A method of obtaining a parameter for radiocolloid trapping by the Kupffer cells using a direct-store video tape data processor is outlined. Activity histograms for 99mTc sulfide colloid are obtained on playback for liver and mediastinal blood pool. A net 4.5 to 1.5 minute count rate ratio for each tracing is procured, and a liver trapping index is calculated by dividing the liver ratio by the mediastinal ratio. This index yields a better separation of patients with impaired liver perfusion than the previously utilized 198Au blood retention method.