How Do Doctors Diagnose ALS?

Medically Reviewed by Arefa Cassoobhoy, MD, MPH on November 17, 2020

If you or your doctor suspect that you may have amyotrophic lateral sclerosis (ALS), also called Lou Gehrig's disease, you naturally want to find out what’s causing your symptoms.

The signs of ALS -- muscle weakness or twitching, slurred speech, or trouble with physical tasks -- can also point to other conditions.

There’s no single test that can diagnose ALS. It takes a variety of exams, tests, and scans to figure out whether you have ALS or something else.

Physical Exam

Your first step is a full exam by a neurologist. This is a doctor who specializes in disorders of the nervous system, including your brain and spinal cord.

This exam will include lots of questions about your symptoms, health history, and family. Bring notes with you so it will be easier to answer their questions.

In a physical exam, your neurologist will also look for the signs of ALS, including:

  • Muscle weakness, often on only one side of the body
  • Slurred or slowed speech and other signs of muscle weakness in your mouth and tongue
  • Muscle twitches
  • Muscles that have shrunk in size, have unusual reflexes, or are tight and rigid
  • Emotional changes such has out-of-control laughing and crying or a loss of good judgment or social skills

Blood and Urine Tests

These won’t detect ALS, but common lab tests can be used to rule out other diseases that have the same kinds of symptoms. Your blood samples and urine may be used to test for:

In some cases, a doctor may also use a what’s called a lumbar puncture, or spinal tap, to take fluid from your spine to look for problems.

In rare cases where ALS runs in a family, genetic tests may be run to figure out whether a gene mutation is linked to your disease.


Scans such as magnetic resonance imaging, or MRI, can’t directly diagnose ALS. That’s because people with the condition have normal MRI scans. But they are often used to rule out other diseases.

For instance, a spinal cord tumor or herniated disk in the neck can cause some symptoms that mimic ALS but will show up on a scan of the spine and neck, ruling out ALS as the cause of the symptoms.

Muscle and Nerve Tests

If basic lab tests don’t point toward a different health issue, your neurologist can use more advanced tests. They’re called “electrophysiological tests,” and doctors can use them to confirm that the way your muscles and nerves are acting fits the definition of ALS.

These tests can show abnormal results if you have ALS, but your doctor could also decide from the results that you have damage to your nerves or a muscle disease that’s not ALS.

These tests include:

Electromyography: EMG is one of the most important tests used to diagnose ALS. Small electric shocks are sent through your nerves. Your doctor measures how fast they conduct electricity and whether they’re damaged.

A second part of the test also checks the electrical activity of your muscles. In both cases, your doctor will be able to see clear abnormal patterns of activity if you have ALS.

A nerve conduction study: This measures the ability of your nerves to send signals. Only about 10% of ALS patients have abnormal nerve conduction study results, but the test can also suggest other diagnoses.

A muscle biopsy. A small sample of your muscle tissue may be taken if your doctor thinks you have a muscle disease other than ALS. You will be given something to numb the area before the tissue is taken.

Second Opinions

There’s no one test that can give you and your doctor a certain diagnosis of ALS. So many of its symptoms can be caused by multiple conditions.

Because of this, many patients like to look for a second opinion after getting an ALS diagnosis. A second neurologist may do a different round of tests that shows something new.

Proof of Progression

Part of the definition of ALS is that it’s a progressive disease -- that is, it gets worse over time.

So once you have your first diagnosis of ALS, your doctor will likely recommend repeating all the tests in 6 or more months to see whether your disease has changed at all.

If the tests show a worsening of the symptoms and underlying muscle and nerve function, your diagnosis will likely be confirmed.

Show Sources


National Institute of Neurological Disorders and Stroke: “Amyotrophic Lateral Sclerosis (ALS) Fact Sheet.”

Massachusetts General Hospital: “Diagnosing ALS.”

ALS Association: “Criteria for the Diagnosis of ALS.”

Mayo Clinic: “Amyotrophic Lateral Sclerosis: Diagnosis.”

Medscape: “Amyotrophic Lateral Sclerosis Workup.”

University of Rochester Highland Hospital: “What is a Neurologist?”

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