The diagnosis of UTI is confirmed by the detection of bacteria in a urine culture, but urine collection is often difficult, especially in infants and very small children. Several factors may alter a urine specimen. Contamination of a specimen by organisms from sources other than the urine is the most common cause of false-positive results. Bag urine specimens are commonly contaminated by perineal and perianal flora and are usually considered inadequate for definitive diagnosis. The American Academy of Pediatrics recommends the urine collected by the bag can be used to determine whether it is necessary to obtain a catherized urine specimen for culture.