ALS is diagnosed primarily through clinical examination and a process of ruling out other conditions, not a single definitive test. The median time from first symptoms to a confirmed diagnosis is about 11.5 months, with many patients waiting well over a year. That delay reflects both the complexity of the disease and the number of conditions that can look similar in the early stages.
What Doctors Look For
ALS causes damage to two distinct sets of nerve cells: upper motor neurons in the brain and lower motor neurons in the spinal cord. A neurologist’s physical exam focuses on finding signs that both systems are breaking down at the same time, which is the hallmark of the disease.
Upper motor neuron damage shows up as abnormally brisk reflexes (including reflexes that appear in muscles that are visibly weak and wasted), stiffness in the limbs, and slow or poorly coordinated movements. Lower motor neuron damage produces muscle weakness, visible muscle wasting, and twitching (fasciculations). Finding both types of damage together, especially when the pattern is spreading over time, is what points a neurologist toward ALS rather than other neurological conditions.
The current diagnostic standard, known as the Gold Coast criteria, requires three things: progressive motor impairment that followed previously normal function, evidence of upper and lower motor neuron problems in at least one body region (or lower motor neuron problems in at least two regions), and test results that rule out other diseases. The body is divided into four regions for this purpose: bulbar (face, tongue, throat), cervical (arms), thoracic (trunk), and lumbosacral (legs).
The Role of EMG and Nerve Conduction Studies
Electromyography, or EMG, is the most important test in the diagnostic workup. A thin needle electrode is inserted into muscles to record their electrical activity. In ALS, the EMG reveals a specific pattern: the remaining motor neurons are firing rapidly to compensate for those that have died, producing unusually large electrical signals. At the same time, the test picks up spontaneous electrical discharges from muscle fibers that have lost their nerve supply. These tiny discharges, called fibrillation potentials and positive sharp waves, are signs of active, ongoing nerve loss.
Fasciculation potentials, the electrical signature of muscle twitches, also carry diagnostic weight equal to other signs of nerve damage. Finding this combination of active nerve loss alongside evidence that the body has been trying to compensate (by enlarging surviving nerve connections) in multiple body regions strongly supports the diagnosis. Nerve conduction studies are typically performed alongside EMG to check for other problems, particularly conditions that cause nerve conduction to slow or block, which would point toward a different diagnosis.
Imaging and Blood Tests
MRI scans of the brain and spinal cord are a standard part of the workup, but not because they confirm ALS. In most people with ALS, the MRI looks normal or shows only subtle, nonspecific changes like faint bright signals along the motor pathways on certain sequences. The real purpose of imaging is to rule out structural problems that can mimic the disease.
Cervical spondylotic myelopathy, where arthritis in the neck compresses the spinal cord and nerve roots, is one of the most important conditions to exclude. Because the upper and lower motor neuron pathways run close together in the cervical spine, compression there can produce a mix of stiffness, weakness, and wasting that closely resembles ALS. Some patients have even undergone spinal surgery before the correct diagnosis was made. Blood and urine tests screen for thyroid disorders, vitamin B12 deficiency, HIV, and other systemic conditions that can affect motor neurons.
Conditions That Mimic ALS
The list of ALS mimics is long, and working through it accounts for much of the diagnostic delay. Multifocal motor neuropathy with conduction block is a particularly important one because it causes progressive weakness, wasting, and fasciculations, usually starting in one hand or arm. Unlike ALS, it responds to treatment, which makes distinguishing between them critical. Nerve conduction studies typically reveal the telltale conduction blocks that separate this condition from ALS.
Other mimics include Kennedy’s disease (a genetic condition affecting lower motor neurons in men), myasthenia gravis, inclusion body myositis, multiple sclerosis, post-polio syndrome, and paraneoplastic syndromes linked to cancer. Each has features that overlap with ALS in some ways but diverge in others. A thorough evaluation often requires weeks or months of repeated exams to document whether symptoms are progressing in the pattern typical of ALS.
Genetic Testing
About 5 to 10 percent of ALS cases are familial, meaning there is a known family history. Genetic testing focuses primarily on two genes. Repeat expansions in the C9orf72 gene are the most common genetic cause, found in roughly 7 to 8 percent of ALS patients in large studies. Mutations in the SOD1 gene account for another 1 to 2 percent. Notably, nearly half of patients found to carry disease-causing gene variants in one large French cohort had no family history of ALS, which means genetic testing can be informative even when there is no obvious hereditary pattern.
Genetic results don’t change the clinical diagnosis itself, but they matter for prognosis, family planning, and increasingly for treatment. Targeted therapies now exist for certain SOD1 mutations, making genetic testing a practical step rather than a purely academic one.
Emerging Blood Biomarkers
Neurofilament light chain (NfL) is a protein released into the blood when nerve cells are damaged. In ALS, levels are typically elevated, and blood tests for NfL have shown diagnostic sensitivity ranging from 76 to 100 percent and specificity between 75 and 92 percent depending on the study and the cutoff used. At a commonly used threshold, the test correctly identifies about 77 to 86 percent of people with ALS while correctly ruling it out in about 75 percent of people without the disease.
Those numbers are promising but imperfect. NfL rises in other neurological conditions too, so an elevated level doesn’t prove ALS on its own. It works best as one piece of the puzzle, particularly in early or uncertain cases where the clinical picture hasn’t fully declared itself.
Why Diagnosis Takes So Long
The median diagnostic delay of 11.5 months (with a quarter of patients waiting 20 months or more) stems from several factors. Early symptoms are often vague: a foot that drags, a hand that fumbles, slurred speech that comes and goes. Primary care doctors may initially attribute these to a pinched nerve, carpal tunnel syndrome, or normal aging. Referral to a neurologist, then to a specialist ALS center, adds time. Once there, the neurologist often needs repeat examinations spaced weeks or months apart to confirm that the weakness is truly progressive and spreading.
Specialist ALS clinics use a multidisciplinary model. During an initial visit, patients typically see a neurologist and clinical nurse specialist, along with physical therapists, occupational therapists, respiratory specialists, and sometimes neuropsychologists. This team approach serves a dual purpose: it helps nail down the diagnosis while simultaneously connecting patients with support services early. Occupational therapy and physiotherapy are the services used most frequently at that first visit, reflecting how quickly the disease can affect daily function even before the diagnosis is finalized.