Accuracy in Specialty Versus Primary Care
When the plasma samples were analyzed in a single batch in the primary care cohort, the AUC was 0.97 when the APS2 was used. In the secondary care cohort, the AUC was 0.96 when the APS2 was used.
When plasma samples were analyzed prospectively (biweekly) in the primary care cohort, the AUC was 0.96 when the APS2 was used. In the secondary care cohort, the AUC was 0.97 when the APS2 was used.
The 2 cutoff-value approach achieved PPVs of 97%-99% in patients with cognitive impairment, which is the target population of currently available antiamyloid treatments.
Although NPVs were slightly lower in these patients (87%-92% using the APS2), “we argue that a very high positive predictive value is probably more important in diagnosing patients as having Alzheimer’s disease, especially before initiating costly and burdensome antiamyloid treatment,” the investigators noted.
The PPVs were less than optimal for accurate identification of Alzheimer’s disease pathology in patients with subjective cognitive decline regardless of the cutoff-value approach used. The researchers pointed out that this could be a disadvantage for clinical trials that include patients with presymptomatic Alzheimer’s disease but not in clinical practice because there are no clinical criteria for diagnosing Alzheimer’s disease at the subjective cognitive decline stage.
The NPVs were higher in patients with subjective cognitive decline (91%-94% for the APS2 or percentage of p-tau217 alone). This indicates the blood test would be more useful for ruling out underlying Alzheimer’s disease when only subtle symptoms are present, the researchers noted.
As for doctors identifying clinical Alzheimer’s disease, primary care physicians had a diagnostic accuracy of 61% (95% CI, 53%-69%) versus 91% (95% CI, 86%-96%) using the APS2. Dementia specialists had a diagnostic accuracy of 73% (95% CI, 68%-79%) versus 91% (95% CI, 86%-95%) using the APS2.
In the overall population, the diagnostic accuracy using the APS2 (90%; 95% CI, 88%-92%) was not different from that using the percentage of p-tau217 alone (90%; 95% CI, 88%-91%).
Very little was known about how a blood test would perform in a primary care setting, said Dr. Palmqvist. “Seeing that the test was just as accurate in primary care (about 90%) as it was in secondary care is really encouraging, especially since primary care is the first, and often final, point of entry into the healthcare system for cognitive evaluations.”
He said he was surprised the biomarkers performed so well in prospective, biweekly analyses throughout the study. “Previous studies have only demonstrated their effectiveness when all collected samples are analyzed at a single time point, which does not reflect how a blood test is used in clinical practice.”
He added that he was surprised that the tests were just as accurate in primary care as in a memory clinic setting with referred patients. This, despite older age and higher prevalence of comorbidities in primary care, such as chronic kidney disease (present in 26% of the primary care cohort), can be a confounding factor causing increased concentrations of p-tau217.
Next Steps
The diagnostic accuracy of the blood tests is on par with FDA-cleared cerebrospinal fluid biomarkers, noted the investigators, led by senior author Oskar Hansson, MD, PhD, Clinical Memory Research Unit, Department of Clinical Sciences Malmö, Faculty of Medicine, Lund University, Lund, Sweden.
As blood tests are “more time effective, cost effective, and convenient” for patients, “they could also potentially replace cerebrospinal fluid tests and PET,” they added.
Dr. Palmqvist emphasized that these tests should not be used as stand-alone diagnostic tools for Alzheimer’s disease but should complement the standard clinical evaluation that includes cognitive testing and a thorough interview with the patient and a spouse or relative.
“This is crucial because Alzheimer’s disease pathology can be asymptomatic for many years, and cognitive symptoms in some patients with Alzheimer’s disease pathology may primarily result from other conditions. Misinterpreting a positive Alzheimer’s disease blood test could lead to underdiagnosis of common non–Alzheimer’s disease conditions.”
With new antiamyloid treatments possibly slowing disease progression by 30%-40% when initiated early on, a blood test for Alzheimer’s disease could lead to more people receiving an accurate and earlier diagnosis, said Dr. Palmqvist. “This could potentially result in a better response to treatment. Results from drug trials clearly indicate that the earlier treatment begins, the more effectively it can slow disease progression.”
The test used in the study is already available in the United States, the investigators said, and a similar test will be accessible in Sweden within a few months. “However, the rollout will probably be gradual and will depend on how international and national guidelines recommend their use, so developing these guidelines will be a crucial next step for widespread implementation, particularly in primary care,” said Dr. Palmqvist.
He also underlined the importance of replicating the findings in more diverse populations. “This will help ensure the tests’ reliability and effectiveness across various demographic and clinical contexts.”
An important next research step is to examine how implementing a blood test for Alzheimer’s disease affects patient care. “This includes looking at changes in management, such as referrals, other examinations, and the initiation of appropriate treatments,” said Dr. Palmqvist.
Another study presented at the meeting showed that a highly accurate blood test could significantly reduce diagnostic wait times.