Many of the features of Congenital fibre type disproportion (CFTD) are also associated with other conditions, in particular other congenital myopathies. For this reason, a diagnosis of CFTD must be confirmed through a muscle biopsy.
Muscle biopsy can be done of two ways: an open biopsy, where a small piece of muscle is taken under a general anaesthetic, or a needle biopsy, where a small muscle sample is removed through a small incision with a special needle, under local anaesthetic. The sample is then analysed under a microscope; muscle from people with CFTD has a characteristic pattern when viewed under a microscope.