There is no single test that confirms ALS. Diagnosis relies on a neurological exam, electrical tests of your muscles and nerves, and a series of tests to rule out other conditions that look similar. The process typically takes about 11 months from the time symptoms first appear, partly because early signs can be subtle and partly because so many other possibilities need to be excluded first.
Start With a Neurologist
Your first step is getting a referral to a neurologist, ideally one who specializes in neuromuscular diseases. A general practitioner can order basic blood work and imaging, but ALS diagnosis requires a specialist’s hands-on clinical exam and expertise in distinguishing motor neuron disease from its many lookalikes. If you’re near an ALS Association Certified Center or clinic, these multidisciplinary teams are set up specifically for this kind of evaluation. You can search for one through the ALS Association’s Center and Clinic Locator on their website.
During your first visit, the neurologist will perform a detailed physical exam looking for the hallmark of ALS: signs of damage to both upper and lower motor neurons at the same time. Upper motor neuron damage shows up as exaggerated reflexes, stiffness (spasticity), and poor coordination. Lower motor neuron damage shows up as muscle wasting, weakness, and visible twitching under the skin called fasciculations. Finding both types of damage in the same person is the central clue.
The neurologist will also check for specific reflex abnormalities. These include the Babinski sign (where the big toe extends upward when the sole of the foot is stroked) and the Hoffman sign (an involuntary finger flick triggered by tapping a fingernail). These reflexes are normal in infants but abnormal in adults, and their presence points to upper motor neuron involvement.
The EMG: The Most Important Diagnostic Test
Electromyography, or EMG, is the single most critical test in an ALS workup. It measures the electrical activity inside your muscles using a thin needle electrode inserted into different muscle groups. The test isn’t exactly comfortable, but it gives the neurologist information no other test can provide.
In a healthy muscle, nerve signals arrive in an orderly pattern. In ALS, the EMG reveals a specific combination of problems: fewer motor units firing (because motor neurons have died), the remaining units firing rapidly to compensate, and abnormally large electrical signals from surviving nerve cells that have taken over the work of dead ones. The test also picks up spontaneous electrical discharges, small bursts of activity called fibrillation potentials that happen when muscle fibers lose their nerve supply. Fasciculation potentials, the electrical signature of visible muscle twitches, carry the same diagnostic weight.
For a diagnosis, the neurologist needs to find these abnormalities in at least two body regions. The body is divided into four regions for this purpose: bulbar (face, tongue, throat), cervical (arms and hands), thoracic (trunk), and lumbosacral (legs and feet). In a limb, the abnormalities must appear in at least two muscles controlled by different nerves and nerve roots, which helps rule out a single pinched nerve or localized problem.
A nerve conduction study is usually done alongside the EMG. Small electrical pulses are sent along your nerves while sensors measure how fast and how strongly the signals travel. In ALS, nerve conduction speed is relatively preserved, but the strength of the signal reaching the muscle may be reduced. This pattern helps distinguish ALS from conditions like multifocal motor neuropathy, where nerve conduction is blocked at specific points along the nerve.
Tests That Rule Out Other Conditions
A significant part of ALS testing is actually about proving you don’t have something else. Many treatable conditions can mimic ALS symptoms, and your neurologist will systematically work through these possibilities.
MRI scans of the brain and spinal cord are standard. Their primary role is exclusion: they can reveal cervical spondylosis (spinal cord compression from degenerative neck changes), syringomyelia (a fluid-filled cyst in the spinal cord), multiple sclerosis plaques, or tumors. Any of these can produce weakness, stiffness, or muscle wasting that overlaps with ALS symptoms. Cervical spondylosis is one of the most common mimics, though it typically comes with prominent neck pain and bladder or bowel issues that ALS does not cause early on.
Blood and urine tests screen for a range of conditions. These may check for vitamin B12 deficiency, thyroid dysfunction, hyperparathyroidism, HIV, and inflammatory markers. In some cases, a lumbar puncture (spinal tap) is done to analyze spinal fluid for signs of infection or inflammation. None of these tests confirm ALS directly, but normal results help narrow the field.
The full list of ALS mimics is long. It includes multifocal motor neuropathy with conduction block, hereditary spastic paraparesis, adrenomyeloneuropathy, and adult polyglucosan body disease. Each has features that set it apart from ALS. Adult polyglucosan body disease, for instance, involves cognitive decline, sensory loss, and bladder problems alongside motor neuron signs. Multiple sclerosis can occasionally affect both upper and lower motor neurons, but imaging will typically show characteristic brain and spinal lesions. Your neurologist considers all of these based on your specific symptom pattern.
Genetic Testing
Current consensus guidelines recommend that all people with ALS be offered genetic testing, regardless of whether there is a known family history of the disease. This is a relatively recent shift. About 1 in 10 ALS cases in people of European ancestry are caused by a repeat expansion in a gene called C9orf72, and many of those individuals have no family history of ALS or related conditions because the gene shows incomplete penetrance (not everyone who carries it develops symptoms).
The recommended panel screens at minimum four genes: C9orf72, SOD1, FUS, and TARDBP. These carry the strongest evidence and the highest diagnostic yield. SOD1 testing is particularly important now because an FDA-approved treatment, tofersen, specifically targets ALS caused by SOD1 mutations. Identifying a genetic cause can directly change your treatment options.
Genetic testing is typically done through a blood draw and can be ordered by your neurologist. Genetic counseling is recommended before and after testing, since results can have implications for blood relatives.
Emerging Blood Biomarkers
A protein called neurofilament light chain (NfL) is increasingly being measured in specialized centers as part of the ALS evaluation. When motor neurons are damaged or die, they release NfL into the spinal fluid and blood. People with ALS tend to have significantly higher NfL levels than people with ALS-mimic conditions, which makes it a useful tool for supporting or casting doubt on a diagnosis.
NfL levels also carry prognostic information: higher levels are associated with faster disease progression and shorter survival. However, NfL is not specific to ALS. It rises in many neurological conditions, including Alzheimer’s disease, multiple sclerosis, and traumatic brain injury. There is also no FDA-cleared assay for routine clinical use yet, so testing is mainly available at academic medical centers and specialized ALS clinics rather than standard labs.
Why Diagnosis Takes So Long
The median diagnostic delay from first symptom to confirmed diagnosis is about 11 months, and for many people it takes longer. There are several reasons for this. Early ALS symptoms, like a foot drop, hand weakness, or slurred speech, overlap with dozens of more common conditions. Many people see orthopedists, physical therapists, or primary care doctors before being referred to a neurologist. The diagnostic criteria themselves require that abnormalities be present in multiple body regions, and in early disease the signs may only be apparent in one.
There is also no single blood test or scan that lights up positive for ALS. The diagnosis is built on accumulating evidence: the right pattern of upper and lower motor neuron signs, EMG findings in multiple regions, and the absence of a better explanation. Neurologists are deliberately cautious because an ALS diagnosis carries enormous weight, and misdiagnosis of a treatable condition would be a serious failure.
If you are in the early stages of evaluation and the neurologist says the picture is not yet clear, that does not mean your symptoms are being dismissed. It often means they need to watch how things evolve over the next few months, repeat testing, or wait for abnormalities to declare themselves in additional body regions. Keeping a written log of your symptoms, including when they started, how they have changed, and which specific muscles are affected, can help your neurologist track progression and reach a diagnosis more efficiently.