How Is the NfL Blood Test Used When Evaluating bvFTD?

The neurofilament light (NfL) blood test is a tool that helps doctors determine if there is active injury to the nerve cells in your brain and nervous system. Because the early symptoms of behavioral variant frontotemporal dementia (bvFTD)—such as personality changes, apathy, or poor judgment—can be difficult to interpret and sometimes look like depression or bipolar disorder, getting an accurate diagnosis is challenging. The NfL test can add an important piece of evidence to the diagnostic puzzle. Elevated levels of NfL suggest there is active neuroaxonal injury (damage to the nerve cells), which can support a diagnosis of a neurodegenerative condition like bvFTD when combined with other tests ^[1]. However, it is not a standalone test: a normal result does not completely rule out bvFTD, and an elevated result does not specifically prove it is bvFTD ^[2].

How the NfL Test Works

To understand the test, it helps to know a little about nerve cell structure. Neurofilaments are structural proteins that act like the internal scaffolding of your nerve cells (neurons) ^[3]. When nerve cells are healthy, they release very little of this protein ^[3]. However, when nerve cells are injured or degenerating, this scaffolding breaks down. Fragments of the NfL protein leak into the cerebrospinal fluid (the fluid surrounding your brain and spinal cord) and eventually cross into the bloodstream ^[4].

Using highly sensitive laboratory tests, doctors can measure the concentration of these protein fragments in a simple blood draw ^[4]. This means the NfL blood test acts as a sensitive biomarker for active neuroaxonal injury (damage to the long extensions of nerve cells) ^[5].

Evaluating Midlife Behavioral Changes

When significant behavioral or personality changes appear in midlife, the most critical step is determining whether they stem from a primary psychiatric, neurological, or mixed cause. These symptoms are real and can be profoundly difficult for individuals and families to navigate, and distinguishing their root cause is essential for proper care.

Research studies show that, on average, blood NfL levels are significantly higher in groups of people with bvFTD compared to those with primary psychiatric conditions ^[1]. A review of multiple studies found that the test’s ability to distinguish bvFTD from psychiatric disorders is moderately to highly accurate in research settings, with sensitivities (ability to correctly identify those with the disease) ranging from 65% to 100%, and specificities (ability to correctly identify those without the disease) ranging from 69% to 96% ^[6].

However, these figures come from specific research studies, and in real-world clinical practice, the test is used to support—not replace—a comprehensive evaluation.

What a Result May Mean

Because different laboratories use different testing methods and units, there is no single, universal “normal” number. Your doctor must interpret your exact number based on your age and the specific laboratory’s reference range ^[7].

Important Limitations of the Test

While the NfL test is a powerful tool, it has important limitations:

• It is not specific to bvFTD: Elevated levels can be caused by Alzheimer’s disease, Lewy body dementia, multiple sclerosis, a recent stroke, seizures, or even a traumatic brain injury ^[8][9].
• Age affects the results: As we get older, our baseline NfL levels naturally rise. Therefore, the test must be interpreted using an age-adjusted laboratory reference range, rather than a simple cut-off ^[7][5].
• Other health factors matter: Your kidney function, body mass index (BMI), and other medical conditions can affect how much NfL is detected in your blood ^[11][12].
• It requires comprehensive context: An NfL result is most useful as one puzzle piece alongside a detailed neurological exam, neuropsychological testing (detailed memory and thinking tests), and brain imaging like an MRI ^[13].