Blood phenylalanine monitoring for dietary compliance among patients with phenylketonuria: comparison of methods

Blood phenylalanine monitoring is critical for the management of phenylketonuria. We compared three methods for measuring blood phenylalanine concentration: the amino acid analyzer, high-performance liquid chromatography with fluorometric detection, and tandem mass spectrometry. Methods: We studied 22 female patients with phenylketonuria, ages 12-48 years, who attended our Metabolic Camp. Blood was collected into heparinized tubes (for analysis by the amino acid analyzer) or filter paper (for analysis by high-performance liquid chromatography With fluorometric detection and tandem mass spectrometry). Results: Blood phenylalanine concentrations of plasma measured by the amino acid analyzer were significantly higher than those obtained from whole blood on filter paper by high-performance liquid chromatography (difference: 102 AM; 95% confidence interval: 23, 181) and tandem mass spectrometry (difference: 137 AM; 95% confidence interval: 58, 216). Phenylalanine concentrations from high-performance liquid chromatography and tandem mass spectrometry were not significantly different (P = 0.5). Conclusions: When monitoring blood phenylalanine concentrations for dietary compliance, clinicians should be mindful of the method being used; analyses of whole blood on filter paper were consistently approximately 15% lower than analyses of plasma.