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PubMed This is a summary of 20 peer-reviewed journal articles Updated
Neurology

Can a Blood Test Diagnose ALS? NfL and Genetic Tests

At a Glance

No single blood test can definitively diagnose ALS. However, neurologists use the Serum NfL blood test to measure nerve damage and predict disease progression, along with genetic testing to identify mutations like SOD1 that may qualify patients for targeted therapies.

In this answer

2 sections

01

The NfL Blood Test: Measuring Nerve Damage

02

Genetic Blood Tests: Finding the Cause

Going through the diagnostic process for ALS can be physically and emotionally exhausting. It is completely natural to wonder if a simple blood draw could replace the daunting and sometimes uncomfortable nerve and muscle tests (like an EMG) you are being asked to undergo.

The direct answer is that no single blood test can definitively diagnose amyotrophic lateral sclerosis (ALS) on its own [1][2]. A blood test can tell your doctor that nerve damage is happening somewhere in your body, but it cannot pinpoint exactly where or how the nerves are failing. That is why the electromyography (EMG) test remains essential—it maps out precisely which nerves and muscles are affected, allowing doctors to confirm the specific pattern of ALS [2][3].

However, blood tests are now a standard and critical part of the modern ALS diagnostic workup. Your neurologist will likely use two specific types of blood tests to help build a complete picture of your health.

The NfL Blood Test: Measuring Nerve Damage

Think of neurofilaments as the internal scaffolding of your nerve cells. When nerve cells are injured or begin to break down, these structural proteins spill into the fluid surrounding your brain and spinal cord, eventually making their way into your bloodstream [4]. The Serum Neurofilament Light (NfL) blood test measures the amount of these proteins.

While elevated NfL levels are a strong indicator of active nerve damage, they are not exclusive to ALS. High levels also show up in other neurological conditions, such as multiple sclerosis or certain types of dementia [5][6]. Because the test is not specific to one disease, it cannot replace a clinical exam [2].

Despite this limitation, your care team may order the NfL test to:

  • Speed up the diagnostic process: Reaching an ALS diagnosis often takes a long time. High NfL levels provide objective evidence of nerve damage, which, when combined with imaging like an MRI, helps doctors rule out other structural causes or “ALS mimics” that typically do not cause elevated NfL levels [7][8].
  • Predict disease progression: Your baseline NfL levels can give your doctor clues about how the disease might behave over time. Higher levels at diagnosis are generally associated with a faster progression of symptoms, while lower levels suggest a slower course [9][10].
  • Monitor treatments: In clinical trials for new therapies, researchers measure NfL to see if a drug is successfully protecting nerve cells [11][12]. For example, the drug tofersen has been shown to lower NfL concentrations in the blood, indicating a reduction in nerve damage [13][14]. (Note that tofersen is an FDA-approved targeted therapy exclusively for ALS patients who have a specific genetic mutation called SOD1 [15]).

Is the NfL test available to me?
The NfL test is transitioning from being primarily a research tool to being available through major commercial labs [16]. Neurologists often order it during the initial workup or shortly after an EMG to help confirm the diagnosis. However, because interpretation requires specific age-adjusted reference ranges [17], it is most commonly ordered by specialized ALS clinics. Insurance coverage can vary depending on whether it is billed for initial diagnosis or ongoing monitoring, so it is worth asking your doctor’s office to check your benefits.

Genetic Blood Tests: Finding the Cause

Another vital blood test you will likely be offered is a genetic panel. While a genetic test cannot diagnose active ALS on its own (since someone can carry a gene without ever developing symptoms), it is a crucial tool if you are already experiencing symptoms [18].

A simple blood draw can check for known ALS-associated mutations, such as C9orf72, SOD1, and FUS [19]. Identifying a specific genetic cause is becoming increasingly important because it determines whether you are eligible for targeted genetic therapies (like tofersen for the SOD1 mutation) and can inform your family about potential hereditary risks [15][20]. Standard clinical practice is moving toward offering genetic testing to all ALS patients, regardless of whether they have a known family history of the disease [18].

Common questions in this guide

Can a blood test definitively diagnose ALS?
No single blood test can diagnose ALS on its own. While blood tests can show that nerve damage is occurring somewhere in the body, doctors still rely on exams like an EMG to map out exactly which nerves and muscles are affected to confirm the diagnosis.
What is the NfL blood test for ALS?
The Serum Neurofilament Light (NfL) test measures structural proteins that leak into the bloodstream when nerve cells break down. It helps doctors confirm active nerve damage, predict disease progression, and monitor how well treatments are working.
Do I need a genetic test if I have ALS?
Standard clinical practice now recommends offering genetic testing to all ALS patients, even without a family history. Identifying a specific genetic mutation, like SOD1, can determine if you are eligible for targeted genetic therapies.
Will the NfL test replace my EMG?
No, an NfL blood test cannot replace an EMG. The blood test confirms that nerve damage is happening, but the EMG is essential to pinpoint exactly where and how the nerves are failing to confirm an ALS diagnosis.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Would an NfL blood test help rule out other conditions in my specific case, or establish a baseline for my progression?
  2. 2.Is genetic testing recommended for me, and does your clinic use a specific specialty lab to run this panel?
  3. 3.How do my blood test results fit together with my EMG and MRI findings?
  4. 4.Are there any clinical trials evaluating new treatments that use NfL levels to track progress, and do I qualify?
  5. 5.Does my insurance typically cover advanced biomarker tests like the serum NfL?

Questions For You

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Related questions

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This page is for informational purposes only and does not replace professional medical advice. Always consult your neurologist for ALS diagnostic testing and interpretation of your results.

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