ALS is detected through a combination of clinical examination, electrical nerve testing, and a process of ruling out other conditions that can mimic its symptoms. There is no single test that confirms ALS on its own. Most people receive a diagnosis 10 to 16 months after their first symptoms appear, partly because the early signs overlap with many other neurological conditions and partly because doctors need to observe progression over time.
What Doctors Look for in a Physical Exam
The clinical exam is the starting point, and it centers on one specific finding: signs that both groups of motor neurons are deteriorating at the same time. Your brain has “upper” motor neurons in the motor cortex that send movement signals downward, and “lower” motor neurons in the brainstem and spinal cord that directly activate your muscles. ALS attacks both.
When upper motor neurons are damaged, you develop stiffness, loss of fine motor control, and exaggerated reflexes. A doctor tapping your knee might get an unusually strong jerk response, for example. When lower motor neurons are lost, the result is muscle weakness, visible wasting, and small involuntary twitches called fasciculations. Finding both sets of signs in the same patient, without any loss of sensation (numbness or tingling), is the hallmark clinical pattern of ALS. Sensation stays intact because the disease targets only the neurons that control movement.
Under the current diagnostic framework known as the Gold Coast criteria, a diagnosis requires three things: documented progressive motor decline that started after a period of normal function, evidence of both upper and lower motor neuron dysfunction in at least one body region, and thorough testing to exclude other causes. These criteria catch about 94% of ALS cases, a significant improvement over older standards that missed roughly 40% of patients who truly had the disease.
Electrical Testing of Nerves and Muscles
An electromyography test, or EMG, is one of the most important tools in the diagnostic process. A thin needle electrode is inserted into different muscles to record their electrical activity at rest and during contraction. In ALS, the EMG picks up abnormal spontaneous firing, including fibrillation potentials (tiny electrical discharges from muscle fibers that have lost their nerve supply) and fasciculation potentials (the electrical signature of those visible muscle twitches).
The test also reveals how the surviving motor neurons are compensating. As some neurons die, the remaining ones sprout new connections to “adopt” orphaned muscle fibers. This shows up on EMG as motor unit signals that are larger than normal, longer in duration, and irregular in shape. The pattern of fewer neurons firing faster to keep up with demand is a telltale sign of ongoing motor neuron loss. Nerve conduction studies are typically performed alongside EMG to confirm that the nerve fibers carrying sensation are working normally, which helps separate ALS from conditions that affect both motor and sensory nerves.
Fasciculations alone are not enough to diagnose ALS. They occur in many completely benign situations, including after too much caffeine or during periods of stress. What matters is whether fasciculations appear alongside other EMG abnormalities and clinical signs of progressive weakness.
Tests That Rule Out Other Conditions
A large part of reaching an ALS diagnosis involves eliminating the long list of conditions that can look similar. The specific tests depend on where symptoms first appeared and how quickly they developed.
- MRI of the brain and spinal cord checks for structural problems like compressed discs in the neck (cervical myelopathy), spinal cord tumors, or demyelinating diseases like multiple sclerosis. Any of these can cause weakness and abnormal reflexes that overlap with ALS symptoms.
- Blood tests screen for infections, inflammatory conditions, thyroid disorders, and other metabolic problems. HIV-related myelopathy, for instance, can mimic upper motor neuron signs.
- Nerve conduction studies help identify conditions like chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) or multifocal motor neuropathy, both of which cause progressive weakness but respond to treatment.
If symptoms come on rapidly over days or weeks rather than months, doctors consider a different set of possibilities entirely, including myasthenia gravis, Guillain-Barré syndrome, and West Nile virus infection. The gradual, relentless progression of ALS is itself a diagnostic clue.
Blood and Spinal Fluid Biomarkers
While no blood test can definitively diagnose ALS, a protein called neurofilament light chain (NfL) is increasingly used to support the diagnosis. Neurofilaments are structural proteins inside nerve cells, and when motor neurons break down, NfL spills into the surrounding fluid. Measured in spinal fluid, NfL levels distinguished ALS patients from healthy people with 97% sensitivity and 95% specificity. Blood-based measurements were slightly less precise but still useful, with about 90% sensitivity.
NfL levels are not exclusive to ALS. They rise in other neurodegenerative diseases too. But in the right clinical context, a very high NfL level adds confidence to the diagnosis, and the test can be especially helpful early in the disease when clinical signs are still limited to one body region. Researchers have also identified several proteins in spinal fluid that are consistently lower in ALS patients, though these are not yet part of routine clinical testing.
Genetic Testing
About 5% of ALS cases are inherited, following a clear pattern within families. The most commonly tested genes include SOD1, C9orf72, FUS, and TDP-43. Mutations in C9orf72 are the most frequent genetic cause and are also linked to frontotemporal dementia, which can overlap with ALS in some patients.
Genetic testing is most relevant when there is a family history of ALS or frontotemporal dementia, but it can also be offered to people with apparently sporadic disease. Identifying a specific mutation matters beyond the diagnosis itself. Some gene-targeted therapies are now in development or already available for certain mutations, making genetic information directly relevant to treatment options. Genetic counseling is recommended alongside testing because a positive result has implications for blood relatives.
Why Diagnosis Takes So Long
The 10-to-16-month gap between first symptoms and diagnosis frustrates patients and families, but several factors make it difficult to shorten. Early ALS often starts with vague complaints: a hand that fumbles with buttons, a foot that drags slightly, a voice that sounds different. These symptoms overlap with far more common conditions like pinched nerves, carpal tunnel syndrome, or normal aging. Many people see their primary care doctor, then an orthopedist or physical therapist, before being referred to a neurologist.
Even once a neurologist is involved, the diagnostic criteria require evidence of progression. A single snapshot of symptoms is rarely enough. Doctors often need repeat visits over weeks or months to document that weakness is spreading to new body regions, which is the pattern that separates ALS from conditions that stay localized. The absence of a single definitive test means the diagnosis is ultimately clinical, built from the accumulation of exam findings, EMG results, and excluded alternatives.