Raising the threshold of bacterial colony counts improves the accuracy of diagnosing a urinary tract infection in children
Article Outline
- Question
- Design
- Setting
- Participants
- Intervention
- Outcomes
- Main Results
- Conclusions
- Commentary
- References
- Copyright
Coulthard MG, Kalra M, Lambert HJ, Nelson A, Smith T, Perry JD. Redefining urinary tract infections by bacterial colony counts. Pediatrics 2010;125:335-41.
Question
Among children with suspected urinary tract infection (UTI), how accurate is a colony count of 106 colony-forming units (cfu)/mL in making the diagnosis?
Design
Prospective study.
Setting
Single hospital in the United Kingdom, providing secondary and tertiary care.
Participants
Two hundred three children (aged 2.0 weeks to 17.7 years) in a hospital setting who were suspected of having UTI for a variety of reasons.
Intervention
Quantitative culture of paired urine samples from children were promptly tested together after serial dilution.
Outcomes
The diagnosis of UTI, defined as having the same uropathogen in both urine samples at concentrations within 25-fold of each other.
Main Results
The 36 children who had a confirmed UTI had a mean colony count of 1.7x107 colony-forming units/mL. Among the 167 children who did not have a urinary tract infection, 12 (7.2%) would have had a false-positive diagnosis made on the first sample, which was revealed because the second sample result was different (n=7) or had a ≥25-fold different colony count (n=5). Raising the threshold from 105 to 106 colony-forming units/mL reduces the false positive rate to 4.8%, and the likelihood ratio for a positive test increases from 13.9 to 20.8. If 2 samples are cultured, the false-positive rates fall to 3.6% and 0.6%, respectively. All 9 children (5.4% of those without a urinary tract infection) who had a mixed culture with ≥105 colony-forming units/mL of a uropathogen (heavy mixed growth) in the first sample had a urine infection excluded by the second sample result.
Conclusions
The minimum urinary bacterial concentration that is used to diagnose a urine infection should be increased from ≥105 to ≥106 colony-forming units/mL, because that would reduce the false-positive rate from 7.2% to 4.8% if 1 sample was cultured and from 3.6% to 0.6% if 2 samples were cultured. Urine samples with heavy mixed growths should be considered contaminated.
Commentary
Urinary tract infections continue to be a cause of serious bacterial infections in children. The pioneering work of Kass1 established the microbiologic definition of a UTI in adults over 50 years ago, and subsequent work by Pryles supported this definition in pediatric patients.2 The present study questions these time-honored definitions and offers some simple advances in laboratory techniques that could help clarify the interpretation of urine culture results. The authors argue that a cutoff value for defining a “true-positive” UTI of 105 cfu/ml is too low and results in many “false-positive” diagnoses of UTIs. They then demonstrate that increasing the cutoff value for defining a “true-positive” UTI to 106 would eliminate many of the “false-positive” UTIs and the subsequent workup of the urinary system. More importantly, they advocate performing serial dilutions of the urine which might be important in preventing a “false-negative” culture, which could result in ignoring a patient that had significant renal disease. Although the present study adds to our understanding of the laboratory techniques for diagnosing UTIs, it is important to evaluate the entire patient. A number of recent studies have applied data on the prevalence of UTIs in various patient populations to help determine who is at risk for having a UTI and requires thorough evaluation.3 Thus, the best approach remains a combination of history, physical examination, and appropriate laboratory evaluation.
References
PII: S0022-3476(10)00403-8
doi:10.1016/j.jpeds.2010.05.007
© 2010 Mosby, Inc. All rights reserved.
