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The appearance of the gallbladder in images of patients undergoing 99Tcm-hexamethylpropylene amine oxime (99Tcm-HMPAO) autologous white cell scans (AWCSs) was compared before and after the introduction of fasting in the white cell scan protocol in our department. Before February 1995, when fasting was not required as a condition of the test, gallbladders were identified in 10 and 13% of 128 patients at 1 and 3 h, respectively. After February 1995, when fasting was imposed for the duration of the AWCS procedure, gallbladders were identified in 44 and 58% of 139 patients undergoing an AWCS at 1 and 3 h, respectively (P < 0.01). While further studies are required to show that fasting affects the false-positive rate, the results suggest that the degradation products of 99Tcm-HMPAO may be retained in the biliary system longer during fasting. Fasting should therefore be included in the protocol for an AWCS, particularly in the evaluation of inflammatory bowel disease (IBD) when 99Tcm-HMPAO is the cell label used.