Learn What ALS Screening Tests Can Reveal

Understanding ALS and Why Screening Tests Matter

Amyotrophic lateral sclerosis, commonly known as ALS, is a progressive neurological disease that affects nerve cells in the brain and spinal cord. These nerve cells, called motor neurons, control voluntary muscles throughout the body. When motor neurons gradually die, the muscles they control weaken and eventually stop working. Most people with ALS die from respiratory failure when the disease affects the muscles used for breathing, typically within 2 to 5 years of symptom onset, though some people live 10 years or longer.

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According to the ALS Association, approximately 5,000 people in the United States are diagnosed with ALS each year, and about 16,000 Americans live with the disease at any given time. ALS can strike anyone—it affects people of all races, ethnicities, and socioeconomic backgrounds. The average age of diagnosis is between 55 and 60 years old, though the disease can occur in people as young as their 20s.

Screening tests and diagnostic procedures play a critical role in identifying ALS because early diagnosis allows patients and their families to plan for care, explore treatment options, and make informed decisions about their future. There is no single test that can definitively diagnose ALS. Instead, doctors use a combination of clinical evaluations, blood tests, imaging studies, and electrical tests to rule out other conditions and confirm ALS. Understanding what these tests measure and how they work can help patients and their families navigate the diagnostic process more effectively.

The diagnostic journey for ALS often begins with a primary care doctor or neurologist noticing symptoms like muscle weakness, muscle twitching (fasciculations), or difficulty speaking or swallowing. From there, patients may be referred to a specialist neurologist who performs more detailed evaluations. Learning about the different screening and diagnostic tests available can reduce confusion and help people understand what to expect during medical appointments.

Practical Takeaway: ALS is a serious disease that requires prompt medical evaluation. If you or someone you know experiences unexplained muscle weakness, persistent muscle twitching, difficulty with speech or swallowing, or difficulty with fine motor tasks that lasts more than a few weeks, schedule an appointment with a neurologist for further evaluation.

Electromyography (EMG) and Nerve Conduction Studies

Electromyography, or EMG, is one of the most important tests used to help diagnose ALS. This test measures the electrical activity of muscles at rest and during contraction. A neurologist or technician inserts a thin needle electrode into the muscle being tested. The electrode picks up electrical signals that the muscle produces when it contracts. These signals are displayed on a monitor and produce a distinctive sound. For someone with ALS, the EMG typically shows signs of denervation—a pattern indicating that motor neurons have degenerated and can no longer send proper signals to muscles.

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During an EMG test, a patient may feel a brief, mild discomfort when the needle enters the skin, similar to a pinprick. The procedure usually lasts 30 to 60 minutes and tests multiple muscles, often in the arms, legs, and face. The neurologist selects muscles based on the patient's symptoms and the suspected pattern of nerve damage. Multiple muscles must be tested because ALS affects different muscles at different rates and in different patterns depending on the type of ALS.

Nerve conduction studies (NCS) often accompany EMG testing. These tests measure how quickly electrical signals move along nerves. Small electrodes placed on the skin deliver mild electrical pulses to stimulate a nerve. Other electrodes then record how quickly the signal travels. In ALS, nerve conduction is typically normal, which helps doctors distinguish ALS from other nerve disorders where conduction is slowed or blocked. This distinction is crucial because conditions like Guillain-Barré syndrome or peripheral neuropathy can mimic some ALS symptoms but have different patterns on nerve conduction studies.

The combination of abnormal EMG findings and normal nerve conduction studies strongly suggests ALS rather than other nerve diseases. However, early in the disease course, EMG results may be normal or only show subtle changes. Patients sometimes need to undergo EMG testing more than once, weeks or months apart, to establish a pattern consistent with ALS. This is why the diagnostic process for ALS often takes time and why multiple tests are necessary.

Practical Takeaway: EMG and NCS are standard tests when ALS is suspected. These tests are uncomfortable but not dangerous and provide critical information about how well nerves and muscles are communicating. Patients should inform their doctor about any medications they take, as some can affect test results.

Blood Tests and Biomarker Screening

Blood tests have become increasingly important in ALS screening and diagnosis over the past several years. Researchers have identified blood biomarkers—measurable substances in the blood that indicate disease activity—that can help support an ALS diagnosis. The most validated biomarker is phosphorylated tau (p-tau), a protein that accumulates in the brains of ALS patients. Another important biomarker is neurofilament light chain (NfL), a protein released from damaged nerve cells into the bloodstream. Elevated levels of these biomarkers in the blood can indicate motor neuron degeneration.

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The advantage of blood biomarker tests is that they are simple, non-invasive procedures requiring only a small blood draw, similar to routine blood work. Unlike EMG or imaging studies, blood biomarker testing can be done in an outpatient clinic or primary care office without specialized equipment. Studies have shown that blood biomarkers can be detected in people with ALS and may help distinguish ALS from other conditions that cause similar symptoms. A 2021 study published in The Lancet found that phosphorylated tau was present in 91 percent of people with ALS, compared to only 1 percent of controls.

However, it is important to understand that blood biomarker tests are not yet standard or widely available in all medical settings. Insurance coverage varies, and some tests must be ordered through specialized research centers or laboratories. Additionally, while elevated biomarkers strongly suggest ALS, they are not present in 100 percent of ALS cases, and having elevated biomarkers alone does not confirm ALS diagnosis. These tests are most useful when combined with clinical evaluation and other diagnostic tests.

Blood tests also reveal information about muscle damage through enzymes like creatine kinase (CK). When muscles break down, they release CK into the bloodstream. Moderately elevated CK levels can support a diagnosis of ALS, though this is a non-specific finding that can occur in other muscle diseases. Routine blood work may also check kidney and liver function, thyroid hormone levels, and vitamin B12 levels to rule out other treatable conditions that can cause ALS-like symptoms.

Practical Takeaway: Blood biomarker tests represent an emerging tool in ALS diagnosis but are not yet standard in all settings. If your doctor mentions blood testing for phosphorylated tau or neurofilament light chain, understand that positive results support but do not confirm ALS diagnosis. Discuss with your doctor whether these tests are covered by insurance and how the results will be used alongside other diagnostic tests.

Imaging Tests: MRI and Their Role in ALS Diagnosis

Magnetic resonance imaging (MRI) is a critical imaging tool used in evaluating suspected ALS. MRI uses powerful magnets and radio waves to create detailed images of the brain and spinal cord without using radiation. When ALS is suspected, an MRI of the brain and cervical (neck) and thoracic (upper back) spinal cord is typically ordered. The MRI helps doctors visualize the motor cortex—the brain region that controls voluntary movement—and check for abnormalities that might explain the patient's symptoms.

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In people with ALS, MRI may show certain characteristic findings, though these are not present in every case. One pattern doctors look for is increased signal intensity in the corticospinal tract, the bundle of nerve fibers that carries signals from the brain to the spinal cord to control movement. Another finding may be atrophy, or shrinkage, of the motor cortex. These findings support an ALS diagnosis, though they may not be visible in the early stages of disease. The primary purpose of brain and spinal cord MRI in ALS evaluation is to rule out other conditions—such as spinal cord compression, multiple sclerosis, stroke, or brain tumors—that can produce similar symptoms.

An MRI scan typically takes 30 to 60 minutes and requires the patient to lie still