Tear Fluid: A Novel Biomarker for Detecting Parkinson’s Disease

Researchers have identified tear fluid as a potential non-invasive specimen for detecting alpha-synuclein ($\alpha$Syn) seeding activity in patients with Parkinson’s disease (PD). This development suggests a possible shift away from the invasive matrices typically required to detect the misfolded protein species associated with the disorder.

Parkinson’s disease is a neurodegenerative disorder characterized by the intraneuronal accumulation of misfolded $\alpha$-synuclein. Detecting the seeding-competent species of this protein is critical for diagnosis, but the methods used to find them often require invasive procedures.

Analysis of Tear Fluid using RT-QuIC

In a study published April 10, 2026, in Nature, researchers evaluated the use of tear fluid (TF) using a technique known as real-time quaking-induced conversion (RT-QuIC). The study included 44 patients with Parkinson’s disease and 32 matched healthy controls.

The results indicated that PD patients exhibited significantly higher fluorescence signals throughout the RT-QuIC amplification curve compared to healthy controls, with a p-value of less than 0.0001. The PD group also showed a shorter lag phase, higher MedianRFU (median relative fluorescence units), higher MaxRFU (maximum relative fluorescence units), and larger AUFC (area under fluorescence curve) values, all with p-values below 0.05.

The study utilized the Youden index to select optimal cut-offs for subject-level readouts. The best trade-offs for these metrics were observed as follows:

• MaxRFU $\ge$ 2/4: 59.1% sensitivity and 65.6% specificity.
• MedianRFU $\ge$ 3/4: 40.9% sensitivity and 78.1% specificity.
• AUFC $\ge$ 3/4: 47.7% sensitivity and 71.9% specificity.

A sensitivity analysis was performed by excluding healthy controls who exhibited potential risk or prodromal features. This adjustment increased the specificity of the tests to 75% for MaxRFU, 86.2% for MedianRFU, and 79.3% for AUFC.

Seeding Amplification and Fibrillar Structures

Separate research published February 18, 2026, in NPJ Parkinsons Dis. Employed an $\alpha$Syn seeding amplification assay ($\alpha$SynSAA) to analyze tear fluid. This study found that seeding activity was detected in 67% of the tear fluid samples from patients with Parkinson’s disease.

Samples from individuals without synucleinopathy remained negative. To verify these findings, researchers used electron microscopy on the seeding-positive end products, which revealed the presence of fibrillar structures.

Clinical Implications and Limitations

The ability to detect $\alpha$Syn seeding activity in a biofluid as accessible as tears could lead to earlier detection and monitoring of the disease. Currently, the lack of reliable biomarkers for Parkinson’s disease complicates early diagnosis and the monitoring of drug-based interventions.

The Nature study noted that while the specificity of the TF-based RT-QuIC is promising, the sensitivity remains suboptimal. The researchers concluded that further optimization is required before the assay can be used as a standard diagnostic tool for synucleinopathies.

Exploratory subgroup analyses from the same study also suggested that there may be phenotype-related variability in how patients respond to the test, indicating that the effectiveness of tear fluid detection may vary across different presentations of the disease.

The underlying hypothesis for this research, as noted in studies funded by the Michael J Fox Foundation, is that because Parkinson’s affects multiple non-motor systems, the secretion of specific proteins into tears may be altered. This profile of protein composition in tear fluid can be measured cost-effectively and non-invasively compared to traditional methods.