The diagnosis of Becker muscular dystrophy (BMD) may be made during childhood, typically after the age of about 7. Sometimes, however, it isn’t made until adolescence or even adulthood, possibly when a young man finds he can’t keep up in physical education classes or military training.
In diagnosing any form of muscular dystrophy, a doctor usually begins by taking a patient and family history, and performing a physical examination. Much can be learned from these, including the pattern of weakness. The history and physical go a long way toward making the diagnosis, even before any complicated diagnostic tests are done.
The doctor also wants to determine whether the patient’s weakness results from a problem in the muscles themselves or in the nerves that control them. Problems in the muscle-controlling nerves, or motor neurons (originating in the spinal cord and brain and reaching out to all the muscles), can cause weakness that looks like a muscle problem but really isn’t.
Other diseases have some of the same symptoms as BMD, and it has sometimes been misdiagnosed as Duchenne muscular dystrophy (DMD)  or limb-girdle muscular dystrophy (LGMD) . For this reason, it’s important to go through a careful diagnostic process, usually involving genetic (DNA) testing, before making an assumption that the disorder is BMD.
Early in the diagnostic process doctors often order a special blood test called a CK level . CK stands for creatine kinase, an enzyme that leaks out of damaged muscle. When elevated CK levels are found in a blood sample, it usually means muscle is being destroyed by some abnormal process, such as muscular dystrophy or inflammation. Therefore, a high CK level suggests that the muscles themselves are the likely cause of the weakness, but it doesn’t tell exactly what the muscle disorder might be.
DNA testing of the dystrophin gene to diagnose BMD is now widely available and is usually done from a blood sample. In many cases, the DNA test alone can tell families and doctors with a fairly high degree of certainty whether the disease course is more likely to be BMD or Duchenne muscular dystrophy (DMD). You can ask your MDA clinic  physician or genetic counselor what tests are available. For more on getting a definitive genetic diagnosis, see The Genie's Out of the Bottle: Genetic testing in the 21st century .
Female relatives of men and boys with BMD  can undergo DNA testing to see if they are carriers of the disease. If they are, they can give birth to children who are themselves carriers or who will develop BMD.
To view a presentation by a genetic counselor, see the August 2012 video Genetics of BMD: Why Your Mutation Matters .
In some cases, to be more certain about the disease and its course, a doctor may suggest a muscle biopsy  in which a small sample of muscle is taken for special examination. Muscle biopsies also may be taken as part of a research study.