Urinary tract infection (UTI), which affects 3% of young children in the United States annually, is the most common serious bacterial infection in children.1 In some children with UTI, infection is localized to the bladder (cystitis). In others, bacteria successfully ascend from the bladder to the kidney causing acute pyelonephritis (APN). APN is one of the principal causes of acquired renal scarring in children. Because it is not possible to differentiate between children with cystitis and APN on clinical grounds, and because markers for APN have not been identified, all children with UTI are currently managed as if they have APN. Current guidelines recommend that all young children with UTI receive treatment with long-course antibiotics (7-14 days) and undergo follow-up imaging tests. This has resulted in an overuse of imaging and antimicrobials for children with UTI. We propose an alternate strategy in which measurement of blood and/or urine biomarkers at the time of presentation is used to stratify children into pathophysiologically distinct subgroups (cystitis vs. APN). This will allow clinicians to individualize the treatment of UTI and to reduce unnecessary use of antimicrobials and imaging tests. As part an NIAID-funded 7-year study designed to compare the efficacy of short-course antimicrobials for children with UTI, we will be enrolling one of the largest samples of children with UTI to date (target n=746). By collecting additional blood and urine samples at the time of presentation from the first 160 children being considered for that study, and by conducting an early DMSA renal scan (to identify APN), we will be able to identify biomarkers that accurately differentiate children with APN from children with cystitis at a minimal additional cost. We will use a step-wise approach to achieve these aims. First, we will determine the accuracy of 5 previously identified proteins (urine IL-6, urine IL-8, urine IL-12, urine macrophage inhibitory factor, and serum procalcitonin) in differentiating APN from cystitis. Next, by examining genes that are differentially expressed in leukocytes from patients with APN and cystitis, we will develop an RNA-based profile that can accurately distinguish APN from cystitis. Finally, we will construct a prediction rule using clinical variables, previously studied proteins, and the RNA-based profile, to differentiate children with APN from children with cystitis. The prediction rules developed in this ancillary study will significantly enhance the parent study by providing an accurate method of stratifying the analysis according to the presence or absence of APN. PUBLIC HEALTH RELEVANCE: The blood and urine tests developed in this grant will allow doctors to differentiate children with kidney infection from children with bladder infection. In this way, children with bladder infection, who are not at risk for permanent kidney damage, can be treated with fewer days of antibiotics and without performing any of the currently recommended imaging tests. This approach could lead to significant reductions in the use of antibiotics and the exposure to radiation in children with urinary tract infections.