Acute UTI: diagnosis and management (CME)

While you wait for results of microscopic analysis of the urine and culture, should you treat this child with antibiotics? If the culture indicates that she does indeed have a UTI, should you order imaging? If so, what kind? Are prophylactic antibiotics appropriate to prevent future UTIs?

UTI is one of the most common bacterial infections of childhood.¹ Nonetheless, clinicians, patients, and families continue to ask questions like those posed by the case described above. Recent studies and publications suggest the best answers to these and other frequently asked questions.

The short answer is to order a urine culture-the gold standard for the diagnosis of UTI. In truth, the answer to this question is more complicated and can be broken down into three separate questions:

From whom should you obtain a urine?²

Obtain a urine from any infant under 2 months of age with fever; infants and children from 2 months to 2 years of age with fever for greater than two days and no other identifiable source of such fever; girls older than 2 years with symptoms that suggest urinary tract involvement (frequency, urgency, hesitancy, dysuria, and lower abdominal pain); and febrile infants and children with a previous UTI history, known urinary tract abnormalities, or positive family history.

A 2003 study validated a previously published clinical decision rule for obtaining a urine culture in young girls using five variables: age below 1 year, white race (although less likely, UTI in non-white children should remain a consideration), temperature ≥39° C, fever for more than two days, and absence of another source of fever on history or physical exam. The presence of three or more of these variables was highly predictive of a positive urine culture.³

And how predictive of UTI is urinalysis? In a series of publications looking at sensitivity, specificity, positive likelihood ratio, and negative likelihood ratio, the most predictive urinalysis study was on an uncentrifuged urine specimen with >10 white blood cells/mm³ plus bacteria seen on Gram stain.^4-6 This combination was 85% sensitive and 99.9% specific, with a positive likelihood ratio of 85 and a negative likelihood ratio of 0.1.

What are the best ways to collect urine?

Bag specimens should not be used for urine culture because of the risk of contaminated culture results.^6,7 The high false-positive rate associated with bag specimens calls into question their use even for dipstick or microscopy. Some observers argue, however, that when the bag is removed shortly after urination, specimens can be used for urinalysis and microscopy as a screening tool, especially to determine if catheterization or suprapubic aspiration is needed.⁸ But do not use urine specimens from a bag for culturing, especially in ill-appearing young children.

Practically speaking, urine cultures should be obtained from midstream clean catch specimens, or by catheterization. In rare instances, a suprapubic bladder tap may be the best way to get urine culture. Significant bacteria indicative of infection on a clean-catch urine is >100,000 colony-forming units per mL of a single uro-pathogen.⁹ For a specimen obtained by catheter, >50,000 colony-forming units per mL of a single pathogen, is considered diagnostic of a UTI.⁹ Most observers agree that any number of a single uropathogen grown from a suprapubic bladder tap indicates infection.⁶