As a new graduate, I recently assisted an RN caring for a patient with suspected occult gastrointestinal bleeding. Before she inserted an indwelling urinary catheter, I prepped the patient's perineum with povidone-iodine (Betadine). The RN cautioned me not to let the prep solution overflow into the anal area because it might interfere with guaiac (Hemoccult) testing for occult blood. Is this correct?-A.T., CALIF.

Yes. A Hemoccult test involves smearing a fecal sample onto reagent paper. If the paper turns blue, the test is considered positive for blood. However, many substances, including povidone-iodine, can cause a false-positive result. Because povidone-iodine is so widely used for skin preps, the Hemoccult product information contains a special reminder not to let it contaminate a fecal test specimen.