Muscle biopsies are often used to measure GAA activity and for histology in patients with muscle weakness. But glycogen accumulation in the muscles of patients varies with biopsy site, so the diagnosis of Pompe disease can be missed by using only a muscle biopsy. Fibroblasts can also serve as a source of material for research but cell culture facilities are not easy for clinicians and it takes several weeks to obtain confluent cultures. Then, assays that use blood to diagnose Pompe disease were developed. Therefore, a group of international clinicians and biologists met together in London in December 2006 and established an agreement concerning the various methods of this enzyme dosage. Recently, a test on a DBS (dried blood spot) has been developed. This test is not invasive, easy to collect and transport, requires small sample volume and provides rapid results. This international consensual group recommends confirming the diagnosis of the Pompe disease after a DBS with a dosage of the enzymatic activity in other tissue. This strategy is currently used in the usual practice.