Survey Result of the Counting Efficiency of Gamma Counter by Certified Reference Materials.

Purpose: In radioimmunoassay (RIA), the gamma counter is the important instrument for the accurate measurement. To manage quality assurance of RIA, the counting efficiency of gamma counter is one of the important parameters. The aim of this study was to evaluate the counting efficiency of gamma counters in multiple institutes on the base of traceability by using the certified reference materials (CRMs).

Methods: Twenty-three institutes that perform RIA were enrolled in this study. I-125 CRMs that were certified by National Institute of Standards and Technology (NIST) were used. Each institute was asked to count the activity of I-125 CRMs at most twice on all gamma counters in use. The counting efficiency of each well of counter was calculated on the base of NIST-certified information, corrected for I-125 decay for date of testing.

Results: From 23 institutes, 44 gamma counters were evaluated. The average counting efficiency of all wells was 85.9% and the standard deviation was 13.5%. As a mean value of each gamma counter, three gamma counters showed poor counting efficiency (less than 70%). The poorest counting efficiency was 7%. The counting efficiency of seven gamma counters was between 70 and 75%. Eight counters had the counting efficiency between 75 and 90%. More than half of counter (26 gamma counters) showed excellent counting efficiency (more than 90%). The standard deviation variation range of inter-well efficiency was from 0 to 11.2.

Conclusion: The first survey on the counting efficiency of gamma counter was performed in South Korea. Most of the RIA laboratories have well managed the quality assurance of gamma counter.