There is no single test that confirms Lou Gehrig’s disease, also known as ALS (amyotrophic lateral sclerosis). Diagnosis relies on a combination of a neurological exam, electrical tests of your nerves and muscles, imaging, blood work, and careful exclusion of other conditions that can look similar. The process takes a median of 11.5 months from the first symptom to a confirmed diagnosis, with most people falling somewhere between 7 and 20 months.
That timeline can feel frustratingly long. The reason is that ALS damages two distinct sets of nerve cells, upper motor neurons in the brain and lower motor neurons in the spinal cord, and doctors need to document damage in both while ruling out every treatable alternative. Here’s what that process actually involves.
The Neurological Exam
The first step is a detailed physical exam by a neurologist, usually over the course of one or more visits. The doctor is looking for a specific pattern: signs of both upper and lower motor neuron damage spreading across different body regions.
Upper motor neuron signs include stiff or spastic muscles, abnormally brisk reflexes, and certain reflex responses like an upgoing toe when the sole of the foot is stroked. Lower motor neuron signs include muscle weakness, visible muscle twitching (fasciculations), shrinking of muscle tissue, and reduced reflexes. The combination of both in the same limb or body region is a hallmark of ALS that most other conditions don’t produce.
The exam covers four regions: the face and throat (bulbar), the arms and shoulders (cervical), the trunk (thoracic), and the legs (lumbosacral). In the bulbar region, the doctor checks for tongue wasting, slurred speech, and difficulty swallowing. In the limbs, they test grip strength, ability to walk, and reflexes. Finding upper and lower motor neuron involvement in multiple regions strengthens the case for ALS. The current Gold Coast diagnostic criteria use a straightforward yes-or-no framework: either the pattern fits ALS, or it doesn’t.
Electromyography and Nerve Conduction Studies
Electrodiagnostic testing is the most important technical step in an ALS evaluation. It consists of two parts, typically done in the same session.
Nerve conduction studies measure how well electrical signals travel along your nerves. Small electrodes are placed on the skin, and brief electrical pulses are sent through motor and sensory nerves. In ALS, motor nerve signals may weaken because nerve cells are dying, but sensory nerves remain normal. That distinction matters because ALS spares the sensory system. If sensory nerves are abnormal, it points toward a different diagnosis like peripheral neuropathy. At minimum, motor and sensory nerves in one upper and one lower limb are tested, along with a specific type of signal called an F wave that checks the health of motor neurons closer to the spinal cord.
The needle EMG portion is more involved. A thin needle electrode is inserted into muscles across at least three limbs, plus a muscle in the tongue or jaw area and muscles along the spine. The needle picks up the electrical activity of individual muscle fibers. In ALS, the test reveals two overlapping patterns. First, there are signs of active nerve damage: spontaneous electrical firing from muscle fibers that have lost their nerve supply, along with visible twitching from motor units firing on their own. Second, there are signs the body has been trying to compensate. Surviving nerve cells sprout new branches to take over orphaned muscle fibers, which creates abnormally large, complex electrical signals. Finding both active damage and this compensatory remodeling across multiple body regions is strong evidence of ALS.
MRI and Imaging
MRI scans of the brain and spinal cord are a standard part of the workup, but their primary role is ruling out other problems rather than confirming ALS. A herniated disc in the neck can compress the spinal cord and cause weakness and stiff reflexes that look remarkably like ALS. Spinal cord tumors, multiple sclerosis lesions, and structural abnormalities can all mimic parts of the clinical picture.
MRI’s greatest contribution to ALS diagnosis has been excluding these spinal “mimic” conditions. Occasionally, high-resolution MRI can pick up changes in the brain’s motor pathways that are consistent with ALS, but this isn’t reliable enough to be diagnostic on its own.
Blood Tests and Lab Work
A panel of blood and sometimes urine tests is drawn to screen for treatable conditions that can masquerade as ALS. These typically include thyroid function (an overactive thyroid can cause muscle weakness and twitching), vitamin B12 levels (deficiency can damage both upper and lower motor neurons), and markers for inflammatory or autoimmune conditions. Specific antibodies, such as anti-GM1 antibodies, help identify multifocal motor neuropathy, a condition that causes progressive weakness but responds to treatment. Levels of very long-chain fatty acids can flag a rare inherited condition called adrenomyeloneuropathy.
None of these blood tests diagnose ALS directly. Their value is in closing the door on alternatives.
Blood Biomarkers
A protein called neurofilament light chain (NfL) has received significant attention as a potential blood-based marker for ALS. When motor neurons are damaged, they release this structural protein into the spinal fluid and bloodstream. People with ALS typically have markedly elevated levels compared to those with other diagnoses.
In studies, blood NfL levels have shown a diagnostic sensitivity of 76 to 100% and specificity of 75 to 92%. A level above a certain threshold carries a positive predictive value of 0.92 for ALS in specialized clinics, meaning 92% of people above that cutoff do have the disease. However, the test performs poorly at ruling ALS out. A level below the threshold had a negative predictive value of only 0.48, meaning about half of people with lower levels still turned out to have ALS. In practice, NfL is largely confirmatory of clinical judgment rather than a standalone diagnostic tool. Its greatest current value is in gauging how aggressively the disease is progressing and in research trials.
Genetic Testing
Current consensus guidelines recommend that all people diagnosed with ALS be offered genetic testing, regardless of whether they have a family history of the disease. About 10% of apparently sporadic ALS cases (those with no known family history) carry identifiable genetic variants, and that number is higher in families where multiple members are affected.
The standard gene panel tests for the four most common ALS-linked genes. The most prevalent is a repeat expansion in a gene called C9orf72, which accounts for roughly 1 in 10 ALS cases in people of European ancestry. The panel also includes SOD1, FUS, and TARDBP. Genetic counseling is recommended before testing, because results can have implications for blood relatives. A positive result doesn’t change the ALS diagnosis itself, but it can influence treatment options, since at least one approved therapy specifically targets a particular genetic variant. It also allows family members to pursue their own testing if they choose.
Muscle Biopsy
Most people being evaluated for ALS will not need a muscle biopsy. It’s reserved for cases where the clinical picture is unclear, particularly when upper motor neuron signs are absent and the pattern could represent either a nerve problem or a muscle disease. Certain conditions like inclusion body myositis (an inflammatory muscle disease) and some forms of muscular dystrophy can look surprisingly similar to ALS in their early stages. A biopsy can distinguish the neurogenic pattern of ALS, where muscle fibers are damaged because the nerves supplying them have died, from the primary muscle destruction seen in myopathies. When the diagnosis is ambiguous, a biopsy can prevent a false ALS diagnosis and identify conditions with different treatments and prognoses.
Conditions That Mimic ALS
Part of what makes ALS testing so thorough is the number of conditions that can imitate it. Kennedy’s disease, a genetic condition affecting men, causes progressive weakness and muscle twitching but progresses much more slowly than ALS. It’s confirmed with a specific genetic test. Multifocal motor neuropathy causes asymmetric weakness, usually in the hands, and can be identified through nerve conduction studies showing conduction block (a specific pattern where the nerve signal drops sharply at one point). Unlike ALS, it responds to immunotherapy.
Cervical spondylotic myelopathy, where arthritis in the neck compresses the spinal cord, is one of the most common mimics. It produces both upper and lower motor neuron signs in the arms and legs, closely resembling ALS, but it shows up clearly on MRI. Vitamin B12 deficiency, thyroid disease, and certain rare metabolic disorders round out the list. Each has its own confirmatory test, and each is treatable, which is exactly why the diagnostic process is so careful.
What the Process Looks Like
If your neurologist suspects ALS, expect the evaluation to unfold over weeks to months rather than days. An initial visit with a neurological exam is typically followed by electrodiagnostic testing, blood work, and an MRI. You may be referred to a specialized ALS center, where neurologists see enough cases to recognize subtle patterns and atypical presentations. Some people receive a clear diagnosis after the first round of testing. Others need follow-up visits spaced weeks or months apart so the doctor can track whether symptoms are progressing in the pattern expected for ALS.
The requirement for progression is built into the diagnostic criteria. ALS spreads from one body region to adjacent ones over time, and documenting that spread is sometimes the final piece of evidence needed. This waiting period is one of the hardest parts of the process, but it exists because misdiagnosis carries serious consequences in both directions: labeling a treatable condition as ALS, or missing ALS when early intervention could matter.