Development and validation of a 2nd tier test for identification of purine nucleoside phosphorylase deficiency patients during expanded newborn screening by liquid chromatography-tandem mass spectrometry.

Background: Purine nucleoside phosphorylase (PNP) deficiency has been recently introduced in the newborn screening program in Tuscany. In order to improve the PNP screening efficiency, we developed a 2nd tier test to quantify PNP primary markers deoxyguanosine (dGuo) and deoxyinosine (dIno). Methods: Dried blood spots (DBS) samples were extracted with 200 mu L of methanol and 100 mu L of water (by two steps). Internal standards were added at a final concentration of 10 mu mol/L. After extraction, samples were analysed by LC-MS/MS. The chromatographic run was performed in gradient mode by using a Synergi Fusion column. Results: The assay was linear over a concentration range of 0.05-50 mu mol/L (R-2 > 0.999) for dGuo and 0.5-50 mu mol/L (R-2 > 0.998) for dIno. Intra-and interassay imprecision (mean CVs) for dIno and dGuo ranged from 2.9% to 12%. Limit of quantitaion (LOQ) were found to be 0.05 mu mol/L and 0.5 mu mol/L for dGuo and dIno, respectively. The reference ranges, obtained by measuring dGuo and dIno concentrations on DBS, were close to zero for both biomarkers. Moreover, DBS samples from seven patients with confirmed PNP were retrospectively evaluated and correctly identified. Conclusions: The LC-MS/MS method can reliably measure dIno and dGuo in DBS for the diagnosis of PNP. Validation data confirm the present method is characterised by good reproducibility, accuracy and imprecision for the quantitation of dIno and dGuo. The assay also appears suitable for use in monitoring treatment of PNP patients.