Variation in gamma counter efficiency used in radioimmunoassay testing for hepatitis B surface antigen and antibody.

Occasional discordant results on testing samples by radioimmunoassay for hepatitis B surface antigen and antibody led to this analysis of gamma counters used by different hospitals and testing laboratories. Eight dilutions of purified 125I were sent under code to 11 institutions doing radioimmunoassay testing. Results demonstrated a wide range in background counts-per-minute (cpm) as well as considerable variation between gamma counters in per cent counting efficiency for 125I (ranging from 20 to 48%). These data support the need for regular monitoring and calibration of gamma counters to ensure low background cpm as well as maximum counting efficiency. This will give reproducible and reliable results in radioimmunoassay testing.