Influence of counting statistics on borderline detection of hepatitis B antigen by a solid-phase radioimmunoassay

The detection of hepatitis B antigen by a solid-phase radioimmunoassay technique is by now well established, particularly where transfusions are concerned. However, it would appear that the use of systematic procedures leading to positive detection, such as the "cutoff" method (mean counts per minute of the negative controls multiplied by a coefficient of 2.1), are open to criticism if elementary precautions concerning the time span and the statistical accuracy of radioactivity counting are not taken. The method may even prove inaccurate in a certain number of borderline cases studies in relation to plasma and different sera, especially if subjected to a single screening process over a large number of samples analyzed by computer. These deficiencies can be avoided by correcting the positive coefficient in relation to the mean counts per minute of negative controls, the statistical accuracy (relative standard deviation) and the distribution of counting times.