Minimum detectable activity when background is counted longer than the sample

This note discusses the use of blank or background counting data that are measured for times that differ from times used for the sample counts. The correct formula for the minimum detectable activity, under this condition, is given as follows: MDA = [3 + 3.29 square root of Rbtg(1 + tg/tb)]/epsilon tg, where Rb denotes background count rate, tb and tg denote background and gross count times, and epsilon denotes counting efficiency. Counting backgrounds for a long time reduces decision levels, uncertainties, and minimum detectable activities. These benefits are fully available only when there is no other source of variability than random fluctuations in count rates.