Children with inborn errors of phenylalanine metabolism: prognosis and phenylalanine tolerance

Phenylalanine Tolerance over Time in Phenylketonuria: A Systematic Review and Meta-Analysis

In phenylketonuria (PKU), natural protein tolerance is defined as the maximum natural protein intake maintaining a blood phenylalanine (Phe) concentration within a target therapeutic range. Tolerance is affected by several factors, and it may differ throughout a person's lifespan. Data on lifelong Phe/natural protein tolerance are limited and mostly reported in studies with low subject numbers. This systematic review aimed to investigate how Phe/natural protein tolerance changes from birth to adulthood in well-controlled patients with PKU on a Phe-restricted diet. Five electronic databases were searched for articles published until July 2020. From a total of 1334 results, 37 articles met the eligibility criteria (n = 2464 patients), and 18 were included in the meta-analysis. The mean Phe (mg/day) and natural protein (g/day) intake gradually increased from birth until 6 y (at the age of 6 months, the mean Phe intake was 267 mg/day, and natural protein intake was 5.4 g/day; at the age of 5 y, the mean Phe intake was 377 mg/day, and the natural protein intake was 8.9 g/day). However, an increase in Phe/natural protein tolerance was more apparent at the beginning of late childhood and was >1.5-fold that of the Phe tolerance in early childhood. During the pubertal growth spurt, the mean natural protein/Phe tolerance was approximately three times higher than in the first year of life, reaching a mean Phe intake of 709 mg/day and a mean natural protein intake of 18 g/day. Post adolescence, a pooled analysis could only be performed for natural protein intake. The mean natural protein tolerance reached its highest (32.4 g/day) point at the age of 17 y and remained consistent (31.6 g/day) in adulthood, but limited data were available. The results of the meta-analysis showed that Phe/natural protein tolerance (expressed as mg or g per day) increases with age, particularly at the beginning of puberty, and reaches its highest level at the end of adolescence. This needs to be interpreted with caution as limited data were available in adult patients. There was also a high degree of heterogeneity between studies due to differences in sample size, the severity of PKU, and target therapeutic levels for blood Phe control.

Twenty-five years of research on neurocognitive outcomes in early-treated phenylketonuria: intelligence and executive function

Studies published from 1980 through 2004, which explored intelligence and executive function outcomes in early-treated PKU, were included in the present meta-analysis. The objective of this study was to examine the degree of difference between early-treated PKU and unaffected control groups (primarily non-familial controls), known as effect size, for various components of measured intelligence (i.e., full-scale, verbal, performance, and "other") and executive function (i.e., planning, working memory, inhibition, flexibility, and "other"). Secondly, in the case of heterogeneity among observed effect sizes, the moderator variables of "years since publication" and "measurement tools" were examined for their potential contribution to this heterogeneity. Thirty-three primary studies, with a total 200 outcomes meeting our inclusionary and exclusionary criteria, were meta-analyzed using Hedges g for effect size to correct for small samples in primary studies. The effect sizes for intelligence outcomes were "small" to "moderate" falling in the range of .20 to .42, with no significant heterogeneity among outcomes. Effect sizes for executive function and its various component processes were in the "moderate" and "large range" falling between .591 (planning) and 1.15 (flexibility). Significant heterogeneity among the executive function effect sizes was identified, and both "years since publication" and "measurement tools" were found to be significant moderators. Studies published more recently exhibited larger effect sizes, and particular executive function tasks demonstrated larger effect sizes than other tests. The results are discussed in terms of the contributions this meta-analysis makes to our understanding of neurocognitive outcomes in early-treated PKU, as well as suggestions for the conduct and reporting of future research in this area.

Secondary analysis of economic data: a review of cost-benefit studies of neonatal screening for phenylketonuria

STUDY OBJECTIVE

To estimate the net financial benefit of neonatal screening for phenylketonuria (PKU): by a simple pooling of cost data from the literature; and by a more complex modelling approach.

DESIGN

A systematic literature review was conducted to identify papers containing data on the monetary costs and benefits of neonatal screening for PKU. The methodological quality of the studies was appraised, and data were extracted on resource use and expenditure. Monetary data were converted to common currency units, and standardised to UK incidence rates. Net benefits were calculated for median, best case and worst case scenarios, and the effect of excluding poor quality studies and data was tested. The net benefit was also estimated from a model based on data from the literature and assumptions appropriate for the current UK situation. Extensive sensitivity analysis was conducted.

MAIN RESULTS

The direct net benefit of screening based on the median costs and benefits from the 13 studies identified was 143,400 Pounds per case detected and treated (39,000 Pounds and 241,800 Pounds for worst case and best case scenarios respectively). The direct net benefit obtained by the modelling approach was lower at 93,400 Pounds per case detected and treated. Screening remained cost saving under sensitivity analysis, except with low residential care costs (less than 12,300 Pounds per annum), or very low incidence rates (less than 1 in 27,000).

CONCLUSIONS

The economic literature on PKU screening is of variable quality. The two methods of secondary analysis lead to the same conclusion: that neonatal PKU screening is worthwhile in financial terms alone in the UK, and that it justifies the infrastructure for collecting and testing neonatal blood samples. This result cannot necessarily be extrapolated to other countries.

Newborn screening for hyperphenylalaninemia on day 5: is 240 mumol/liter the most appropriate cut-off level?

In most countries, including the UK, Sweden, Japan, and the FRG, blood samples for newborn screening are not taken until Day 5, in contrast to Day 3 in the United States. This results in fundamentally different conditions with respect to the amount of phenylalanine ingested by the newborn until the day of sampling. Nevertheless those countries who routinely screen on Day 5 have adopted the U.S. cut-off level of 240 mumol/liter (4 mg/dl). This study is the first to compare, via direct evaluation of a regional screening program, whether a higher or a lower cut-off level would be more appropriate for phenylalanine screening on Day 5. In a prospective study on phenylalanine screening with 132,638 infants (cut-off level 120 mumol/liter) the number of recalls was 152. Among them 14 cases with a permanent disorder of phenylalanine metabolism were detected. With the officially recommended cut-off level of 240 mumol/liter as well as with the 360 mumol/liter level used in some centers, the recall rate would have been lower, specificity higher, and the predictive values of a positive result higher. On the other hand sensitivity would have been significantly lower (only 80%). Because in newborn screening the crucial point is sensitivity, our data suggest that in phenylalanine screening done on Day 5 cut-off levels of 240 or 360 mumol/liter are less appropriate than a lower cut-off level. With higher cut-off levels a large proportion of patients (one-fifth) with a permanent disorder of phenylalanine metabolism would be missed.