False positive diagnosis of lysosomal storage disease with dried blood spot sample; leucocyte number as a challenging factor

INTRODUCTION: Recently Dried Blood Spot (DBS) samples have been recommended as screening test for Lysosomal storage diseases. Although DBS samples have many advantageous including non-invasiveness, costing, transportation, usage of these samples is limited by high false positive rate. We aimed to investigate any possible effect of leucocyte number on enzyme activity in dried blood samples in a retrospective study.
METHODS: Data collected from the subjects (n=263) whom hematological parameters are available in database of Ege University Hospital. The lysosomal enzyme activity results (alpha glycosidase, glycocerebrosidase, alpha galactosidase, sphingomyelinase and galactocerebrosidase) were re-evaluated in regard to leucocyte number. Enzyme activities had measured by using fluorometric and LC MSMS methods.
RESULTS: All enzyme activities were closely correlated with the number of total leucocyte, since leucocytes are the main source of lysosomal enzymes. While Glycocerebrosidase and Galactocerebrosidase presented positive correlation with the number of neutrophils and Sphingomyelinase showed positive correlation with the number of lymphocytes. When we recalculated the lysosomal enzyme activities with regard to the leucocyte number, the false positive rates for glycocerebrosidase, sphingomyelinase and alpha galactosidase decreased from 20%, 10.5% and 10.8% to 4.5%, 4.4% and 4.2%, respectively.
DISCUSSION AND CONCLUSION: Our data indicated that the enzyme activity in dried blood samples including low leucocyte number might be found lower than reference intervals resulting in false positive diagnosis. We concluded that the calculation of enzyme activity in regard to the number of leucocytes might achieve the more reliable results and might be helpful to decrease the false positive rate.