[The traps of Guthrie's test].

Presently, 90% of the newborn infants are undergoing screening for phenylketonuria before discharge from the maternity hospital. The tests of detection on urine (search for phenylpyruvic acid) and blood (evaluation of serum phenylalanine) have to be interpreted according to the fetal age, the age at sampling, the conditions of anabolism and the protid intake.