A CMT blood test is a genetic test that checks for inherited mutations linked to Charcot-Marie-Tooth disease, a group of disorders that damage the peripheral nerves in your arms and legs. A simple blood draw is sent to a specialized genetics lab, where technicians analyze your DNA for specific gene changes known to cause the condition. It’s the most definitive way to confirm a CMT diagnosis and identify the exact subtype you have.
What the Test Is Looking For
Charcot-Marie-Tooth disease isn’t a single condition. It’s a family of inherited neuropathies caused by mutations in dozens of different genes, each affecting the peripheral nerves in slightly different ways. The blood test scans your DNA for the most common of these mutations.
The single most frequent cause is a duplication of a gene called PMP22, which provides instructions for building a protein in the nerve’s protective coating. This one duplication accounts for roughly 50% of all CMT cases and 70% of the most common subtype, CMT1. Labs typically check for this duplication first. If that comes back normal, testing may expand to a broader panel that includes other genes like MPZ, GJB1, and MFN2, which are responsible for rarer subtypes. Some labs offer panels covering 60 or more genes in a single test.
Why Your Doctor Ordered It
Doctors order CMT genetic testing when a person shows a pattern of symptoms that points toward inherited nerve damage, especially when those symptoms started gradually and have been getting worse over time. The typical picture includes muscle weakness and wasting in the feet and lower legs, difficulty lifting the front of the foot (sometimes called foot drop), reduced or absent reflexes, numbness or tingling in the hands and feet, and foot deformities like unusually high arches or curled toes.
A family history of similar symptoms makes testing even more likely. If a first- or second-degree relative already has a confirmed CMT mutation, your doctor may order a targeted test looking specifically for that known family variant rather than a broad panel.
How It Works With Other Tests
The blood test doesn’t usually happen in isolation. Before or alongside genetic testing, most people undergo a nerve conduction study, which measures how fast electrical signals travel through the nerves. This is a critical first step because the speed of nerve conduction helps doctors determine which type of CMT is most likely, which in turn guides which genes the lab should prioritize.
Slow nerve conduction suggests a demyelinating form (CMT1), where the insulating sheath around nerves is damaged. Normal-speed conduction with reduced signal strength suggests an axonal form (CMT2), where the nerve fibers themselves are degenerating. This distinction matters because different subtypes involve different genes, and testing the right ones first saves time and money.
What to Expect During the Test
The test itself is a standard blood draw. No fasting or special preparation is needed. A nurse or phlebotomist collects a blood sample, which is then shipped to a genetics laboratory for analysis.
Results take longer than a typical blood test. If the lab is only checking for the common PMP22 duplication, you can expect results within about two weeks. If that initial test is negative and the lab moves on to a broader gene sequencing panel, the total wait stretches to roughly 6 to 8 weeks. Your doctor’s office will typically call you to discuss results rather than releasing them through a patient portal, since genetic findings often need explanation.
Understanding Your Results
Results fall into a few categories. A positive result means the lab found a mutation known to cause CMT. This confirms the diagnosis and identifies your specific subtype, which can help predict how the disease may progress and whether your children could inherit it.
A negative result means no known CMT mutation was found. This doesn’t always rule out the disease. Current genetic testing identifies a cause in about 63% of people who clinically have CMT. The remaining cases involve mutations in genes that haven’t been discovered yet or aren’t included on standard panels. If your symptoms strongly suggest CMT but the test is negative, your doctor may still diagnose you based on your clinical picture and nerve conduction results.
There’s also a middle category: a variant of uncertain significance, or VUS. This means the lab found a genetic change but scientists don’t yet have enough evidence to say whether it actually causes disease. VUS results are common with large gene panels and can be frustrating. They don’t confirm or rule out a diagnosis, and your doctor may recommend retesting in the future as scientific understanding improves.
Cost and Insurance Coverage
Genetic testing for CMT can be expensive. A targeted test for the PMP22 duplication alone costs less, while a full multigene panel runs in the range of $1,200 or more. Some comprehensive genome-level tests can exceed $2,000. Insurance coverage varies widely. Many insurers require pre-authorization and a letter of medical necessity before they’ll reimburse genetic testing. If you’re concerned about cost, contact your insurance company before the test is ordered and ask specifically about pre-authorization requirements and what portion you’ll be responsible for.
What a Positive Result Means for Your Family
Because CMT is inherited, a confirmed genetic result has implications beyond just you. Most forms follow an autosomal dominant pattern, meaning a single copy of the mutated gene from one parent is enough to cause the disease. If you carry a CMT mutation, each of your biological children has a 50% chance of inheriting it. Some rarer forms are X-linked or autosomal recessive, which changes the math. A genetic counselor can walk you through what your specific mutation means for family planning and whether other relatives should consider testing.
Targeted testing for a known family mutation is faster and cheaper than a full panel, so once one person in a family has a confirmed result, it simplifies the process for everyone else.