Testing for nerve damage typically starts with a physical exam and then moves to electrical tests that measure how well your nerves send signals. Depending on your symptoms, your doctor may also order blood work, imaging, or a small skin biopsy. The specific combination depends on whether the suspected damage involves large nerve fibers, small nerve fibers, or both, and whether the goal is confirming damage or finding its underlying cause.
The Neurological Physical Exam
Before any lab work or specialized testing, a neurological exam gives your doctor a baseline picture of how your nerves are functioning. This in-office evaluation covers several systems: mental status (memory, alertness, orientation to time and place), cranial nerves (the 12 nerves connecting your brain to your eyes, ears, face, and throat), motor strength, reflexes, and sensory perception.
You might be asked to smile, close your eyes tightly, or read letters on a chart to check specific cranial nerves. For motor testing, the doctor may ask you to spread your fingers apart while they gently push them together, or to relax your arm while they move it back and forth. Sensory testing often involves a tuning fork pressed against your skin to check vibration sense, a pinprick to test pain perception, or a light touch with a cotton swab. Reflexes are tested with a small hammer at your knee, ankle, or elbow.
This exam helps the doctor determine which nerves are affected, whether the problem is sensory or motor (or both), and how severe the damage appears. It also guides which follow-up tests to order.
Nerve Conduction Studies
A nerve conduction study (NCS) measures how fast and how strongly electrical signals travel through your nerves. A damaged nerve produces a slower, weaker signal than a healthy one. During the test, electrodes are placed on your skin above the nerve being checked. These deliver a mild electrical pulse, while recording electrodes on nearby muscles capture the response. The time it takes for your muscle to respond is called the conduction velocity.
Healthy nerves in the arm typically carry signals at 43 to 52 meters per second, depending on the specific nerve. Nerves in the lower leg are somewhat slower, with normal speeds ranging from about 38 to 42 meters per second. When results fall below these thresholds, it points to damage in the nerve’s insulating coating (the myelin sheath). If the signal strength is reduced but the speed is normal, the issue is more likely damage to the nerve fibers themselves.
The test feels like brief, mild electric shocks. Most people find it uncomfortable but tolerable, and the entire study usually takes 30 to 60 minutes.
Electromyography (EMG)
An EMG is often done alongside a nerve conduction study and focuses on the muscles rather than the nerves directly. A small needle electrode is inserted into the muscle to record its electrical activity while you contract it and while it’s at rest. A healthy muscle at rest produces no electrical signals. If the muscle shows spontaneous electrical activity when you’re not moving it, or abnormal patterns when you are, that suggests the nerve supplying that muscle is damaged.
The electrical activity appears on screen as wavy and spiky lines that the specialist interprets in real time. Together, the NCS and EMG can pinpoint where along a nerve the damage is located, whether it affects sensory nerves, motor nerves, or both, and whether the pattern suggests a single injured nerve or widespread neuropathy.
Preparing for Electrical Testing
If you’re scheduled for an NCS or EMG, skip lotions, creams, and perfumes on the day of the test. These products create a barrier on the skin that can interfere with electrode readings. Wear loose, comfortable clothing so the technician can access your arms and legs easily. Let your provider know beforehand if you take blood thinners or have a pacemaker or other implanted electrical device.
You may also be asked to avoid smoking and caffeine for two to three hours before the test, since both can affect nerve and muscle activity. No fasting is required unless your doctor specifically says otherwise.
Blood Tests to Find the Cause
Electrical testing confirms that nerve damage exists, but blood work helps explain why. Since diabetes is the most common cause of peripheral neuropathy in developed countries, a fasting blood glucose test is standard. An oral glucose tolerance test is the most rigorous option: you fast overnight, have blood drawn in the morning, drink a precise glucose solution, and then have your blood drawn again two hours later. This can catch prediabetes that a single fasting test might miss.
Vitamin levels matter too. Low vitamin B12 can directly cause neuropathy along with anemia and weakness. Elevated vitamin B6 can also cause nerve damage, which surprises many people since B6 is widely available in supplements. Thiamine (B1) deficiency tends to cause motor neuropathy, meaning weakness rather than numbness or tingling.
A protein electrophoresis test examines proteins in your blood or urine to screen for cancerous or precancerous blood conditions that sometimes trigger neuropathy, including multiple myeloma and related disorders. Your doctor may also check thyroid function, kidney function, and markers of inflammation depending on your specific symptoms.
Skin Biopsy for Small Fiber Neuropathy
Standard nerve conduction studies only detect damage to large nerve fibers, the ones responsible for motor control and vibration sense. Small nerve fibers, which carry pain and temperature signals, can be damaged without showing up on those tests. If your symptoms include burning pain, prickling sensations, or abnormal sensitivity to temperature but your NCS results come back normal, a skin punch biopsy can fill the gap.
This is a simple office procedure. A tiny circular tool removes a small plug of skin, usually from your lower leg. Pathologists then count the nerve fibers that cross from the deeper skin layer into the outer layer under a microscope. That count, reported as fibers per millimeter, is compared against published norms matched to your age and sex. A density below the expected range confirms small fiber neuropathy even when all other electrical tests are normal.
Imaging: Ultrasound and MRI
When doctors suspect a structural problem, such as a nerve being compressed, trapped, or affected by a tumor, imaging comes into play. High-resolution ultrasound and MRI can both visualize nerves directly rather than just measuring their function.
Research comparing the two found that ultrasound detected nerve problems with 93% sensitivity compared to 70% for MRI, with both equally specific at 83%. Ultrasound was particularly better at spotting nerve enlargement, peripheral nerve sheath tumors, and compression from abnormal muscle tissue. In cases where both methods identified a single lesion, ultrasound also picked up additional lesions, such as multiple neurofibromas and areas of fibrous thickening, that MRI missed entirely.
Ultrasound has practical advantages too: it’s done in real time, costs less, and lets the examiner move the probe dynamically to follow a nerve along its path. MRI remains useful for deeper structures that ultrasound can’t reach well, such as nerves within the spinal canal or deep in the pelvis.
How These Tests Work Together
No single test diagnoses nerve damage on its own. The process typically follows a logical sequence. The physical exam narrows down which nerves are likely involved. Nerve conduction studies and EMG confirm whether damage exists and characterize its type and location. Blood work identifies treatable underlying causes like diabetes, vitamin deficiencies, or blood disorders. If large-fiber tests come back clean but symptoms persist, a skin biopsy checks for small fiber involvement. Imaging is added when a structural cause like compression or a mass is suspected.
For something like diabetic neuropathy, the workup might be straightforward: abnormal glucose tolerance plus reduced nerve conduction velocities in the feet. For unexplained neuropathy with normal electrical studies, it could take the full range of testing before an answer emerges. The timeline from first appointment to diagnosis varies, but most people complete their core testing within a few weeks.