Interlaboratory surveys of the quantitation of thyroxin and thyrotropin (thyroid-stimulating hormone) in dried blood spot specimens.

Screening of newborns for hypothyroidism is mandated by law in most states. The screening usually consists of measuring thyroxin in dried blood spot specimens followed by measurement of thyrotropin if the results for thyroxin are suggestive of hypothyroidism. The first nationwide interlaboratory surveys designed to assess the proficiency of screening laboratories in the identification of euthyroid and hypothyroid specimens are reported here. Each survey consisted of three specimens mailed quarterly from late 1979 through 1980. A total of 88 laboratories participated in at least one of the four surveys. More than 17 different methods or diagnostic kits were used by participants in the surveys. Coefficients of variation ranged from 20% to 41% for thyroxin data and from 23% to 63% for thyrotropin. Despite the large analytical interlaboratory variation, most participants assigned clinical classifications consistent with the survey design. The incidence of mis-classifications was low.