Increased genital uptake of 99mTc red blood cells: a potential cause of false-positive studies for gastrointestinal bleeding.

We have recently seen several 99mTc labeled red blood (Tc-RBC) cell studies, for the localization of gastrointestinal (GI) bleeding, which were difficult to interpret because of increased activity in the low abdomen, which was found due to male genital activity. To examine the extent of this problem six recent Tc-RBC studies for GI bleeding in males were examined for the presence of penile activity. Penile activity was seen in all males and was marked enough to require additional views in four. Lateral views readily separated penile from rectal activity, showing that the activity was not posteriorly located. Twenty-four Tc-RBC studies on females were also examined. Although modest levels of perineal activity could be seen, this did not cause problems in diagnosis. Attention to this common problem in males should prevent false-positive diagnoses of GI bleeding from the rectum.