In many previous blogs about Alzheimer’s Disease and dementias, I have said that Alzheimer’s was a diagnosis of exclusion. That means all other causes of dementia had to be ruled out before the diagnosis of Alzheimer’s could be made. Using this method, the “diagnosis was right only 60% of the time.” I also said it was a diagnosis that could only be made accurately after death at the time of an autopsy. Characteristic microscopic alterations of the brain had to be present to say the deceased truly had Alzheimer’s. That, of course, was right 100% of the time.
Later, though, analysis of the cerebrospinal fluid (CSF) of a patient suspected of Alzheimer’s was used to make the presumptive diagnosis. Obtaining CSF required the patient to have a lumbar puncture (spinal tap), the “mother of all tests.” A lot of patients refused to have that. MRI imaging of the brain using special techniques is also useful, diagnostically.
But now, or at least as soon as they are approved, blood-based biomarkers (tests) are available that will “rule in” Alzheimer’s Disease. These tests are recommended for use in patients with definite symptoms of dementia and not for asymptomatic individuals. These tests are thus not recommended as a screening tool. The tests are designed to detect evidence of the following abnormalities in the brain:
Amyloid beta 42
Hybrid amyloid beta 42/40
Phosphorylated-tau protein (p-tau)
From these results, Calculated ratios will yield a percent likelihood of the person having Alzheimer’s.
These tests, two tau proteins and two amyloid proteins, were “accurate in identifying Alzheimer’s Disease more than 90% of the time.”
The good news here is when patients begin to show cognitive decline, these tests can be done. If they are positive, the patient can be referred to determine if they are candidates for the new monoclonal antibodies. These biomarker tests will change the diagnostic approach to patients with early cognitive impairment in the hope that progression of the disease can be halted if not reversed.
Reference: Skolnik N. New Diagnostic Criteria for Alzheimer’s Disease will Change Practice. Medscape Family Medicine 2024 September 10. https://www.medscape.com/viewarticle/new-diagnostic-criteria-for-Alzheimers-will-change-practice.
