Biomedical subjects
Publications and source records attributed to C C Mabry.
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Screening newborns for phenylketonuria (PKU) is a mandatory practice based on measuring a raised blood phenylalanine level. Many factors influence the rate of blood phenylalanine rise so that there are many pitfalls in detecting the 1:10,000 affected infant. About one percent of all babies tested proves to be "false positives." Two-thirds of those with persistent hyperphenylalaninemia prove to have classic PKU. Non-classic PKU with less intense, persistent hyperphenylalaninemia is due to different alterations in the enzyme, phenylalanine hydroxylase. Additionally, about one percent of the confirmed positive patients is due to either a defect in the synthesis or regeneration of the cofactor, tetrahydrobiopterin; these latter forms are not amenable to treatment with the low phenylalanine diet. Screening programs have developed directives regarding the timing and conditions for obtaining the specimens for testing. Specific confirmatory tests of those with positive results must be performed. Even so, about one in 70 affected babies is "missed," resulting in mental retardation, seizures, and neurologic deficits.
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Subtypes of primary congenital hypothyroidism (CHT) showed different degrees of impairment of thyroid function at screening and confirmation. Patients with athyrosis showed the greatest impairment with mean value for screening 1-thyroxine (ScrT4) of 2.73 micrograms/dl (p < 0.0001 vs E group), screening thyroid-stimulating hormone (ScrTSH) of 327.6 microIU/ml (p < 0.0001 vs E group), confirmatory L-thyroxine (ConT4) of 1.21 micrograms/dl (p < 0.0001 vs E group), and confirmatory thyroid-stimulating hormone (ConTSH) of 572 microIU/ml (p < 0.0001 vs E group). Infants with dysgenetic ectopic thyroids (E group) showed the best preservation of function with mean values for ScrT4 of 6.0 micrograms/dl, ScrTSH of 229 microIU/ml, ConT4 of 6.23 micrograms/dl, and ConTSH of 324.8 microIU/ml. Generally, intermediate mean values were found for infants with normal or goitrous thyroids with values for ScrT4 of 4.26 micrograms/dl, ScrTSH of 205 microIU/ml, ConT4 of 3.21 micrograms/dl, and ConTSH of 428 microIU/ml. The risk for CHT being missed appeared to be greatest in patients with dysgenetic ectopic thyroids with the mean difference between ScrT4 and the L-thyroxine cutoff value (ScrT4Diff) being -2.2 micrograms/dl; five cases fell within 0.5 microgram/dl of the cutoff. The risk was least in infants with athyrosis with ScrT4Diff of -4.64 micrograms/dl (p < 0.0001 vs E group); no cases were within 1 microgram/dl of missing the cutoff. For infants with normal or goitrous thyroids ScrT4Diff was -3.44 micrograms/dl; one case was initially missed and three fell within 0.5 microgram/dl of the cutoff. The risk for missing CHT in screening is apparent. Clinical vigilance must be maintained to detect missed cases as early as possible.