To diagnose a urinary tract infection in children, a positive urinary culture is required. Contamination poses a frequent challenge so a cutoff of 10 CFU/mL is used for a "clean-catch" mid-stream sample, 10 CFU/mL is used for catheter-obtained specimens, and 10 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.