Low biotinidase activity in plasma of some preterm infants: possible source of false-positive screening results.

Screening for biotinidase deficiency has been added recently to some national screening programmes. To clarify the problem of false-positive screening tests in premature infants, we have studied biotinidase activities in the plasma of this population in more detail. In 64 newborns (premature and term babies) biotinidase activities correlated positively with gestational age from the 2nd to the 30th day of life. During the 1st-3rd day the activities were below the normal adult range in all 64 infants. In 56 infants the activities subsequently increased gradually and reached the normal adult range during the 4th-40th day of life. In contrast, the biotinidase activities in eight preterm infants dropped during the 3rd-7th day of life. Impaired liver function as a possible cause for this finding could be ruled out in these infants. The lowest activities in these infants were measured during the 4th-6th day of life, i.e. unfortunately at a time when samples for the screening are normally taken. According to our data, 4-8 out of 48 preterm or small-for-date infants with biotinidase activities ranging from 4.7%-26% of the mean adult value would have given false-positive screening tests. A positive screening test was also obtained in a newborn and in an older unrelated child with a partial biotinidase deficiency. In these children the biotinidase activity did not rise but remained slightly below or at the lower range for heterozygotes (at 31% and 38% of the mean adult value). Currently we do not know whether such individuals are heterozygotes, or whether they have a variant of biotinidase deficiency. However, these children have developed normally without biotin therapy.