The most common test for nerve damage is an electrodiagnostic study, which combines a nerve conduction study (NCS) with electromyography (EMG). These two tests are often performed together in the same appointment and can pinpoint where a nerve is injured, how severely, and whether the damage affects the nerve’s insulating coating or the nerve fiber itself. But depending on the type of nerve damage suspected, your doctor may also order blood work, imaging, a skin biopsy, or other specialized tests.
Nerve Conduction Studies
A nerve conduction study measures how fast and how strongly electrical signals travel along a nerve. Small electrode stickers are placed on your skin, and a mild electrical current is sent between them. You’ll feel a quick twinge or muscle twitch with each pulse. The test is uncomfortable but brief, and each nerve takes only a minute or two to assess.
Three key measurements come from this test. Latency tells how long it takes a signal to travel between two points, measured in milliseconds. Conduction velocity captures the speed of that signal in meters per second. And amplitude reflects how many nerve fibers are actually functioning. Together, these numbers reveal two distinct types of damage. When the nerve’s insulating layer (myelin) is injured, signals slow down: latency increases and conduction velocity drops, but amplitude stays normal because the underlying fibers are intact. When the nerve fibers themselves are lost, amplitude drops. This distinction matters because myelin injuries often recover well, while fiber loss can be more permanent.
Carpal tunnel syndrome is a good example of how this plays out. In early stages, it shows up as slowed signal speed at the wrist, a sign of myelin disruption from compression. In later stages, the amplitude drops too, meaning nerve fibers have started dying off from prolonged pressure.
Electromyography (EMG)
EMG is usually performed right after the nerve conduction study. A thin needle electrode is inserted into a muscle, and it records the electrical activity both at rest and during a gentle contraction. Healthy muscle tissue is electrically silent when relaxed. If a nerve supplying that muscle is damaged, the muscle starts producing spontaneous electrical signals on its own. The neurologist reads these signals as graphs, sounds, and numerical values to determine whether a nerve injury is active, healing, or long-standing.
While nerve conduction studies evaluate the nerve itself, EMG shows what’s happening downstream in the muscles the nerve controls. This makes the two tests complementary. A nerve conduction study might confirm that a nerve signal is weak, and the EMG can show exactly which muscles are affected, helping map the location and extent of the damage.
Preparing for Electrodiagnostic Testing
Don’t apply lotion, oil, or makeup to your skin on the day of the test, because residue on the skin interferes with electrode contact. Showering beforehand helps. If you take blood thinners or have a bleeding disorder, let your neurologist know ahead of time since the EMG involves a needle. Certain medications can also alter results and may need to be held the morning of the test, but only after checking with your doctor.
The full appointment typically takes 30 to 60 minutes. You won’t need sedation, and you can drive yourself home afterward.
Blood Tests
Electrodiagnostic studies confirm that nerve damage exists, but blood work helps explain why. A standard neuropathy panel includes a complete blood count, a comprehensive metabolic panel, hemoglobin A1c (to check for diabetes or prediabetes), thyroid hormone levels, vitamin B12, and a serum protein electrophoresis with immunofixation. That last test screens for abnormal proteins in the blood that can sometimes attack nerves.
Diabetes is the single most common cause of peripheral neuropathy, so A1c is one of the first things checked. Vitamin B12 deficiency is another treatable cause that’s easy to miss, especially in older adults or people taking certain acid-reducing medications. An abnormal protein result doesn’t automatically mean it’s causing the neuropathy, though. About 3 to 4 percent of people over 50 have a harmless monoclonal protein in their blood, so further testing is needed to determine whether it’s related.
Skin Biopsy for Small Fiber Neuropathy
Standard nerve conduction studies only detect damage to large nerve fibers. The smallest fibers, which carry pain and temperature signals, don’t show up on NCS or EMG. If you have burning, tingling, or stabbing pain but your electrodiagnostic tests come back normal, a skin punch biopsy can catch what those tests miss.
The procedure is straightforward. A tiny circular punch (about 3 millimeters) removes a small sample of skin, usually from the lower leg near the ankle. The sample is stained and examined under a microscope to count the number of nerve fibers that have grown up into the outer layer of skin. A diagnosis of small fiber neuropathy is made when the nerve fiber density falls in the lowest 5th percentile for your age and sex. Normal values decline naturally with age, dropping by about 0.54 fibers per millimeter for each decade, and women tend to have slightly higher counts than men. The European Federation of Neurological Communities considers this biopsy a top-tier diagnostic tool for small fiber neuropathy.
Quantitative Sensory Testing
Quantitative sensory testing (QST) measures your ability to detect specific sensations, including light touch, vibration, warmth, cold, and pain. A device applies a precisely controlled stimulus to your skin, and you press a button the moment you feel it. For vibration testing, a probe vibrates at a fixed frequency (typically 100 to 125 Hz) and gradually increases in intensity. For thermal testing, a small metal plate on your skin slowly warms or cools from a baseline of around 34°C until you notice the change.
QST is painless and noninvasive, but it depends on your subjective response, which means results can vary with attention and motivation. It’s most useful when combined with other tests rather than used alone. QST can track how neuropathy is progressing over time or whether a treatment is helping restore sensation.
Nerve Ultrasound
High-resolution ultrasound lets a doctor see the nerve directly, measuring its size and looking for structural changes. The most useful measurement is the cross-sectional area of the nerve at specific points. A nerve that’s being compressed, as in carpal tunnel syndrome, swells just before the compression site. This swelling is visible on ultrasound and can be compared to established reference values for that nerve at that location.
Nerve ultrasound is painless and quick, often done in the same clinic visit as electrodiagnostic testing. It’s especially helpful for entrapment neuropathies and can detect nerve enlargement that increases with age and repeated mechanical stress at common compression points. The technology is also expanding into broader use for inherited and acquired polyneuropathies, where widespread changes in nerve size can support a diagnosis.
Magnetic Resonance Neurography
Magnetic resonance neurography (MRN) is a specialized form of MRI designed to visualize nerves in high detail. Standard MRI can show the spine and surrounding tissue, but MRN uses specific sequences that suppress signals from blood vessels and highlight nerve tissue, making it possible to trace individual nerves along their path and assess their internal structure.
MRN is particularly useful for nerve injuries in the shoulder area (brachial plexus) and lower back (lumbosacral plexus), where electrodiagnostic testing can be limited. It can identify nerve compression from masses or scar tissue, pinpoint hourglass-shaped constrictions within a nerve, and help classify the severity of an injury. This information is often used to determine whether surgery would help, and MRN can also guide targeted injections by showing exactly where along a nerve the problem lies.
Autonomic Testing
If nerve damage affects the autonomic nervous system, which controls sweating, blood pressure, heart rate, and digestion, specialized tests can detect it. The most widely used is the quantitative sudomotor axon reflex test (QSART), which evaluates the small nerve fibers that control sweat glands.
During QSART, a chemical is applied to the skin through a mild electrical current to stimulate sweat production. The signal travels along the local nerve fiber, branches, and activates a nearby group of sweat glands. A sensor measures the resulting sweat output over time, including how quickly sweating begins and how much sweat is produced. Reduced or absent sweating at a test site indicates damage to the postganglionic sympathetic nerves in that area. QSART is one of the few objective tests for small autonomic fibers and is commonly used alongside skin biopsy to evaluate small fiber neuropathy.
Spinal Fluid Analysis
A lumbar puncture (spinal tap) is not a routine test for most types of nerve damage, but it plays an important role when certain inflammatory or autoimmune conditions are suspected. Guillain-Barré syndrome and multiple sclerosis are two conditions where spinal fluid analysis provides key diagnostic information.
The test involves inserting a needle into the lower back to collect a small sample of cerebrospinal fluid. Lab technicians check protein levels, white blood cell counts, and the presence of specific proteins. A classic finding in Guillain-Barré syndrome is elevated total protein (above 45 mg/dL) with a normal white blood cell count, typically five or fewer cells per microliter. This pattern, called albuminocytologic dissociation, points to inflammation at the nerve roots without infection.