Accuracy of single concentration estimations of platelet angiotensin II receptor number. Its usefulness in screening for pregnancy-induced hypertension.

Platelet angiotensin II (AngII) receptor number has been suggested as a screening test for pregnancy-induced hypertension. However, markedly different false-positive rates have been reported, perhaps the result of differing methods used. We sought therefore to compare the two methods. Platelet AngII receptor number was determined by saturation analysis with computerized curve fitting and specific binding at a single radioligand concentration. The two methods were compared by correlation and by plotting their differences v their means, to determine their limits of agreement. There were significant correlations between the value obtained by saturation analysis and each of the three single ligand concentrations studied (1 nmol/L, P < .001; 500 pmol/L, P < .001; and 250 pmol/L, P < .01). However, for none of the three did the regression line approach the line of equality. Assessment of agreement by comparing differences and means for each subject showed increasing scatter with increasing receptor number and 95% confidence intervals too large to be clinically relevant. We conclude that the receptor number estimated from specific binding at one ligand concentration differs significantly from that obtained by saturation analysis. The limits of agreement of the two methods are wide and we urge caution in the use of single ligand concentration methods for estimating binding site densities.