Acid Alpha-Glucosidase, Blood Spot
Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test
Pompe disease, also known as glycogen storage disease type II, is an autosomal recessive disorder caused by a deficiency of the lysosomal enzyme alpha-glucosidase (GAA) leading to an accumulation of glycogen in the lysosome causing swelling, cell damage, and progressive organ dysfunction. Pompe disease is caused by mutations in the GAA gene, and it is characterized by muscle hypotonia, weakness, cardiomyopathy, and eventually death due to either cardiorespiratory or respiratory failure. The clinical phenotype, in general, appears to be dependent on residual enzyme activity, with complete loss of activity causing onset in infancy leading to death, typically within the first year of life. Juvenile and adult-onset forms are characterized by later onset and longer survival. Treatment by enzyme replacement therapy, recently available, makes early diagnosis of Pompe disease desirable, as early initiation of treatment may improve the prognosis. The estimated incidence is 1 in 40,000 live births.
Since Pompe disease is considered a rare condition that progresses rapidly in infancy, the disease, in particular the juvenile and adult-onset forms, is often considered late, if at all, during the evaluation of patients presenting with muscle hypotonia, weakness, and/or cardiomyopathy. Testing traditionally required a skin or muscle biopsy to establish cultures for enzyme testing. More recently, molecular genetic testing of the GAA gene (GAAMS/89898 Pompe Disease, Full Gene Sequencing) became clinically available. Determination of the enzyme assay in dried blood spot specimens can be performed in a timely fashion and provide better guidance in the decision to submit samples for further confirmatory testing by molecular genetic analysis (GAAMS/89898 Pompe Disease, Full Gene Sequencing).
Evaluation of patients of any age with a clinical presentation suggestive of Pompe disease (muscle hypotonia, weakness, and/or cardiomyopathy)
Normal results (>7.4 pmol/dried blood spot punch/hour) in properly submitted specimens are not consistent with classic Pompe disease. Affected individuals typically show <2.5 pmol/dried blood spot punch/hour; however, some later onset cases may show higher enzyme activity.
Results <7.5 pmol/dried blood spot punch/hour can be followed up by molecular genetic analysis of the GAA gene (GAAMS/89898 Pompe Disease, Full Gene Sequencing) to determine carrier or disease status.
Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances
Specimens exposed to heat >25 degrees C for more than 48 hours will yield higher activity levels than properly submitted specimens. This may cause false-normal (false-negative) results in affected patients.
Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.
Normal >7.4 pmol/punch/h
Clinical References Provides recommendations for further in-depth reading of a clinical nature
1. Kishnani PS, Howell RR: Pompe disease in infants and children. J Pediatr 2004;144:S35-S43
2. Winkel LP, Hagemans ML, van Doorn PA, et al: The natural course of non-classic Pompe's disease; a review of 225 published cases. J Neurol 2005;252:875-884
3. Katzin LW, Amato AA: Pompe disease: a review of the current diagnosis and treatment recommendations in the era of enzyme replacement therapy. J Clin Neuromuscul Dis 2008 Jun;9(4):421-431
4. Enns GM, Steiner RD, Cowan TM: Lysosomal disorders. In Pediatric Endocrinology and Inborn Errors of Metabolism. Edited by K Sarafoglou, GF Hoffmann, KS Roth. McGraw-Hill, Medical Publishing Division, 2009, pp 750-751