There is no single test that confirms ALS. The diagnosis relies on a combination of clinical examination, electrical nerve and muscle testing, blood work, and imaging, all aimed at finding evidence of motor neuron damage while ruling out other conditions that can look remarkably similar. This process typically takes about 11.5 months from when symptoms first appear, though it can stretch well beyond 20 months for some people.
The reason no definitive test exists is that ALS presents in a wide variety of ways, and there are at least 77 other conditions that can mimic its symptoms. Some of those mimics are treatable, which makes the process of elimination not just thorough but essential.
What Doctors Are Looking For
ALS damages two types of nerve cells: upper motor neurons in the brain and lower motor neurons in the spinal cord. A diagnosis requires evidence that both are deteriorating and that the damage is spreading over time. The most current diagnostic framework, known as the Gold Coast criteria, requires three things: progressive worsening of motor function after a period of normal function, signs of both upper and lower motor neuron damage in at least one body region (or lower motor neuron damage in two or more regions), and thorough testing to rule out anything else that could explain the symptoms.
Upper motor neuron signs include stiff, spastic muscles and exaggerated reflexes. Lower motor neuron signs include muscle wasting, weakness, and visible twitching (fasciculations). A neurologist looks for these during a detailed physical exam, testing reflexes, muscle tone, strength, and coordination across different body regions: the face and throat, arms, trunk, and legs.
Electromyography: The Closest Thing to a Core Test
If one test comes closest to being central to ALS diagnosis, it’s electromyography, or EMG. This test uses a thin needle electrode inserted into muscles to measure their electrical activity. In ALS, the EMG reveals a specific pattern: fewer motor units firing than normal (because nerve cells have died), with the remaining units firing rapidly to compensate. The surviving motor units also tend to be abnormally large, a sign that healthy nerves have taken over territory from dying ones.
The EMG also picks up spontaneous electrical activity in the muscle fibers that have lost their nerve supply. These signals, called fibrillation potentials and positive sharp waves, indicate active, ongoing nerve damage. Fasciculation potentials, the electrical signature of visible muscle twitches, carry the same diagnostic weight. Finding both chronic nerve remodeling and active denervation across multiple body regions strongly supports an ALS diagnosis.
A companion test, nerve conduction studies, sends small electrical impulses along the nerves to measure how fast and completely signals travel. In ALS, sensory nerves remain normal, which helps distinguish it from conditions like peripheral neuropathy. Motor nerve conduction is also largely preserved early on, without the signal blockages or slowing seen in treatable conditions like multifocal motor neuropathy. This distinction matters because multifocal motor neuropathy can look like ALS but responds to treatment.
Blood Tests and Spinal Fluid Analysis
Blood tests in an ALS workup aren’t looking for ALS itself. They’re screening for other explanations: thyroid disorders, vitamin B12 deficiency, infections, and autoimmune conditions that can all cause weakness or muscle wasting. These are routine and inexpensive, and catching one of these conditions early can change the outcome entirely.
A lumbar puncture (spinal tap) is sometimes performed to analyze cerebrospinal fluid. This can help rule out inflammatory conditions like multiple sclerosis or certain infections. Researchers have found that levels of neurofilament light chain, a protein released when nerve fibers break down, are significantly elevated in people with ALS. In one study, spinal fluid levels of this protein distinguished ALS patients from healthy individuals with 97% sensitivity and 95% specificity. Blood levels were also elevated, though somewhat less precise. However, neurofilament levels also rise in other neurological diseases like Parkinson’s and multiple sclerosis, so the test signals nerve damage broadly rather than pointing specifically to ALS.
MRI and Imaging
MRI scans of the brain and spine are a standard part of the workup, but again, their purpose is exclusion rather than confirmation. A brain MRI can reveal strokes, tumors, or demyelinating plaques from multiple sclerosis. A spinal MRI can show herniated discs, spinal cord compression, or structural abnormalities at the base of the skull, all of which can produce symptoms that overlap with ALS.
The overlap between spinal problems and ALS is a particularly common source of misdiagnosis. Herniated discs and spinal stenosis are extremely common in older adults, and when someone with early ALS also happens to have a bulging disc on their MRI, the weakness in their arm or leg may be attributed to the disc rather than to a motor neuron disease. In documented case series, patients with ALS were initially diagnosed with cervical or lumbar radiculopathy based on imaging findings that turned out to be incidental. The key clue that something else is going on is when the pattern of weakness doesn’t quite fit the nerve root that the disc is compressing, or when symptoms progress to areas the disc can’t explain.
Genetic Testing
Current guidelines recommend that all people diagnosed with ALS be offered genetic testing, regardless of whether anyone else in their family has had the disease. About 10% of ALS cases run in families, but genetic mutations also appear in at least 10% of people with no family history (so-called sporadic ALS). The most common genetic cause in people of European ancestry is a repeat expansion in the C9orf72 gene, which accounts for roughly 1 in 10 ALS cases overall.
Genetic testing doesn’t typically play a role in making the initial diagnosis. Its value comes after diagnosis, for two reasons. First, certain genetic mutations are now linked to specific treatments. Second, knowing your genetic status has implications for family members who may want to understand their own risk. Genetic counseling is recommended alongside testing to help make sense of the results, especially since some mutations show incomplete penetrance, meaning not everyone who carries the mutation develops the disease.
Why Diagnosis Takes So Long
The median time from first symptoms to confirmed diagnosis is 11.5 months, with a range that spans from about 7 months on the faster end to 20 months or more for others. Several factors stretch this timeline. People over 60 take about 51% longer to receive a diagnosis than younger patients. Sporadic cases (no family history) take 46% longer than familial ones. And limb-onset ALS, which starts with weakness in an arm or leg, takes 45% longer to diagnose than bulbar-onset ALS, which starts with speech or swallowing difficulties.
Part of the delay is unavoidable. Doctors need to document that symptoms are progressing, which requires time and repeat visits. Early symptoms like a weak grip, a dragging foot, or slurred speech can each point to dozens of other conditions. And because there’s no single confirmatory test, the diagnosis often isn’t secure until enough alternative explanations have been investigated and dismissed. The challenge for neurologists is balancing thoroughness against the reality that every month of delay is a month without access to treatment and support planning.