Identification of an unusual gamma counter fault using Trigg's technique for trend detection.

A simultaneous deterioration in performance of several radioimmunoassays suggested that equipment was malfunctioning. Replicate counts from an automated gamma counter showed unacceptable variation during a 24-hour period; this variation was detected using Trigg's technique, a sensitive method for trend analysis, and was not identified by the usual laboratory testing procedures. A defective capacitor was found in the voltage stabilisation unit in the gamma counter, which probably made it responsive to small variations in the power supply to the laboratory. It is suggested that the analysis of replicate counts using Trigg's technique is a valuable test of gamma counter function which should become routine.