Why Is Amyloid-β PET Requested After Performing CSF Biomarkers?

Alzheimer's disease (AD) diagnosis often relies on amyloid-β (Aβ) biomarkers obtained through cerebrospinal fluid (CSF) analysis or amyloid-β positron emission tomography (PET). While these methods are considered interchangeable for diagnostic purposes, their clinical application can vary. Prior research has shown that CSF Aβ42 levels are a reliable indicator of amyloid pathology. However, uncertainty remains regarding when clinicians choose to request additional amyloid-β PET scans after already performing CSF biomarker testing. This gap motivated a closer examination of the clinical reasoning behind such diagnostic decisions.

The study aimed to investigate the clinical rationale for ordering amyloid-β PET scans after CSF biomarker analysis in patients undergoing diagnostic evaluation for cognitive decline. Specifically, it sought to identify the factors that lead clinicians to request amyloid-β PET despite having CSF results. The researchers also assessed whether these decisions aligned with established appropriate-use criteria (AUC) for amyloid-β PET. The goal was to better understand how diagnostic uncertainty is managed in real-world clinical settings.

The researchers conducted a retrospective chart review of 72 memory clinic patients who underwent amyloid-β PET scans after CSF biomarker analysis between 2011 and 2019. Patient records were analyzed to determine the clinical factors that prompted the additional PET scan. The study team also evaluated whether the PET requests followed the AUC guidelines. Data collected included patient age, cognitive status, CSF biomarker results, and clinical presentation. The primary focus was on identifying patterns in diagnostic decision-making.

The most common reason for requesting amyloid-β PET scans was a mismatch between the clinical diagnosis and CSF Aβ/tau results, observed in 74% of cases. Incongruent MRI findings and unusual clinical presentations were also noted as contributing factors. Only 7% of PET scans were requested in patients with CSF Aβ+/tau+ status. Additionally, 15 cases did not align with AUC criteria, particularly when CSF results conflicted with the clinical diagnosis. Among patients diagnosed with AD after PET, 47% presented with non-amnestic symptoms. These findings suggest that amyloid-β PET is often used to resolve diagnostic uncertainty when CSF biomarkers and clinical features do not align.

The study found that the primary clinical reason for requesting amyloid-β PET after CSF biomarker analysis was a discrepancy between the clinical diagnosis and CSF Aβ/tau results. This suggests that amyloid-β PET is used to clarify diagnostic uncertainty when CSF results are incongruent with the patient's clinical presentation. The researchers propose that amyloid-β PET is particularly valuable in cases where CSF findings do not support the initial clinical impression. However, the study also highlights that some PET requests did not follow established appropriate-use criteria, indicating variability in clinical decision-making. These findings may inform future guidelines on the use of amyloid-β PET in clinical practice.