Objective: An assay for measuring globotriaosylceramide (Gb3Cer) levels in plasma and urine of patients with Fabry disease (FD) has been established. Settings: Institute of Inherited Metabolic Disorders, General Faculty Hospital and 1st Faculty of Medicine of Charles University, Prague. Material and Methods: Gb3Cer concentrations were measured in 31 FD hemizygotes with classic form of the disease, 4 FD hemizygotes with cardiac variant, 33 FD female carriers and 6 carriers of cardiac variant. The control group consisted of 40 healthy men and 22 healthy women. Urine or plasma are diluted with acetone : methanol : water (45 : 45 : 10) and centrifuged to remove protein. The detection of relevant analytes is accomplished with flow injection analysis-electrospray ionization by triple quadrupole mass spectrometer operating in the multiple-reaction monitoring mode. The calibration standard C17:0-Gb3Cer is used. Results: The assay is linear up to 20 mg/l urine and 50 mg/l plasma. The limits of detection and quantitation of Gb3Cer are 0.04 mg/l and 0.13 mg/l, respectively. The repeatability and reproducibility are under 5%. Established reference ranges in urine of males are up to 119 mg Gb3Cer/mol creatinine, in plasma 4–11 mg Gb3Cer/l. The ranges for men with FD are 440– 1586 mg Gb3Cer/mol creatinine in urine and in plasma 13–50 mg Gb3Cer/l. Reference range in urine of females is up to 179 mg Gb3Cer/mol creatinine, in FD carriers the range is 31–667 mg Gb3Cer/mol creatinine. Fourteen Fabry patients have been tested both before treatment and in the course of the enzyme replacement therapy. Decreasing Gb3Cer concentration was found in all investigated patients in plasma whereas in urine only in eight of them. Conclusion: Compared to the formerly used and relatively insensitive chromatographic techniques, no solvent extraction is needed and only a single dilution step is required to prepare sample for MS/MS analysis. Because of its specificity, sensitivity, and high-throughput, the assay provides useful diagnostic mean in classic form of Fabry disease and a model of detection strategy for other lysosomal disorders.

        Key words: Fabry disease, globotriaosylceramide, tandem mass spectrometry, enzyme replacement therapy.