ALS is diagnosed through a combination of neurological exams, electrical tests of muscle and nerve function, and imaging or lab work to rule out other conditions. There is no single test that confirms it. The process typically takes about 11.5 months from when symptoms first appear, though it can range anywhere from 7 to 20 months. That delay isn’t due to negligence. ALS shares features with dozens of other conditions, and the diagnosis requires watching how symptoms progress over time.
What Doctors Look for on Exam
The defining feature of ALS is that two separate systems of motor neurons break down at the same time. Upper motor neurons run from the brain down the spinal cord, and lower motor neurons extend from the spinal cord out to the muscles. In most neurological conditions, only one of these systems is affected. In ALS, both are.
Upper motor neuron damage shows up as overactive reflexes, stiff or spastic muscles, and loss of coordination or fine motor control. If the nerves controlling speech and swallowing are involved, slurred speech and difficulty swallowing appear. Lower motor neuron damage looks different: muscles visibly shrink (atrophy) and twitch spontaneously (fasciculations). Weakness can come from either type of nerve damage, which is why it’s often the first symptom people notice, whether in a hand, foot, or the muscles of the mouth and throat.
A neurologist will test reflexes, muscle tone, strength, and coordination across four body regions: the head and neck (bulbar region), the arms, the trunk, and the legs. The pattern of findings across these regions matters. Under the current Gold Coast diagnostic criteria, a person needs upper and lower motor neuron signs in at least one body region, or lower motor neuron problems in at least two regions, along with clear evidence that the impairment is progressive and started after a period of normal function. Crucially, other explanations for the symptoms must be thoroughly excluded.
Electromyography and Nerve Conduction Studies
Electrical testing of the nerves and muscles is one of the most important steps. It can detect motor neuron damage that isn’t yet visible on a physical exam, which is especially valuable early in the disease.
Nerve conduction studies send small electrical impulses along nerves to measure how well signals travel. In ALS, the speed of these signals is usually normal or only slightly reduced, because the insulation around nerve fibers (myelin) stays intact. What does change is the strength of the electrical signal reaching the muscle, which drops as motor neurons die. One key finding: sensory nerves should test completely normal. ALS affects only the motor system, so if sensory nerves are impaired, the diagnosis points elsewhere, toward conditions like peripheral neuropathy. The absence of conduction blocks (spots where a signal stops along a nerve) also helps distinguish ALS from a treatable condition called multifocal motor neuropathy.
The needle EMG portion is more revealing. A thin needle electrode is inserted into muscles across at least three limbs, plus the trunk and sometimes the tongue or jaw muscles. The test looks for two patterns happening simultaneously. Active denervation, meaning muscles are currently losing their nerve supply, shows up as spontaneous electrical discharges: fibrillation potentials, positive sharp waves, and fasciculation potentials firing on their own. Chronic reinnervation, where surviving motor neurons have sprouted new branches to take over orphaned muscle fibers, produces motor unit potentials that are larger and more complex than normal, with decreased recruitment. Finding both active and chronic denervation spread across multiple body regions is highly characteristic of ALS.
Imaging and Blood Tests
MRI scans of the brain and spinal cord are standard in the workup, but their primary purpose is to rule out other diagnoses rather than confirm ALS. An MRI can identify cervical spinal cord compression, multiple sclerosis lesions, tumors at the base of the skull, or other structural problems that could mimic ALS symptoms. Once those are excluded, subtle MRI findings can sometimes support the diagnosis, but they aren’t reliable enough to stand on their own.
Blood and urine tests serve a similar exclusionary role. They screen for conditions like thyroid disease, vitamin B12 deficiency, and autoimmune disorders. In some cases, a lumbar puncture (spinal tap) is performed to check the cerebrospinal fluid. None of these tests will be positive “for” ALS. Their value is in closing the door on other possibilities.
Genetic Testing
Current evidence-based guidelines recommend that all people with ALS be offered genetic testing, regardless of whether they have a known family history of the disease. About 5 to 10 percent of ALS cases are familial, but genetic mutations also appear in people with no family connection to the disease. Testing typically includes a panel of ALS-associated genes, with a particular focus on the C9orf72 gene, which is the most common genetic cause. The rationale is practical: the yield of genetic testing is high enough that results may directly influence treatment decisions as gene-targeted therapies become available.
Conditions That Mimic ALS
Part of the reason diagnosis takes so long is the number of conditions that look like ALS but aren’t. Some of these are treatable, which makes the distinction especially important.
- Multifocal motor neuropathy with conduction block causes progressive weakness without sensory loss, closely resembling ALS, but responds to immune therapy. Nerve conduction studies can usually tell the two apart.
- Kennedy’s disease is a genetic condition affecting lower motor neurons in men, often with facial twitching and breast tissue enlargement. It progresses much more slowly than ALS.
- Cervical spondylotic myelopathy occurs when age-related spinal cord compression produces upper and lower motor neuron signs in the arms and legs. MRI of the spine identifies it.
- Myasthenia gravis can cause weakness and difficulty speaking or swallowing, but it involves the junction between nerve and muscle, not the motor neurons themselves. Specific blood antibody tests and response to medication distinguish it.
- Other mimics include vitamin B12 deficiency, hyperthyroidism, multiple sclerosis, and rare metabolic conditions like adrenomyeloneuropathy.
Why Diagnosis Takes So Long
In a study of over 300 patients, the median time from first symptoms to confirmed diagnosis was 11.5 months. Some people received their diagnosis in 7 months; for others, it stretched past 20. Several factors drive this delay.
Early ALS often affects only one limb or one body region. A person might notice a weak grip, a foot that drags, or slurred speech. These symptoms overlap with far more common conditions, so initial evaluations often head down other diagnostic paths. Carpal tunnel syndrome, pinched nerves, and benign fasciculation syndrome are frequent early misdiagnoses. The diagnostic criteria themselves require evidence that symptoms are progressive, which by definition means watching the disease over time. A single visit rarely provides enough information.
Older classification systems (the revised El Escorial criteria) used a tiered approach, categorizing patients as having “possible,” “probable,” or “definite” ALS based on how many body regions showed upper and lower motor neuron signs. Definite ALS required involvement in three regions. This system was useful for research but created confusion in clinical care, sometimes delaying access to treatment for people who clearly had the disease but didn’t yet meet the highest tier. The newer Gold Coast criteria, now widely adopted, simplified this into a single diagnostic category, which has helped reduce unnecessary delays.
Blood Biomarkers in ALS Diagnosis
A blood protein called neurofilament light chain (NfL) has received significant attention as a potential diagnostic marker. When motor neurons are damaged, they release this structural protein into the bloodstream and spinal fluid. People with ALS tend to have substantially elevated NfL levels.
In research settings, NfL has shown diagnostic sensitivity between 76 and 100 percent and specificity between 75 and 92 percent. Those numbers sound promising, but they come largely from studies comparing ALS patients to healthy controls or people with conditions that are already easy to distinguish from ALS clinically. In a real-world study at a specialized referral clinic, NfL levels above 110.9 pg/ml had a positive predictive value of 0.92 for ALS, meaning 92 percent of people above that threshold did have the disease. But levels below that cutoff had a negative predictive value of only 0.48, meaning NfL couldn’t reliably rule ALS out. In practice, the test mostly confirmed what the clinical evaluation already suggested. Its current strength lies in tracking disease progression and stratifying patients in clinical trials, not in making the initial diagnosis.
The Diagnostic Team
ALS diagnosis and care increasingly happens through multidisciplinary clinics that bring together neurologists, physical therapists, occupational therapists, respiratory therapists, speech-language pathologists, dietitians, social workers, and mental health professionals. The neurologist drives the diagnostic process, but the broader team becomes essential immediately after diagnosis, establishing baselines for breathing capacity, nutrition, mobility, and communication that guide ongoing care. If your evaluation is happening through a general neurology practice, a referral to a specialized ALS or motor neuron disease center can both speed up the diagnostic process and connect you with this full team from the start.