There is no single test that confirms ALS (Lou Gehrig’s disease). Diagnosis relies on a neurologist finding evidence that both upper and lower motor neurons are degenerating, while systematically ruling out other conditions that can look similar. The process typically takes 10 to 16 months from when symptoms first appear, with a median delay of about 12 months. That long timeline reflects the complexity of the diagnosis, not medical negligence.
What Neurologists Look for on Exam
ALS damages two distinct sets of nerve cells: upper motor neurons, which send signals from the brain down the spinal cord, and lower motor neurons, which carry those signals from the spinal cord out to the muscles. A diagnosis requires evidence that both are failing, and this combination is what sets ALS apart from most other neurological conditions.
Signs of upper motor neuron damage include abnormally brisk reflexes, stiff or tight muscles that resist being stretched, 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 a healthy adult, the toes curl downward. Another hallmark is the “clasp knife” phenomenon: when a doctor quickly bends your arm or leg, the muscle suddenly locks up with resistance before releasing, almost like opening a pocketknife.
Signs of lower motor neuron damage are different. These include visible muscle wasting, weakness, and fasciculations, the small involuntary twitches you might notice under the skin. As motor neurons die, the muscles they controlled lose their nerve supply and begin to shrink.
Under the current Gold Coast diagnostic criteria, the standard used internationally, the diagnosis comes down to a straightforward question: does this person have progressive motor neuron dysfunction involving both upper and lower motor neurons, with no better explanation? If yes, the diagnosis is ALS. If not, it isn’t. The older classification system used confusing categories like “possible” or “probable” ALS, which caused unnecessary distress. The Gold Coast criteria eliminated those labels.
The Role of EMG Testing
Electromyography, or EMG, is the most important test in the diagnostic workup. It involves inserting thin needle electrodes into muscles to record their electrical activity, and it can detect lower motor neuron damage that isn’t yet visible on a physical exam.
In ALS, EMG reveals two things happening simultaneously. First, there is ongoing denervation: muscle fibers that have lost their nerve connection fire off tiny spontaneous electrical signals called fibrillation potentials. Second, there is chronic reinnervation: surviving motor neurons sprout new branches to take over the orphaned muscle fibers, creating abnormally large, complex electrical signals. This pattern of active damage alongside attempted repair is characteristic of ALS.
To count as meaningful, EMG abnormalities need to show up across multiple body regions. A thorough study samples muscles in at least three limbs, each supplied by different nerve roots, along with muscles in the trunk and the bulbar region (tongue, throat, and jaw). Finding widespread changes across regions that couldn’t be explained by a single pinched nerve or localized problem is what points toward ALS rather than something more contained.
Ruling Out Conditions That Mimic ALS
Several treatable conditions can produce symptoms that closely resemble ALS, and excluding them is a critical part of the diagnostic process. This is one reason the workup takes time.
- Cervical spondylotic myelopathy. Degenerative changes in the neck spine can compress both the spinal cord and nerve roots, producing a mix of upper and lower motor neuron signs that looks remarkably like ALS. Since spinal arthritis is common in the same age group that develops ALS (average onset around 65), incidental findings on imaging can further complicate things. Clues that point away from spondylosis include fasciculations in distant areas like the tongue or legs, and emotional changes like involuntary laughing or crying.
- Multifocal motor neuropathy. This rare immune-mediated condition causes slowly progressive, asymmetric weakness, often starting with wrist or finger drop. It can look like early ALS, but sensory function stays intact, bulbar muscles are spared, and nerve conduction studies typically reveal characteristic “conduction blocks” where the signal stalls along a nerve. Identifying this condition matters because it responds to treatment with immunoglobulin therapy.
- Kennedy’s disease. This genetic condition, which affects men in their 30s and 40s, causes weakness and wasting in the face, throat, and limb muscles with prominent tongue twitching. Unlike ALS, it progresses much more slowly and is accompanied by signs of hormonal disruption like breast tissue enlargement. A genetic test confirms it.
MRI and Blood Tests
MRI scans of the brain and spine do not diagnose ALS directly. Their primary role is to rule out structural problems, including spinal cord compression, tumors, multiple sclerosis lesions, and other causes of myelopathy. In some ALS patients, MRI may show subtle changes along the motor pathways in the brain or slight brain atrophy, but these findings are not specific enough to confirm the diagnosis on their own.
Blood tests serve a similar exclusionary purpose. They check for conditions that can cause ALS-like weakness, including thyroid disorders, vitamin B12 deficiency, infections, and certain cancers. Nerve conduction studies, which measure how fast electrical signals travel along nerves, help distinguish ALS from peripheral neuropathies and conditions like multifocal motor neuropathy. In ALS, nerve conduction speeds generally remain normal because the problem lies in the motor neuron cell bodies, not in the nerve insulation.
Blood Biomarkers
Neurofilament light chain (NfL) is a protein released into the blood when nerve cells are damaged. People with ALS tend to have significantly elevated levels, with a median around 218 pg/ml in one large study, compared to about 45 pg/ml in people with alternative diagnoses. Levels above roughly 111 pg/ml had a 92% chance of correctly indicating ALS in a specialized clinic setting.
That sounds promising, but the test has real limitations. It cannot reliably rule ALS out: a level below that threshold had only a 48% chance of correctly identifying someone who did not have the disease. In practice, NfL largely confirms what an experienced neurologist already suspects rather than providing independent diagnostic power. Its greatest current value is in measuring how aggressively the disease is progressing and as a tracking tool in clinical trials, not as a standalone screening test.
Why Diagnosis Takes So Long
The 10 to 16 month average delay between first symptoms and diagnosis frustrates patients and families, but several factors make it difficult to shorten. Early ALS symptoms, like a weak grip, a foot that drags, or slightly slurred speech, overlap with dozens of more common conditions. Neurologists need time to observe whether weakness is progressive and spreading to new body regions, which is the hallmark that separates ALS from static problems. The longest documented median delay in one U.S. study was 27 months, while the shortest, from a French national database linked to specialized ALS clinics, was just over 9 months.
Referral to a specialized ALS center can meaningfully reduce that wait. These multidisciplinary clinics bring together neurologists, respiratory therapists, physical therapists, speech pathologists, and other specialists under one roof. Studies show they diagnose ALS faster than general neurology pathways, communicate the diagnosis more effectively, and coordinate care better from the start. Patients at these clinics are typically reviewed every two to three months, with the team staying in contact between visits. If you or someone you know is being evaluated for possible ALS, getting to one of these centers early in the process is one of the most impactful steps you can take.