In these cases, the diagnosis of ‘manifesting carrier’ is based on a physical examination by an expert clinician to confirm the signs of muscle weakness. If the carrier status is not known but there is a family history of Duchenne or Becker muscular dystrophy, the diagnosis can easily be confirmed by genetic testing, usually on a blood sample.
The diagnosis might be more difficult in the absence of a family history of Duchenne or Becker muscular dystrophy. As carriers of Duchenne and Becker muscular dystrophy do not usually have any muscle problems, other diagnoses are often considered.
If a blood test is done, high levels of a protein called creatine kinase (CK) might be seen. CK is normally found in muscle, but when muscles are damaged, such as in Duchenne and Becker muscular dystrophy, it leaks into the bloodstream. The liver enzymes (aminotransferases, ALT and AST) are also often found to be high but this is a consequence of the muscle damage and not of a liver problem. However, normal CK levels do not completely exclude a diagnosis of ‘manifesting carrier’.
A muscle biopsy is often required to reach the diagnosis in a person without a family history of the condition. The muscle biopsy can show an abnormal expression of the protein dystrophin and it is very useful in excluding other forms of muscular dystrophy.
However, the diagnosis of a carrier has to be confirmed by genetic testing, usually on a blood sample. Different types of genetic tests can provide specific and more detailed information about the DNA mutation. A genetic diagnosis is important as it alerts the carrier to initiate adequate management, to seek genetic advice and to identify other family members who could be at risk of carrying the mutation.