AJP - Endocrinology and Metabolism, Vol 251, Issue 3 357-E361, Copyright © 1986 by American Physiological Society
Variance weighting functions in radioimmunoassay calibration
T. W. Gettys, P. M. Burrows and D. M. Henricks
Software packages for radioimmunoassay calibration assume that expected
counting rate is a function of ligand dose. Previous studies have indicated
that variances of counting rate are also related to dose, but the structure
of individual assays does not permit precise estimation of counting rate
variances at individual doses. The method described here uses results from
an accumulation of assays to characterize the relationship between mean and
variance of counting rate, thus providing a variance weighting function for
the calibration. Analysis of 140 cortisol assays, all with two replicates
of each of 50 sources (9 standard doses, 3 quality control preps, and 38
unknowns), led to an asymmetric rising ogive relating variances to means of
counting rates. A rectangular hyperbola provided an adequate
characterization of this relationship in an accumulation of 21 testosterone
assays. Relationships between mean and variance of counting rate in 19
growth hormone assays and 7 triiodothyronine assays were characterized by a
straight line and a rising exponential curve, respectively. Calibration
curves, such as the commonly adopted logistic ogive in relation to log
dose, are fitted by weighted least squares to observed counts directly
using empirical weights proportional to the reciprocal of estimated
counting variance. The advantage of this method is that all observations
contribute to the calibration in accordance with their merit.
