To compare the phenylalanine(Phe) concentration in the sample of dried blood spot, measured by four different methods: fluorescence assay, tandem mass spectrometry (MS/MS), including MS/MS Derivatized, MS/MS Non-Derivatized and MS/MS-Standard Curve.

A total of 204 dried blood spot (DBS) samples of phenylketonuria (PKU) patients from Shanghai Xinhua Hospital were collected in this study. Phe concentration in DBS was measured by fluorescence assay and MS/MS assay, including MS/MS Derivatized, MS/MS Non-Derivatized and MS/MS-Standard Curve. The samples were divided into low, middle and high concentration groups according to Phe concentration, which were under the 360 μmol/L, between 360 and 600 μmol/L, and over 600 μmol/L respectively. The differences among the groups were analyzed by consistency check and non-parametric test.

The within-day and between-day precisions of MS/MS-Standard Curve assay were 4.0%-7.2%, the recoveries were 93.2%-97.3%. Consistency check showed that less than 8.1% of Phe value were out of the 95% limit of agreement among these four methods. In the low and middle concentration groups, the quartile of Phe value measured by fluorescence assay, MS/MS Standard Curve and MS/MS Derivatized were 176 (118, 251)μmol/L, 174 (94, 273)μmol/L, 153 (94, 242)μmol/L; and 540 (478, 578)μmol/L, 485 (414, 529)μmol/L, 466 (402, 513)μmol/L, respectively. Phe value measured by fluorescence assay was significantly higher than that by MS/MS Standard Curve, the difference was 4.8%-9.0%, (Z=-3.787 to -2.674, P<0.01). Phe value obtained from MS/MS Non-Derivatized was less than that by MS/MS-Standard Curve, the difference was 3.9%-5.2%, (Z=-7.474 to -5.747, P<0.01). In the high concentration group, the quartile of Phe value measured by MS/MS-Standard Curve, MS/MS Derivatized, MS/MS Non-Derivatized and fluorescence assay were 807 (695, 924)μmol/L, 700 (575, 785)μmol/L, 680 (623, 771)μmol/L and 711 (674, 794)μmol/L, respectively. Phe value obtained from MS/MS Standard Curve was significantly higher than those from the other methods and the difference was 10.9%-16.2%, (Z=-4.458 to -4.356, P<0.01).

The MS/MS Standard Curve assay showed good specificity and accuracy. These four assays displayed good agreement. Although there was difference in measuring the Phe concentration among these methods, they could be used in blood Phe concentration monitoring for PKU patients.(Chin J Lab Med, 2018, 41: 361-365)

Phenylketonurias; Phenylalanine; Dried blood spot testing; Tandem mass spectrometry