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QUESTION When collecting a midstream urine sample from toilet-trained children, does cleaning of the perineal/genital area before collection prevent sample contamination? METHODS Design: cluster randomised (by week) controlled trial. Allocation: not concealed. Blinding: blinded (microbiologists). Follow-up period: 48 hours. Setting: emergency department of a tertiary care paediatric centre in Montreal, Quebec, Canada. Patients: 350 toilet-trained children 2-18 years of age (45% 2-5 y, 37% 6-12 y, and 18% 13-18 y; 60% girls) who had a midstream urine sample requested for any medical reason. Children with developmental delay were excluded. Intervention: cleaning of the perineal/genital area twice, using liquid soap and gauze pads (n = 179) or no cleaning (n = 171) before collecting a midstream urine sample in a sterile container. Only samples with a positive urinalysis result were sent to the microbiology laboratory for culture. Outcomes: positive urinalysis (positive leukocyte esterase or nitrites on dipstick or ≥5 white blood cells per high-powered field after centrifuging) and contaminated urine culture (<108 colony-forming units of a single organism or a mix of ≥2 organisms per litre of urine). Urinary tract infection was diagnosed by a positive urine culture (≥108 colony-forming units of a single pathogenic organism per litre of urine). Patient follow-up: 100% (intention-to-treat analysis). MAIN RESULTS Cleaning of the perineal/genital area reduced the risks of a positive urinalysis result and contamination of the urine culture (table). The intervention was more beneficial in children 6-18 years of age (6.3% v 29% contamination) than in those 2-5 years of age (10% v 15% contamination). The prevalence of urinary tract infection was 7%. The predictive value of a positive urinalysis result was 41% in the cleaning group and 13% in the no cleaning group (p = 0.001). 93% of contaminated specimens and 96% of positive cultures were from girls. CONCLUSION When collecting a midstream urine sample from toilet-trained children, cleaning of the perineal/genital area before collection reduced the frequency of sample contamination and a positive urinalysis result. [ABSTRACT FROM AUTHOR]