There is no single test that confirms ALS. Diagnosis relies on a combination of a neurological exam, electrical tests of your muscles and nerves, blood work, imaging, and sometimes a spinal tap, all aimed at finding a specific pattern of nerve damage while ruling out other conditions that can look similar. The process typically takes weeks to months and involves multiple appointments.
Why ALS Is Hard to Diagnose
ALS destroys two types of motor neurons: upper motor neurons in the brain and lower motor neurons in the spinal cord. The hallmark of ALS is damage to both types at the same time, which sets it apart from dozens of other neurological conditions. But early on, symptoms often affect just one area, like a weak hand or slurred speech, which can look like a pinched nerve, a spinal cord problem, or an autoimmune condition.
Because no blood test or scan can definitively confirm ALS, doctors use a process of elimination combined with clinical criteria. The newest standard, called the Gold Coast criteria (introduced in 2020), simplified earlier diagnostic frameworks and catches about 94% of ALS cases while still correctly identifying non-ALS conditions roughly 82% of the time. Older criteria were more conservative, correctly diagnosing only 56 to 60% of cases at the “probable” level, which meant many patients waited longer for answers.
The Neurological Exam
The first step is a detailed physical exam by a neurologist. They’re looking for a specific combination of signs that point to both upper and lower motor neuron damage happening together. Upper motor neuron signs include stiff, tight muscles, exaggerated reflexes, and slow movements. Lower motor neuron signs include muscle wasting, weakness, and visible twitching (fasciculations) under the skin.
ALS often starts in the hands, feet, arms, or legs and then spreads. Some people first notice trouble swallowing or slurred speech, which points to the bulbar region (the nerves controlling the mouth and throat). The neurologist will test strength, reflexes, coordination, and speech across multiple body regions, because ALS needs to show progression across regions over time. A single weak limb isn’t enough.
EMG and Nerve Conduction Studies
Electromyography, or EMG, is the most important diagnostic test for ALS. A thin needle electrode is inserted into muscles in several regions of your body, usually the arms, legs, and sometimes the tongue or back muscles. The test picks up electrical signals your muscles produce at rest and during contraction.
In ALS, the EMG shows two things happening simultaneously. First, there’s evidence of ongoing nerve death: the muscle fibers that have lost their nerve supply fire off tiny, abnormal electrical bursts called fibrillation potentials and positive sharp waves. Second, there’s evidence that surviving nerves are trying to compensate by taking over orphaned muscle fibers. This creates abnormally large electrical signals when you contract the muscle, because each remaining nerve is now controlling a bigger group of fibers than normal. Finding both patterns together, active nerve loss and chronic remodeling, is the electrical signature of ALS.
A nerve conduction study is usually done at the same appointment. Small electrical pulses are sent along your nerves through electrodes on the skin, and the speed and strength of the signals are measured. In ALS, the sensory nerves (the ones that carry feeling) are normal. If those nerves are damaged, it points away from ALS and toward a different condition. Motor nerve signals may be slightly reduced in ALS simply because motor neurons have died, but the pattern is distinct from conditions like peripheral neuropathy.
Blood Tests and Imaging
Blood tests don’t diagnose ALS directly, but they help rule out conditions that mimic it. Your doctor will typically check for thyroid problems, vitamin deficiencies, infections like HIV or Lyme disease, and inflammatory markers that could point to autoimmune conditions. In some cases, antibody tests help exclude myasthenia gravis or chronic inflammatory neuropathies, both of which can cause progressive weakness.
An MRI of the brain and spinal cord is standard. It’s used to rule out structural problems like a herniated disc pressing on the spinal cord (cervical myelopathy), spinal cord tumors, or areas of demyelination that could indicate multiple sclerosis. In ALS, the MRI is often normal or shows only subtle changes, which is itself a useful clue. If the MRI reveals a clear structural cause for the symptoms, the diagnosis shifts away from ALS.
Lumbar Puncture
A spinal tap isn’t always required, but it’s sometimes done to analyze cerebrospinal fluid, the liquid surrounding your brain and spinal cord. The main purpose is to look for signs of inflammation, infection, or cancer that could explain the symptoms. Elevated white blood cells or abnormal protein levels can point toward inflammatory conditions like Guillain-Barré syndrome or chronic inflammatory demyelinating polyneuropathy. In ALS, the fluid is typically unremarkable.
Genetic Testing
About 5 to 10% of ALS cases run in families (familial ALS). Variants in over 30 genes have been linked to the disease, but four genes account for the majority. The most common is C9orf72, responsible for up to 40% of familial cases and about 7% of cases with no family history. SOD1 accounts for roughly 12% of familial cases. Two others, TARDBP and FUS, each contribute about 4% of familial cases.
Genetic testing is offered when there’s a family history of ALS or a related condition called frontotemporal dementia. It’s also increasingly offered to people with no family history, since some gene variants show up in apparently sporadic cases. The test is a blood draw. Results can affect treatment options, since certain therapies target specific genetic mutations, and they carry implications for family members who may want to know their own risk.
Conditions That Must Be Ruled Out
The list of ALS mimics is long, and it varies depending on your symptoms. If your main problem is weakness and stiffness in the limbs, doctors need to exclude cervical myelopathy (spinal cord compression in the neck), spinal cord tumors, hereditary spastic paraplegia, and HIV-related spinal cord damage. If the weakness is more floppy than stiff, the possibilities include pinched nerve roots, peripheral neuropathies, and muscle diseases like inclusion body myositis.
When symptoms come on rapidly over days or weeks rather than months, the differential shifts dramatically. Myasthenia gravis, Guillain-Barré syndrome, West Nile virus, and even botulism can all cause sudden motor neuron problems. ALS, by contrast, progresses gradually over months, so the timeline of your symptoms is one of the most important diagnostic clues.
Multifocal motor neuropathy deserves special mention because it can closely resemble ALS, with progressive weakness and muscle wasting, but it’s treatable with immunoglobulin therapy. Nerve conduction studies can usually distinguish the two, which is one reason the electrical testing is so critical.
What the Timeline Looks Like
From the first appointment to a confirmed diagnosis, the process often takes three to six months, sometimes longer. Part of this is practical: you need multiple tests scheduled across different visits. But part of it is intentional. Neurologists often want to observe whether symptoms are progressing and spreading, because that progressive spread is one of the defining features of ALS. A single snapshot may not be enough.
You’ll likely have at least two or three visits with a neurologist, an EMG session lasting one to two hours, an MRI, and several rounds of blood work. If the picture is still unclear, a repeat EMG a few months later can show whether the pattern of nerve damage is advancing. Some patients are referred to specialized ALS centers where neurologists who see the disease regularly can weigh the full picture more quickly.