Effect of lot-to-lot variability in filter paper on the quantification of thyroxin, thyrotropin, and phenylalanine in dried-blood specimens.

We prepared whole-blood pools to contain various concentrations of phenylalanine (Phe), thyroxin (T4), and thyrotropin (TSH) and applied them to six different lots of Schleicher & Schuell Grade 903 filter paper, two of which represented extremes for serum-absorbancy. Individual measured T4 values showed minimal overlap among all pools for each individual filter-paper lot and for all lots combined, but Phe values overlapped considerably among the high-concentration pools within and among lots. Individual TSH values also showed considerable overlap among the high-concentration pools for all lots combined, but little overlap within each lot. Maximum differences in mean observed values among lots ranged from 6% to 36% for all analytes. Assay results from hemolyzed blood specimens generally were lower than from intact-cell blood specimens for T4 and TSH, but slightly higher for Phe. Maximum among-lot differences in mean values ranged from 13% to 29% for all analytes when each tested lot was used for assay calibration. Lot-to-lot differences in measured values were not strongly related to serum absorbancy values. We conclude that routinely encountered within- and among-lot filter paper variability, as measured by serum-absorbancy, is not alone sufficient to cause gross quantification errors in neonatal screening programs.