For Doctors in a Hurry
- Clinicians need to understand why older age and male sex correlate with lower tau positron emission tomography positivity in Alzheimer disease.
- The researchers analyzed 672 PET scans, 945 autopsy cases, and 85 combined cases to evaluate tau pathology and imaging sensitivity.
- Tau PET sensitivity for advanced neuropathology dropped from 92% in younger patients to 42% in those aged 83 or older.
- The study concludes that lower tau tangle density in older males explains the reduced sensitivity of current PET imaging techniques.
- Physicians should interpret negative tau PET results with caution in older male patients, as these may reflect lower tangle density.
Refining the Clinical Utility of Tau PET Imaging
The diagnostic framework for Alzheimer’s disease has transitioned from a symptom-based syndrome to a biological construct defined by the presence of amyloid plaques, tau tangles, and neurodegeneration [1]. While amyloid PET identifies early pathology, flortaucipir PET imaging has become a vital tool for clinicians because its signal correlates more closely with regional atrophy and the onset of symptomatic impairment [2, 3]. This imaging modality is now frequently used to determine eligibility for disease-modifying therapies, where the magnitude of tau burden can predict the degree of clinical benefit from treatment [4, 3]. However, the reliability of these biomarkers can be influenced by biological heterogeneity, potentially complicating the assessment of near-term clinical progression [5, 6]. A new study now offers fresh insights into how age and sex-related differences in underlying neuropathology may impact the sensitivity of these essential diagnostic tools.
Demographic Variations in Tracer Uptake
To determine whether low Tau PET signals in older men represent false negatives or a true reflection of biological differences, researchers investigated amyloid-positive individuals with symptomatic Alzheimer’s disease across three distinct cohorts: PET-only, autopsy-only, and PET-to-autopsy. The PET-only cohort consisted of 672 amyloid-β-positive participants presenting with either mild cognitive impairment or dementia, with a mean age of 71.9 (± 8.2) years and a 52.8% male composition. By examining this large clinical sample, the study sought to clarify why certain demographic groups consistently show lower tracer retention despite meeting the clinical criteria for Alzheimer’s disease. The researchers utilized [18F]flortaucipir-PET to measure tau burden, defining Tau-PET-positivity using a predefined threshold of 1.36 SUVr (standardized uptake value ratio, a measure of tracer concentration in the brain relative to a reference region like the cerebellum). In the PET-only cohort, the analysis revealed that older age and male sex were associated with a lower prevalence of Tau-PET-positivity and lower Tau-PET SUVr (all p < 0.05). These findings suggest that the standard imaging threshold may capture different levels of underlying pathology depending on the patient's age and sex, potentially impacting how clinicians interpret scans in the oldest-old and male populations.
Neuropathological Evidence for Lower Tangle Burden
Direct physical evidence from an autopsy-only cohort of 945 participants (mean age 82.7 ± 7.9 years, 54.6% male) provided a biological basis for the imaging trends. All individuals in this cohort had been diagnosed with either mild cognitive impairment or dementia and exhibited moderate-to-frequent scores on the Consortium to Establish a Registry for Alzheimer's Disease (CERAD) scale, which is a standardized neuropathological measure of neuritic plaque density. By utilizing postmortem tissue, the study could correlate clinical demographics with the actual physical presence of tau tangles, bypassing the potential limitations of PET tracer binding kinetics. The analysis revealed that older age and male sex were associated with a lower prevalence of Braak V-VI neuropathology (both p < 0.05). Braak staging is a method used to classify the degree of neurofibrillary tangle spread, where stages V and VI represent advanced, widespread tau pathology throughout the neocortex. Furthermore, among the 598 cases classified as Braak V-VI, older age was significantly associated with lower tangle density (β = -0.38, p < 0.001). This suggests that even when the disease has spread to the same anatomical regions, the absolute concentration of tau tangles is lower in older patients compared to younger ones, providing a biological explanation for the reduced sensitivity of tau-targeted PET scans in the elderly.
Quantifying the Sensitivity Gap in Clinical Practice
Establishing the direct relationship between antemortem imaging and postmortem pathology required a PET-to-autopsy cohort of 85 participants (mean age 81.7 ± 9.2 years, 52.9% male). The temporal reliability of these findings is supported by a median PET-to-post-mortem interval of only 2.6 months, ensuring that the imaging results closely reflected the pathological state of the brain at the time of death. Within this group, a subset of 63 participants also had available data regarding tangle density, allowing for a granular comparison between tracer binding and the physical concentration of neurofibrillary tangles. The study found that Tau-PET demonstrated 100% specificity for detecting Braak V-VI neuropathology across all age-stratified and sex-stratified models, indicating that a positive scan is a highly reliable indicator of advanced Alzheimer pathology. However, the sensitivity of the tracer was significantly compromised by age and sex. For patients under 83 years old, the sensitivity for detecting Braak V-VI stages was 92% (95% confidence interval [CI]: 80% to 100%), but this figure dropped to 42% (95% CI: 23% to 62%) in patients aged 83 and older. A similar disparity was observed between sexes, as Tau-PET sensitivity was 85% (95% CI: 69% to 96%) in females compared to just 48% (95% CI: 28% to 68%) in males. These data suggest that while a positive flortaucipir PET scan confirms advanced pathology, a negative scan in an older male patient may frequently represent a false negative caused by tangle densities falling below the detection threshold.
Clinical Implications for Patient Selection
The reduction in tracer sensitivity among specific demographics appears to be a direct reflection of biological variation in tangle burden rather than technical tracer failure. Among participants confirmed to have advanced Braak V-VI neuropathology, older age was significantly associated with lower Tau-PET SUVr (β = -0.46, p = 0.003), and male participants exhibited lower Tau-PET SUVr compared to females (β = -0.97, p = 0.001). Within the same PET-to-autopsy cohort, older participants demonstrated a trend toward lower tangle density (β = -0.31, p = 0.053), providing a pathological explanation for the diminished PET signal. Crucially, the study found a lack of age and sex interactions, which indicates that the fundamental relationship between [18F]flortaucipir-PET binding and actual tangle density remains consistent across different groups. This confirms that lower [18F]flortaucipir-PET uptake in older men is explained by lower tangle densities rather than a failure of the tracer to bind to available tau. For the practicing clinician, these results necessitate a nuanced interpretation of negative tau PET scans in symptomatic elderly or male patients. Because the tracer accurately reflects tangle density, a negative scan in these populations may not rule out advanced Braak V-VI pathology, but instead may indicate a lower-density burden that falls below the detection threshold of 1.36 SUVr. This has significant implications for treatment eligibility, particularly for emerging therapies that require PET-confirmed tau pathology for enrollment. Clinicians should recognize that the sensitivity of [18F]flortaucipir-PET for detecting advanced tau pathology is biologically limited by the lower tangle density characteristic of older age and male sex, suggesting that a negative result in an 85-year-old man carries different diagnostic weight than the same result in a 65-year-old woman.
References
1. Jack CR, Bennett DA, Blennow K, et al. NIA‐AA Research Framework: Toward a biological definition of Alzheimer's disease. Alzheimer s & Dementia. 2018. doi:10.1016/j.jalz.2018.02.018
2. Malpetti M, Joie R, Rabinovici G. Tau Beats Amyloid in Predicting Brain Atrophy in Alzheimer Disease: Implications for Prognosis and Clinical Trials. Journal of Nuclear Medicine. 2022. doi:10.2967/jnumed.121.263694
3. Malpetti M, Rabinovici G. From Clinical Trials to Memory Clinics, Tau-PET Visual Reads Can Help Diagnosis and Patient Stratification. Neurology. 2023. doi:10.1212/WNL.0000000000207935
4. Jones K, Aakre J, Castillo AM, et al. Eligibility for donanemab trial in a population-based study of cognitive aging. The journal of prevention of Alzheimer's disease. 2025. doi:10.1016/j.tjpad.2025.100088
5. Shand C, Markiewicz PJ, Cash DM, et al. Heterogeneity in Preclinical Alzheimer's Disease Trial Cohort Identified by Image-based Data-Driven Disease Progression Modelling.. medRxiv : the preprint server for health sciences. 2023. doi:10.1101/2023.02.07.23285572
6. Lu M, Pontecorvo MJ, Devous MD, et al. Aggregated Tau Measured by Visual Interpretation of Flortaucipir Positron Emission Tomography and the Associated Risk of Clinical Progression of Mild Cognitive Impairment and Alzheimer Disease: Results From 2 Phase III Clinical Trials.. JAMA neurology. 2021. doi:10.1001/jamaneurol.2020.5505
