COMPARISON OF A SINGLE G-M TUBE METHOD AND A MULTITUBE METHOD OF ASSAYING I131UPTAKE BY THE HUMAN THYROID

Abstract

The efficacy of a single G-M tube and a four-tube multicounter method is assaying the I131 content of the human thyroid gland were compared. The mean error of triplicate readings by the single-tube method was 3.3% compared to a mean error of 5.4% by the multitube method. As recorded by the single-tube method, thyroid gland uptake was 20 to 33% greater than that recorded by the multicounter method. When the source of radiation was a vial containing I131 and was strapped to the neck, the single tube method recorded an increment of 5 to 10%, whereas the multicounter method recorded a decrement of 24 to 30%. When the neck was interposed between the source and the single G-M tube, only 35% of the control count was recorded. Despite a 1 cm. variation in the position of the source of radiation, the single tube method recorded counts with an error of less than 5% when the G-M tube was at least 30 cm. from the source. It is concluded that the single G-M tube method is satisfactory, provided that sensitive gamma-counting tubes are used and provided that the distance between the G-M tube and the patients neck is not less than 30 cm. According to our data, the single G-M tube method, if used as described, is preferable to the multitube method used in these expts.