Lou Gehrig’s disease, known medically as ALS (amyotrophic lateral sclerosis), is diagnosed through a combination of neurological exams, electrical nerve tests, and the systematic exclusion of other conditions that can look similar. There is no single blood test or scan that confirms it. The median time from first symptoms to a confirmed diagnosis is about 11.5 months, and roughly 4 in 10 patients are initially given a different diagnosis before ALS is identified.
Why Diagnosis Takes So Long
ALS is fundamentally a diagnosis of exclusion. The disease attacks both upper motor neurons (which run from the brain down the spinal cord) and lower motor neurons (which run from the spinal cord out to the muscles). Many other conditions can damage one or both of these pathways, and doctors must rule those out before confirming ALS. Early symptoms like a weak grip, tripping while walking, or slurred speech overlap with dozens of other neurological problems.
In a large study of 304 patients, the median total diagnostic time was 11.5 months, with a wide range. Some people received a diagnosis within 7 months, while others waited 20 months or longer. Part of this delay comes from the disease itself: symptoms often start in one small area, like a hand or foot, and it takes time for the pattern of widespread motor neuron damage to become clear enough to distinguish from other causes.
The Neurological Exam
The physical exam is the cornerstone of an ALS evaluation. A neurologist looks for the hallmark combination: signs of upper motor neuron damage and lower motor neuron damage occurring together in the same person.
Lower motor neuron signs include muscle weakness, visible shrinking (atrophy) of muscle tissue, muscle twitching (fasciculations), and reduced muscle tone. These reflect the death of nerve cells that directly control muscle fibers. Upper motor neuron signs point to damage higher up in the nervous system: abnormally brisk reflexes, stiff or spastic muscles, and slow, poorly coordinated movements. One particularly telling finding is a reflex that remains strong or even exaggerated in a muscle that is visibly weak and wasted. In most conditions, a weak muscle has weak reflexes. In ALS, the upper motor neuron damage keeps those reflexes firing even as the lower motor neurons die off.
The neurologist also checks for specific pathological reflexes, like the Babinski sign, where the big toe fans upward when the sole of the foot is stroked. In a healthy adult, the toe curls downward. An upward response signals upper motor neuron damage.
The Gold Coast Diagnostic Criteria
Doctors use a formal set of requirements called the Gold Coast criteria to confirm an ALS diagnosis. These criteria require three things to be present simultaneously:
- Progressive motor decline: The person must have worsening motor function over time, starting from a period of previously normal movement.
- Upper and lower motor neuron involvement: Both types of damage must be detectable in at least one body region (bulbar, cervical, thoracic, or lumbosacral). If only lower motor neuron signs are found, they must appear in at least two separate body regions.
- No better explanation: Testing must rule out other diseases that could account for the symptoms.
These criteria simplified an older, more complex system and made it easier to reach a diagnosis earlier in the disease course, which matters for getting patients into treatment and clinical trials sooner.
Electromyography and Nerve Conduction Studies
Electromyography (EMG) is one of the most important tests in an ALS evaluation. A thin needle electrode is inserted into muscles in multiple body regions to record their electrical activity. In ALS, the EMG reveals a distinctive two-part pattern: evidence of ongoing nerve damage and evidence that remaining healthy nerves are trying to compensate.
The ongoing damage shows up as fibrillation potentials and positive sharp waves, which are spontaneous electrical discharges from muscle fibers that have lost their nerve supply. Fasciculation potentials, the electrical signature of muscle twitches, are also common. The compensatory changes appear as abnormally large motor unit potentials. These occur because surviving nerve cells sprout new branches to take over muscle fibers abandoned by dying neurons, creating oversized electrical signals. To be thorough, the test samples muscles across at least three limbs plus the trunk and bulbar (mouth and throat) muscles.
Nerve conduction studies are performed alongside EMG, and they serve a different purpose. By stimulating nerves with small electrical pulses and measuring the response, doctors can check whether the nerve’s insulating coating (myelin) is intact. In ALS, myelin is preserved, so nerve conduction speeds are generally normal. This helps distinguish ALS from conditions like multifocal motor neuropathy, where the myelin is damaged and electrical signals get blocked at specific points along the nerve. Finding a “conduction block” on this test is a red flag that something other than ALS is causing the weakness.
Sensory nerve conduction should be normal in ALS because the disease spares the nerves responsible for sensation. If sensory responses are abnormal, it suggests a different diagnosis like peripheral neuropathy.
MRI and Lab Work
MRI scans of the brain and spinal cord are a standard part of the workup, though they are primarily used to rule out other diagnoses rather than to confirm ALS. An MRI can reveal spinal cord compression from herniated discs, tumors, or spinal stenosis, all of which can mimic ALS symptoms. Degenerative changes in the cervical spine are an especially common source of diagnostic confusion, since the upper and lower motor neuron pathways run close together in the neck. Compression there can produce a mix of weakness, atrophy, and brisk reflexes that looks very similar to ALS. High-resolution MRI can sometimes detect ALS-related changes directly, but this is not reliable enough to serve as a standalone diagnostic tool.
Blood and urine tests check for other treatable conditions. These typically screen for thyroid dysfunction, vitamin deficiencies, infections, and inflammatory markers. In cases where multifocal motor neuropathy is suspected, testing for specific antibodies (anti-GM1) can help distinguish it from ALS. A lumbar puncture may be performed in some cases to analyze spinal fluid.
Conditions That Mimic ALS
Several conditions can closely resemble ALS, and some of them are treatable, which makes accurate diagnosis critical. Cervical spondylotic myelopathy, where arthritis in the neck compresses the spinal cord and nerve roots, is one of the most common mimics. It can cause weakness, muscle wasting, and brisk reflexes in the arms and legs. Clues that point toward ALS over neck problems include fasciculations appearing in areas far from the neck, like the tongue or legs, and the presence of emotional lability (sudden, involuntary laughing or crying).
Multifocal motor neuropathy typically starts with weakness in a single hand or forearm, often with muscle twitching and cramps. Unlike ALS, it responds to treatment, which is why identifying conduction blocks on nerve studies and checking for antibodies is so important. Other mimics include Kennedy disease (a genetic condition causing slowly progressive weakness), myasthenia gravis, inclusion body myositis, and certain spinal muscular atrophies.
Genetic Testing
Current evidence-based guidelines recommend that all people diagnosed with ALS be offered genetic testing, regardless of whether they have a family history of the disease. This is a relatively recent shift. In populations of European ancestry, a genetic repeat expansion in a gene called C9orf72 is the most common genetic cause, accounting for about 1 in 10 ALS cases. Genetic variants in SOD1, FUS, and TARDBP are also tested. Identifying a specific genetic cause matters for two practical reasons: an FDA-approved therapy now exists for people with SOD1 mutations, and knowing your genetic status may open the door to clinical trials targeting specific genetic pathways.
Genetic testing is most commonly done through a panel that screens multiple ALS-associated genes at once. Genetic counseling is recommended alongside testing to help patients and family members understand the implications of results, particularly since some mutations show incomplete penetrance, meaning not everyone who carries the variant will develop the disease.
Emerging Blood-Based Markers
A protein called neurofilament light chain (NfL) is showing promise as a blood-based marker for ALS. Neurofilaments are structural proteins inside nerve cells, and when those cells are damaged or dying, NfL spills into the blood and spinal fluid at elevated levels. In people who carry certain high-risk genetic mutations, NfL levels rise 6 to 12 months before symptoms appear and predict the onset of clinical ALS with high accuracy. While NfL testing is not yet part of routine diagnostic criteria, it is increasingly used in research settings and clinical trials to track disease activity and may eventually help shorten the diagnostic timeline.