In our previous case-based review, I teased the opportunity to use biomarkers to increase the accuracy and expediency of the diagnosis of Alzheimer's disease (AD). These tests are no longer confined to the research setting but are now available to specialists and primary care clinicians alike. Given that most cognitive disorders are first identified in primary care, however, I believe that their greatest impact will be in our clinical space.
The pathologic processes associated with AD can be detected approximately two decades before the advent of clinical symptoms, and the symptomatic period of cognitive impairment is estimated to occupy just the final third of the disease course of AD. Using imaging studies, primarily PET, as well as cerebrospinal fluid (CSF) and even blood biomarkers for beta amyloid and tau, the pathologic drivers of AD, clinicians can identify patients with AD pathology before any symptoms are present. Importantly for our present-day interventions, the application of biomarkers can also help to diagnose AD earlier.
Amyloid PET identifies one of the earliest markers of potential AD, but a barrier common to advanced diagnostic imaging has been cost. Medicare has now approved coverage for amyloid PET in cases of suspected cognitive impairment. In a large study of more than 16,000 older adults in the United States, PET scans were positive in 55.3% of cases with mild cognitive impairment (MCI). The PET positivity rate among adults with other dementia was 70.1%. The application of PET resulted in a change in care in more than 60% of patients with MCI and dementia. One quarter of participants had their diagnosis changed from AD to another form of dementia, and 10% were changed from a diagnosis of other dementia to AD.
Liquid biomarkers can involve either cerebrospinal fluid or blood samples. To date, CSF testing has yielded more consistent results and has defined protocols for assessment. Still, collection of CSF is more challenging than collection of blood, and patients and their families may object to lumbar puncture. CSF assessment therefore remains generally in the province of specialists and research centers.
Primary care clinicians have been waiting for a reliable blood-based biomarker for AD, and that wait may be about to end. A study published in July 2024 included 1213 adults being evaluated for cognitive symptoms in Sweden. They completed a test measuring the ratio of phosphorylated tau 217 vs non-phosphorylated tau 217, with or without a test for serum amyloid ratios as well. These tests were compared with clinicians’ clinical diagnoses as well as CSF results, which were considered the gold standard.
Using only clinical tools, primary care clinicians’ and specialists’ diagnostic accuracy for MCI and dementia were just 61% and 73%, respectively. These values were substantially weaker vs the performance of either the serum tau or amyloid ratios (both 90% accurate). The authors concluded that serum testing has the potential to improve clinical care of patients with cognitive impairment.
Where does that leave us today? Commercially available blood biomarkers are available now which use different tests and cutoff values. These may be helpful but will probably be difficult to compare and interpret for primary care clinicians. In addition, insurance is less likely to cover these tests. Amyloid PET scans are a very reasonable option to augment clinician judgment of suspected cognitive impairment, but not all geographic areas will have ready access to this imaging study.
Still, it is an exciting time to have more objective tools at our disposal to identify MCI and AD. These tools can only be optimized by clinicians who recognize symptoms and perform the baseline testing necessary to determine pretest probability of MCI or dementia.
Note: This article originally appeared on Medscape.
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