Objective To assess whether antibiotic reduces voiding cystourethrogram (VCUG)-associated urinary tract infection (UTI).

Design Open-labelled randomised controlled trial.

Setting Tertiary paediatric nephrology centre.

Patients 120 children (age 2 months–5 years) undergoing VCUG.

Interventions Children were randomised into group A (antibiotic, n=72) or group B (no antibiotic, n=48) in 3:2 ratio. Group A received oral antibiotic (cephalexin if <6 months or co-trimoxazole if >6 months old) a day prior to VCUG and continued for 1 day post VCUG.

Main outcome measures The main outcome measure is incidence of VCUG-associated UTI. Urine was checked on day 3 after VCUG and UTI was defined as significant growth of a single organism in a symptomatic child.

Results The median age was 8 months (IQR 13 months) with 68% male. Indication for undertaking VCUG was history of UTI (first UTI in infancy=43, recurrent UTI=49) and congenital anomaly of kidney and urinary tract without any UTI (n=28). Post-VCUG UTI was significantly higher among group B in comparison to group A (17% (n=8) vs 1.4% (n=1); p=0.01, OR=14.2 (95% CI 1.7 to 117)). Multivariate binary logistic regression analysis found an abnormal pre-VCUG ultrasound scan to be a significant independent risk factor for post-VCUG UTI (p=0.02, OR=9.51, 95% CI 1.43 to 63.4). The number needed to treat with antibiotic to prevent one post-VCUG UTI was 6.5, which reduced to 4 if only the group with abnormal pre-VCUG ultrasound scan was included.

Conclusions Antibiotic significantly reduces post-VCUG-acquired UTI especially in those with abnormal ultrasound scans.

Trial registration number Clinical Trial Registry of India: CTRI/2017/03/00824.