Today, the most reliable way to diagnose facioscapulohumeral muscular dystrophy (FSHD) is with a test for a tiny missing section of DNA on chromosome 4. This test, which is performed on blood cells, is considered highly accurate for FSHD, even though no specific gene has been identified as being associated with the disorder.
In people who have a family history of the disease and are showing signs of it, a DNA test  is generally all that need be done to confirm whether FSHD is likely to develop. (See Causes/Inheritance .)
In many cases, however, people with no family history are suspected of having either FSHD or some other neuromuscular disorder. In these situations, less expensive and less specific tests than the FSHD DNA test may be done first.
One test is a creatine kinase level . This test, also performed on a blood sample, measures the amount of an enzyme known as creatine kinase in the blood. When muscle cells breakdown, as they do in muscular dystrophies and some other disorders, the creatine kinase, or CK, level is elevated. Creatine kinase was formerly called creatine phosphokinase, or CPK.
Another type of diagnostic test is the electromyogram, or EMG , which measures the electrical activity in the muscles.
A nerve conduction velocity, or NCV, test may also be done. This involves measuring how fast signals travel from one part of a nerve to another.
Another diagnostic procedure sometimes undertaken is the muscle biopsy . In this procedure, a small piece of muscle is taken, under local anesthesia, usually from the arm or leg. Biopsy samples reveal cellular and molecular abnormalities that suggest certain muscle disorders and rule out others.
Muscle biopsies are less often performed today than in the past, especially when there’s a DNA test for the disease the doctor suspects is causing the symptoms — as there is for FSHD. Muscle biopsy samples, however, are extremely valuable to researchers seeking to understand the relationship between the DNA results and what actually happens inside the muscle.
FSHD can be confused with polymyositis , which is neither a genetic disease nor a muscular dystrophy. It also can be confused with certain conditions of the nervous system that aren’t muscle disorders.
Seeing a neurologist who specializes in neuromuscular disorders at an MDA clinic  or major medical center, and agreeing to a full assortment of diagnostic procedures, will increase the possibility of an accurate diagnosis.
Diagnoses made many years ago (for example, in older family members) may be worth revisiting. Many DNA tests and other diagnostic approaches became available during the 1990s. For more on getting a definitive genetic diagnosis, see The Genie's Out of the Bottle: Genetic testing in the 21st century .