PRINCIPLE OF METHOD: Hydrolysis of the radiolabelled substrate [14C]-sphingomyelin (labelled in the choline moiety) is followed by measuring release of [14C]-phosphoryl choline which is determined by scintillation counting.

USES AND LIMITATIONS OF THE METHOD: Deficiency of sphingomyelinase is the primary defect in Niemann-Pick disease types A and B, and these disorders can be diagnosed by demonstration of such a deficiency in white cells or fibroblasts. Prenatal diagnosis can also be achieved by showing deficiency in chorionic villi or cultured amniotic cells. Heterozygote detection is not reliable and is not offered by the SAS laboratory. In Niemann-Pick disease type C (NPC) deficiency of sphingomyelinase is not the primary defect, but in some patients there is a secondary deficiency of this enzyme observed in cultured fibroblasts only. In practice other methods are used for diagnosis of NPC.

The assay is included in our lysosomal enzyme screening procedure for patients with hepato(spleno)megaly.

SPECIMEN REQUIREMENTS: Blood. 5 ml lithium heparin (orange capped tube) unseparated and unfrozen. Send at room temperature to arrive at the laboratory within 24h of venepuncture. THE LABORATORY RECOMMENDS USE OF A COURIER SERVICE OR ROYAL MAIL SPECIAL DELIVERY FOR SENDING ALL SPECIMENS TO THE LABORATORY.

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