Brazilian reference values for MPS II screening in dried blood spots--a fluorimetric assay

Mucopolysaccharidosis II (MPS II), or Hunter Syndrome, is a lysosomal storage disorder that is caused by the deficiency or absence of iduronate-2-sulfatase (IDS) enzyme; in this disease, early diagnosis is essential to provide higher life expectancy for patients. This study validates a fluorimetric assay that is used to assess IDS enzyme activity using dried blood spot (DBS) samples and presents the reference interval for the Brazilian population.

Venous blood sample was collected in heparin tubes for leukocyte extraction and DBS preparation. IDS activity in the leukocytes was analyzed, and the results were considered the gold standard reference for the categorization of volunteers as positive or negative controls (PC and NC, respectively). IDS activity in the DBS was analyzed using an adapted version of the leukocyte assay. Statistical analyses were performed using a ROC curve to determine cutoff values and using a parametric Student's t test to compare values between genders. To verify that the assay yielded consistent results, a Bland-Altman plot was prepared.

Leukocyte IDS activity values ranged between 2.71 and 17.36 nmol/mg protein/h in the NC group and between 0 and 0.11 nmol/mg protein/h in the PC group. Based on the DBS assay, activities ranged between 1.83 and 16.86 μmol/L blood/h in the NC group and between 0.58 and 4.32 μmol/L blood/h in the PC group.

Reference values of IDS activity were determined in DBS with acceptable sensitivity and specificity. Therefore, the DBS assay described in this work may be a useful tool to screen MPS II patients in the Brazilian population.