There is no single test that confirms ALS. Diagnosis relies on a combination of clinical examination, electrical nerve testing, imaging, and blood work, all aimed at finding a pattern of progressive motor nerve damage while ruling out other conditions that look similar. The process takes a median of 11.5 months from the first symptom to a confirmed diagnosis, with most people waiting somewhere between 7 and 20 months.
That long timeline reflects both the complexity of the disease and the fact that early symptoms, like a weak grip or slurred speech, overlap with dozens of other conditions. Understanding what goes into the diagnostic process can help you know what to expect and why each step matters.
What Doctors Look for on Physical Exam
ALS damages two types of motor neurons: upper motor neurons (which run from the brain down to the spinal cord) and lower motor neurons (which run from the spinal cord out to the muscles). A neurologist looks for signs of damage to both systems, because that combination is the hallmark of the disease.
Upper motor neuron damage produces overactive reflexes, stiffness or spasticity in the limbs, and a positive Babinski sign, where stroking the sole of the foot causes the big toe to extend upward and the other toes to fan out. In healthy adults, the toes curl downward. Lower motor neuron damage shows up as muscle wasting, visible twitching (fasciculations), weakness in individual muscles, and reduced reflexes in the affected area. Finding both sets of signs in the same body region is a strong signal.
The neurologist examines multiple body regions: the arms, legs, the muscles controlling speech and swallowing (bulbar region), and the trunk. The current Gold Coast diagnostic criteria require progressive motor impairment plus upper and lower motor neuron signs in at least one body region, or lower motor neuron signs in at least two regions. Every other plausible explanation for the symptoms must also be investigated and excluded.
Electromyography and Nerve Conduction Studies
Electromyography (EMG) is the single most important test in the ALS workup. A thin needle electrode is inserted into muscles in at least three limbs, the trunk, and sometimes the muscles of the tongue or jaw. The test picks up electrical patterns that reveal whether motor neurons are dying and whether surviving neurons are trying to compensate.
In ALS, EMG typically shows two overlapping patterns. Acute denervation appears as spontaneous electrical discharges from muscles that have lost their nerve supply: fibrillations, positive sharp waves, and fasciculation potentials. Chronic denervation shows up as abnormally large, complex motor unit potentials, a sign that remaining healthy nerve cells have sprouted new branches to take over orphaned muscle fibers. Seeing both acute and chronic changes spread across multiple body regions is a strong indicator.
Nerve conduction studies run alongside the EMG. In ALS, the speed at which nerves carry signals is usually normal or only mildly slowed, because the insulating coating around nerve fibers stays intact. What changes is the strength of the signal reaching the muscle, which can drop as motor neurons die. Critically, sensory nerves remain normal in ALS. If the test shows sensory nerve damage, the neurologist looks for a different diagnosis, such as peripheral neuropathy.
These electrodiagnostic findings matter for more than confirmation. Earlier diagnostic criteria (the revised El Escorial criteria) had a sensitivity of only about 40% at the first visit, meaning they missed more than half of true cases. Newer criteria that give more weight to EMG findings, like the Awaji criteria, raised that sensitivity to roughly 69%. The improvement is especially dramatic early in the disease: within the first six months of symptoms, the newer approach catches 61% of cases compared to just 12% with the older system.
MRI and Lab Work: Ruling Out Mimics
MRI of the brain and spinal cord does not diagnose ALS directly. Its role is to rule out conditions that can mimic it. A herniated disc pressing on the spinal cord in the neck, for instance, can cause weakness and stiffness in the arms and legs that looks very much like ALS. Spinal cord tumors, multiple sclerosis, and structural abnormalities can all produce overlapping symptoms. MRI identifies or eliminates these possibilities.
Blood and urine tests screen for metabolic and inflammatory conditions. The list of ALS mimics is long and includes diseases as varied as multisystem atrophy, certain genetic ataxias, enzyme deficiencies, and chronic inflammatory nerve conditions. Some of these are treatable, which is one reason a thorough exclusion process is not just a formality but a genuine safeguard.
Blood-Based Biomarkers
A protein called neurofilament light chain (NfL) has attracted significant attention as a potential blood-based marker for ALS. When motor neurons break down, NfL spills into the bloodstream. People with ALS show median blood levels around 218 pg/ml, compared to roughly 45 pg/ml in people with other diagnoses. At a cutoff of about 111 pg/ml, the test correctly identifies approximately 77% of ALS cases and has a positive predictive value of 92%, meaning that when it’s elevated, there’s a high chance the person truly has ALS.
The limitation is on the other side: a level below that cutoff does not reliably rule ALS out. Slower-progressing cases can have NfL levels that overlap with non-ALS conditions, giving the test a negative predictive value of only about 48%. For now, NfL is used as a supporting piece of evidence rather than a standalone diagnostic tool. It can add confidence to an uncertain clinical picture, but it cannot replace EMG and clinical examination.
Genetic Testing
About 5 to 10% of ALS cases run in families, but genetic factors also play a role in a portion of cases with no family history. Four genes account for most known genetic ALS. A repeat expansion in the C9orf72 gene is the most common, found in roughly 40% of familial cases and 6% of cases without a family history. Mutations in the SOD1 gene appear in about 15% of familial and 2% of sporadic cases. Two other genes, TARDBP and FUS, each account for about 3% of familial cases and under 1% of sporadic ones.
Some ALS centers now recommend testing all newly diagnosed patients for C9orf72 and SOD1 variants regardless of family history, because targeted therapies for SOD1-related ALS already exist and clinical trials increasingly select patients based on genetic profile. A positive genetic result can also have implications for family members who may want to pursue their own testing or genetic counseling.
Why Diagnosis Takes So Long
The median diagnostic delay of nearly a year is not primarily caused by slow testing. Most of that time is consumed by the disease itself needing to declare its pattern. ALS often starts with a single vague symptom: a foot that drags, a hand that fumbles, speech that occasionally slurs. At that stage, it can resemble carpal tunnel syndrome, a pinched nerve, or dozens of other common conditions. Patients often see an orthopedist or primary care physician first and may undergo unrelated treatments before a neurologist enters the picture.
Even once a neurologist is involved, the diagnostic criteria require evidence that the disease is progressive and spreading. A single abnormal EMG in one limb is not enough. The neurologist needs to see the involvement expand over time, through repeated clinical exams or follow-up EMG studies. This is inherently a waiting game, though the newer Gold Coast criteria have simplified the categories to a straightforward “ALS or not ALS” decision, which may help reduce delays at the margins.
Tracking Progression After Diagnosis
Once ALS is confirmed, disease progression is tracked using staging systems. The King’s College staging system divides ALS into five stages. Stages 1 through 3 correspond to how many body regions show weakness, wasting, spasticity, or difficulty with speech and swallowing. Stage 1 means one region is affected; stage 3 means three. Stage 4 is reached when a person needs a feeding tube for nutritional support or noninvasive ventilation for breathing. Stage 5 is death. This framework helps clinicians, patients, and families understand where someone stands in the disease course and plan care accordingly.