False Positive Diagnosis of Lysosomal Storage Disease Based on Dried Blood Spot Sample; Leucocyte Number of a Challenging Factor

Aim: Recently dried blood spot (DBS) samples have been recommended as a screening test for Lysosomal Storage diseases. Although DBS samples have many advantages including non-invasiveness, cost and transportation, usage of these samples is limited by its high false positive rate. We aimed to investigate any possible effect of the leucocyte number on enzyme activity in dried blood samples in a retrospective study. Materials and Methods: Data was collected from subjects (n=263) for whom hematological parameters were available in the database of Ege University Hospital. The lysosomal enzyme activity results (alpha glycosidase, glycocerebrosidase, alpha galactosidase, sphingomyelinase and galactocerebrosidase) were re-evaluated with regard to the leucocyte number. Enzyme activities were measured using fluorometric and liquid chromatography-tandem mass spectrometry methods. Results: All enzyme activities closely correlated with the total number of leucocyte, since leucocytes are the main source of lysosomal enzymes. Glycocerebrosidase and galactocerebrosidase presented a positive correlation with the number of neutrophils and sphingomyelinase showed a 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. Conclusions: 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 with regard to the number of leucocytes might produce more reliable results and might be helpful in decreasing the false positive rate.