. However, there are lots of "ifs" here. The imaging test described above is not pathognomonic and the Lilly drug, solanezumab, has not yet been approved.