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a Children's Hospital, Birmingham, b University of Birmingham, Institute of Child Health,
Birmingham
Correspondence to: Mrs A MacDonald, Dietetic Services, Children's Hospital, Ladywood Middleway, Birmingham B16 8ET.
Accepted 26 September 1997
A 1993 MRC working group on phenylketonuria suggested
standardising blood phenylalanine measurements by taking blood samples at the same time each day. Since it is not known how representative of
a 24 hour period a single phenylalanine concentration is, the aim of
this study was to investigate the 24 hour variability of plasma
phenylalanine in well controlled children with phenylketonuria. Sixteen
subjects, 12 girls and four boys aged 1 to 18 years, had hourly venous
blood samples collected for 13 hours between 09.00 and 21.00 on one
day. Serial skin puncture blood specimens were then collected at 24.00, 03.00, and 06.00 within the same 24 hour period. All food and drink was
weighed. The median variation in plasma phenylalanine concentration was
155 µmol/l/day, with a minimum of 80 and a maximum of 280. The
highest concentration occurred in the morning between 6.00 and 9.00 in
63% of subjects; the lowest occurred between midday and midnight in
94%. Concentrations < 100 µmol/l occurred in 46% of children
below 11 years, three having concentrations < 30 µmol/l for two,
six, and seven hours respectively. Three of five subjects had
concentrations above the MRC guidelines for 24% of the period studied.
Except in two subjects, the blood concentrations did not rise in
response to phenylalanine consumption. However, the greater the
quantity of protein substitute taken between waking and the 16.00 specimen, the larger the decrease in daytime phenylalanine
concentration (r = 
0.7030) (p < 0.005). There is
therefore wide variability in phenylalanine concentrations in a 24 hour
period in children with phenylketonuria which is not reflected in a
single observation. Further study is needed to investigate the effects
of timing of protein substitute on the stability of phenylalanine concentrations.
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