Critical analysis of the accuracy of a combined test (75Se-HCAT and 58Co-B12) of ileal function using a whole-body counter.

The accuracy of a combined test (75Se-HCAT + 58Co-B12) of ileal function by whole-body counting has been assessed with particular attention being paid to the reliability of the 100% retention measurements, the accuracy of the crossover of 58Co into the 75Se channel and the effects of variable counting geometry. A linear relationship between 58Co crossover and body weight improved the accuracy of determining the crossover value for individual patients. Non-geometrical errors such as those associated with counting statistics and crossover were found to be small, with a coefficient of variation usually less than +/- 2% except in cases of very low 75Se-HCAT retention in the presence of substantial amounts of 58Co. The main sources of error were due to problems of variable counting geometry but there was very good agreement between 75Se and 58Co retentions measured by whole body counting and by excreta analysis which suggested that the overall error of the test was unlikely to exceed +/- 10%. A protocol is presented for use of the combined test on an outpatient basis and includes retention measurements at 4, 7, and 10 d post administration.