Bag-Collected Urine Specimens Cannot Be Used to Diagnose UTI in Febrile Children
A bag-collected urine specimen should never be used to diagnose a urinary tract infection in a febrile 2-year-old child because the false-positive rate is extremely high, and the diagnosis of UTI cannot be established reliably through culture of urine collected in a bag. 1
Why Bag Specimens Are Unreliable
The American Academy of Pediatrics issues a strong recommendation (evidence quality A) that if a febrile infant requires antimicrobial therapy, a urine specimen must be obtained through catheterization or suprapubic aspiration (SPA) before antibiotics are administered, because the diagnosis of UTI cannot be established reliably through culture of urine collected in a bag. 1
Bag-collected specimens have an extremely high false-positive rate—only 15% positive predictive value—meaning that 85% of positive bag cultures do not represent true infection. 2
If a bag culture is positive, the likelihood of a false positive is extremely high, so the result must be confirmed by culturing urine obtained by catheterization or SPA; if an antimicrobial agent is present in the urine, the opportunity for confirmation is likely to be lost. 1
In one study of children under 2 years, none of the children with positive urine culture from a plastic collection bag had urinary tract infection confirmed by suprapubic puncture or catheterization (0% confirmation rate). 3
Another study found false-positive and false-negative rates of 18% and 24%, respectively, when using a bag specimen cutoff of ≥10⁵ CFU/mL compared to catheterized specimens. 4
Proper Specimen Collection Methods
First-Line Collection Techniques
Urethral catheterization has a sensitivity of 95% and specificity of 99% compared to suprapubic aspiration, making it the preferred method when SPA is not feasible. 1
To avoid contamination during catheterization, two practical steps should be implemented: (1) discard the first few milliliters obtained by catheter (allow them to fall outside the sterile collecting vessel) and culture only the subsequent urine; and (2) if the catheterization attempt is unsuccessful, use a new, clean catheter (aided, in girls, by leaving the initial catheter in place as a marker). 1
Suprapubic aspiration provides the most reliable specimen but is more invasive and technically challenging. [1, 5
When Preparation Stimulates Voiding
When catheterization or SPA is being attempted, the clinician should have a sterile container ready to collect a urine specimen, because the preparation for the procedure may stimulate the child to void. 1
Whether the urine is obtained through catheterization or is voided, the first few drops should be allowed to fall outside the sterile container to reduce urethral flora contamination. 1
Acceptable Use of Bag Specimens
Urine collected in a bag or via clean catch method is suitable for urinalysis (screening purposes), but such specimens—especially urine collected in a bag—are less appropriate for culture. 1
A negative urinalysis from a bag specimen can help rule out UTI (combined negative leukocyte esterase and nitrite has 90.5% negative predictive value), but a positive result requires confirmation with a properly collected specimen. 2, 6
The American Academy of Pediatrics recommends using bag specimens for urinalysis screening only, never for definitive culture-based diagnosis. 1, 5
Diagnostic Criteria for UTI in This Age Group
To establish the diagnosis of UTI in febrile infants and children 2–24 months, clinicians should require both urinalysis results that suggest infection (pyuria and/or bacteriuria) and the presence of at least 50,000 colony-forming units (CFU) per milliliter of a uropathogen cultured from a urine specimen obtained through transurethral catheterization or SPA. 1, 6
The thrust of this key action statement is that the diagnosis of UTI in febrile infants is signaled by the presence of both bacteriuria and pyuria; in general, pyuria without bacteriuria is insufficient to make a diagnosis of UTI because it is nonspecific and occurs in the absence of infection (e.g., Kawasaki disease). 1
10–50% of culture-proven UTIs in febrile infants have false-negative urinalysis, so culture is mandatory regardless of urinalysis results. 2, 7
Management Algorithm for This Clinical Scenario
Step 1: Assess Clinical Urgency
If the 2-year-old appears ill or toxic and requires immediate antimicrobial therapy, obtain a urine specimen by catheterization or SPA before administering antibiotics. 1
Once antimicrobial therapy is initiated, the opportunity to make a definitive diagnosis is lost because multiple studies show that urine may be rapidly sterilized within 24–48 hours. 1, 2
Step 2: Obtain Proper Specimen
Perform urethral catheterization (preferred in most settings) or suprapubic aspiration to obtain a specimen suitable for both urinalysis and culture. 1
Do not delay antibiotic therapy in a toxic-appearing child, but ensure the specimen is collected first. 1
Step 3: Interpret Results
Require both abnormal urinalysis (pyuria ≥10 WBCs/HPF or positive leukocyte esterase, and/or bacteriuria) and culture showing ≥50,000 CFU/mL of a single uropathogen to diagnose UTI. 1, 6
If the bag specimen was already collected and is positive, do not treat based on that result alone—obtain a catheterized or SPA specimen for confirmation before starting or continuing antibiotics. 1, 3
Common Pitfalls to Avoid
Never diagnose UTI from a bag specimen culture alone—the 85% false-positive rate means most positive results do not represent true infection. 2, 3
Do not assume a negative bag urinalysis completely excludes UTI in a febrile child—if clinical suspicion remains high, obtain a catheterized specimen because 10–50% of true UTIs have false-negative urinalysis. 2, 7
Avoid treating empirically based on bag culture results without confirmation, as this leads to unnecessary antibiotic exposure, imaging studies, and potential harm from overdiagnosis. 1, 3
Do not use bag specimens for culture in children under 2 years when UTI diagnosis is being considered—studies show 0% confirmation rate when bag cultures are verified by catheterization or SPA. 3
Quality-of-Life and Morbidity Considerations
Prompt and accurate diagnosis of UTI reduces renal scarring, which can lead to long-term complications including hypertension and chronic kidney disease. 8
Overdiagnosis from unreliable bag specimens leads to unnecessary hospitalization, invasive imaging procedures (voiding cystourethrography), and potentially harmful antibiotic treatment in children who do not have UTI. 3
Missed diagnosis due to false-negative bag cultures can result in untreated pyelonephritis, increasing the risk of permanent renal damage. 8, 5