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In infants and young children, the only cardinal feature of UTI is a febrile illness without other definitive source. The approach to neonates and infants <3 months of age with fever and no identifiable source is discussed in detail in chapter 116, Fever and Serious Bacterial Illness in Infants and Children. Urine testing (including urine chemical strip testing, microscopy, and culture) is an important part of a more comprehensive evaluation in this age group. As mentioned earlier, UTI should be considered in infants with bronchiolitis, particularly in the presence of high fever (temperature of 40°C [104°F]).16,17 In verbal children, dysuria combined with suprapubic tenderness on examination is the classic constellation of symptoms and signs.
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There are no clinical criteria that confirm the diagnosis of UTI in children without urinary testing and culture.18 Evidence-based clinical practice guidelines for the evaluation and treatment of pediatric UTI from the American Academy of Pediatrics are limited to infants and young children 2 to 24 months of age and require both pyuria and bacteriuria with ≥50,000 colonies/mL of a single uropathogenic organism (in a properly collected specimen, <1 hour old at room temperature and <4 hours old refrigerated) for definitive diagnosis of UTI.19 Positive urine cultures in the absence of pyuria/bacteriuria may represent asymptomatic bacteriuria. For infants <2 months old, a positive urine culture is the gold standard for diagnosis.
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In adolescents, symptoms of dysuria without vaginal or urethral discharge, or an examination consistent with UTI/pyelonephritis, such as suprapubic or costovertebral angle tenderness, in the presence of a positive urine chemical strip for pyuria and/or nitrites, allow a presumptive diagnosis of UTI. A careful sexual history (with assurance of confidentiality and respect for privacy) is important in this age group, because urethral symptoms (such as dysuria) may predominate in both UTI and sexually transmitted infections. Urine culture remains important for definitive diagnosis, and pyuria without uropathogenic culture growth may suggest sexually transmitted infection. Consider pelvic examination for sexually active girls and appropriate testing in both boys and girls with dysuria who are sexually active (see chapter 149, Sexually Transmitted Infections).
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DIFFERENTIAL DIAGNOSIS
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UTI is a possible diagnosis in all infants with fever. In children with dysuria but no fever, the most common concerns are listed in Table 132-2.
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LABORATORY EVALUATION
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Urine Sample Collection
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If children can void on command, then attempt collection of a spontaneously voided specimen. Perineal cleaning before voiding reduces the rate of false-positive urinary dipstick tests and the rate of contaminated culture results.20
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In infants and children who are not able to void on command, bladder catheterization is the preferred method for urine collection. Suprapubic aspiration, although invasive, is also acceptable. The value of perineal bag specimens is limited by the high false-positive results and low specificity. Although the sensitivity of perineal bag specimens for UTI is generally similar to that collected from catheterized or suprapubic specimens, the specificity is low, and a positive culture result from a perineal bag specimen has a high likelihood of significant contamination with perineal bacterial flora. The only (rare) circumstance where a perineal bag specimen may be used is to exclude disease when the pretest probability of UTI is very low, in which case a negative test rules out disease; if the results from a perineal bag specimen are positive, confirmation before giving antibiotics requires culture of a specimen collected in a sterile manner. Due to this diagnostic delay, many clinicians prefer obtaining a definitive specimen initially.
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The definitive test for UTI is a urine culture, and colony counts indicating infection are based on the type of sample collection (Table 132-3). Do not give antibiotics until a urine culture is obtained using a sterile method (bladder catheterization or suprapubic aspiration; see Procedures later). Based on the clinical scenario, length of illness, and urinalysis results, lower colony counts or mixed-growth cultures cannot necessarily be dismissed. Gram-negative urine culture results are usually available within 24 hours of the time that the culture plate is prepared.
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Colorimetric or Chemical Test Strip Testing and Microscopic Analysis
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Urine culture results are not available at the first ED visit, so chemical test strips that can detect leukocyte esterase and urinary nitrites, in conjunction with microscopic urinalysis, help predict the results of the urine culture. Leukocyte esterase is an enzyme found in some white blood cells, and its presence on testing suggests pyuria. Nitrite in the urine depends on coagulase-splitting bacteria that can reduce urinary nitrate to nitrite. Enterobacteria generally reduce nitrate to nitrite if the urine has been in the bladder long enough (about 4 hours), but most gram-positive bacteria do not reduce nitrate to nitrite, so nitrite testing has insufficient sensitivity for detection of UTI (53%; range, 15% to 82%).19 Urinary nitrites and leukocyte esterase alone are not sensitive markers for children who empty their bladders frequently. Urinary nitrites are highly specific (98%; range, 90% to 100%)19 and are therefore helpful when positive.21 Microscopy of spun or unspun urine for leukocytes and bacteria and/or Gram stain of unspun urine are also helpful for immediate diagnosis; urine Gram stain performs as well as a chemical test strip for urine infection.22 Gram stain can also identify the bacterial morphology and help guide appropriate initial antibiotic therapy. Table 132-4 summarizes the test characteristics of the urinalysis.
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Regardless of the results from chemical testing strip or microscopic analysis, send the sample for a urine culture. There are two situations where one might not send urine for culture: (1) in low-risk patients with a completely normal urinalysis and another explanation for the symptoms; and (2) in older adolescent females with a very high posttest probability of UTI, without severe illness or complicating medical problems, and in an area with a predictable antibiotic resistance pattern. Because pyuria alone does not confirm a UTI, refer to Table 132-2 for some causes of culture-negative pyuria.
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Acute imaging of the urinary tract is rarely necessary in UTI assessment or treatment. If children have persistent fever or are worsening despite appropriate therapy or symptoms are unusually severe, then renal US is indicated to rule out abscess, stone, or obstruction. A renal US and bladder US are recommended for children 2 to 24 months old after the first UTI.19 Routine urethrocystography after first UTI is not recommended.19 Voiding cystourethrography is indicated if renal-bladder US reveals hydronephrosis, scarring, or other findings that would suggest either high-grade vesicourethral reflux or obstructive uropathy. This testing is arranged on an outpatient basis or is performed during hospitalization and is not typically facilitated from the ED. Some expert groups recommend against imaging after the first UTI, if the infection follows a typical clinical course. In Britain, US is recommended in the setting of atypical response to therapy or recurrent infection.23,24
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Other Suggested Testing
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Testing beyond urinalysis and culture is not necessary for afebrile children with isolated UTI before initiating treatment. Children with atypical presentations or significant comorbidities may require further investigation as directed by the clinical picture. Febrile neonates and young infants (<2 months of age) require further evaluation prior to initiating antibiotics. Approximately 10% of young infants with febrile UTI admitted to the hospital demonstrate a sterile cerebrospinal fluid pleocytosis thought to be due to systemic release of inflammatory mediators.27,28 Less than 1% of febrile infants with UTI will also have bacterial meningitis. Perform lumbar puncture and obtain blood cultures in febrile infants <1 month old with UTI before starting empiric antibiotics (see chapter 116). Lumbar puncture is not needed in febrile older infants and children with UTI unless there are signs or symptoms suggestive of meningitis, or if the child is clinically ill or the illness does not respond to treatment.29
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Most febrile infants with UTI have upper tract involvement, and laboratory tests are unlikely to differentiate those with bacteremia from those without bacteremia.30 About 5% to 10% of febrile infants with UTI have bacteremia.27 There is low risk of bacteremia and adverse events in well-appearing infants and children with febrile UTI.29,31,32 For older patients being sent home for UTI treatment who have been initially committed to oral antibiotics, a blood culture is not likely to be helpful.