to diagnose a urinary tract infection in children, a positive urinary culture is required. contamination poses a frequent challenge so a cutoff of 105 cfu/ml is used for a "clean-catch" mid-stream sample, 104 cfu/ml is used for catheter-obtained specimens, and 102 cfu/ml is used for suprapubic aspirations (a sample drawn directly from the bladder through the stomach wall with a needle). the world health organization does not like the use of “urine bags” to collect samples because there is a high rate of contamination when that urine is cultured. catheterization is preferred if an individual is unable to use a toilet. some medical groups, such as the american academy of pediatrics, recommend renal ultrasound and voiding cystourethrogram (watching a person's urethra and urinary bladder with real time x-rays while they urinate) in all children who are younger than 2 years old and have had a urinary tract infection.other medical groups such as the national institute for clinical excellence recommend routine imaging only in babies younger than 6 months old or who have unusual findings.