Allowable bias derived from the NOBIDA reference values.

Figures of allowable bias are used to make rational choices of quality control rules, to judge the validity of published reference values, and to determine the stability of sample materials. Usually, allowable bias is parametrically defined as 0.25 times the total biological standard deviation, because that is half the width of the 90% confidence interval of parametrically estimated reference limits from 120 reference values. The published figures are mostly derived from very small populations, less than 120. We estimated allowable bias non-parametrically as the least of 4 percentile differences in distributions of reference values from the large Nordic reference interval project biobank and database (NOBIDA). The percentile differences are equivalent to 0.25 times the total biological standard deviation in Gaussian distributions. We also estimated allowable bias from the distributions of non-parametrically estimated reference limits after resampling 120 reference values from the same datasets. Clearly larger allowable bias was derived from the resampling method than from the percentile difference method, showing that non-parametric estimation of reference limits from 120 reference values implies a larger allowable bias than 0.25 times the normal biological standard deviation. With some exceptions, the figures of allowable bias using the percentile difference method were in the same order of magnitude as parametrically derived figures in other studies, and lend some support to the results from those smaller studies. Whether such bias specifications, if met, guarantee measurements of sufficient clinical quality is unknown.