Blood phenylalanine concentrations in patients with PAH-deficient hyperphenylalaninaemia off diet without and with three different single oral doses of tetrahydrobiopterin: Assessing responsiveness in a model of statistical process control

Tetrahydrobiopterin (BH₄) cofactor loading is a standard procedure to differentiate defects of BH₄ metabolism from phenylalanine hydroxylase (PAH) deficiency. BH₄ responsiveness also exists in PAH-deficient patients with high residual PAH activity. Unexpectedly, single cases with presumed nil residual PAH activity have been reported to be BH₄ responsive, too. BH₄ responsiveness has been defined either by a ≥30% reduction of blood Phe concentration after a single BH₄ dose or by a decline greater than the individual circadian Phe level variation. Since both methods have methodological disadvantages, we present a model of statistical process control (SPC) to assess BH₄ responsiveness. Phe levels in 17 adult PKU patients of three phenotypic groups off diet were compared without and with three different single oral dosages of BH₄ applied in a double-blind randomized cross-over design. Results are compared for ≥30% reduction and SPC. The effect of BH₄ by ≥30% reduction was significant for groups (p < 0.01) but not for dose (p = 0.064), with no interaction of group with dose (p = 0.24). SPC revealed significant effects for group (p < 0.01) and the interaction for group with dose (p < 0.05) but not for dose alone (p = 0.87). After one or more loadings, seven patients would be judged to be BH₄ responsive either by the 30% criterion or by the SPC model, but only three by both. Results for patients with identical PAH genotype were not very consistent within (for different BH₄ doses) and between the two models. We conclude that a comparison of protein loadings without and with BH₄ combined with a standardized procedure for data analysis and decision would increase the reliability of diagnostic results.