Comparison of nonisotopic and radioimmunoassay techniques for digoxin: a study based on external quality assurance measurements.

A comparison of the precision and accuracy of three nonisotopic and of radioimmunoassay (RIA) techniques for digoxin was made using data from 141 samples of spiked human serum assayed between 1984 and 1987 by members of the Heathcontrol external quality assurance scheme. There were significant differences between techniques in the number of observations rejected as outliers greater than 3 SD from the sample mean, the percentages of rejected measurements being Abbott TDX 1.4, RIA 3.3, Ames TDA 4.2, and Syva EMIT 6.3. Ames TDA and Syva EMIT were shown to have a significantly lower precision than the other techniques in measurements of digoxin concentrations less than 1 nmol/L. A significant 4% negative bias was observed for Ames TDA measurements of digoxin concentrations between 1 and 2.6 nmol/L. A concentration-related bias was demonstrated for Abbott TDX measurements that varied from 14% overestimates at concentrations less than 1 nmol/L to 6.7% underestimates of digoxin at concentrations greater than 2.6 nmol/L.