This post is a review of a recent paper from PEDIATRICS that came from the PECARN research network.
Accuracy of the Urinalysis for Urinary Tract Infections in Febrile Infants 60 Days and Younger
Leah Tzimenatos, MD, Prashant Mahajan, MD, MPH, MBA, Peter S. Dayan, MD, MSc, Melissa Vitale, MD, James G. Linakis, MD, PhD, Stephen Blumberg, MD, Dominic Borgialli, DO, MPH, Richard M. Ruddy, MD, John Van Buren, PhD,i Octavio Ramilo, MD, Nathan Kuppermann, MD, MPH, for the Pediatric Emergency Care Applied Research Network (PECARN)
What are the test characteristics of the urinalysis in diagnosing urinary tract infection in infants less than 60 days of age with fever?
This is a planned secondary analysis of a prospective study conducted between 2008-13, from the PECARN network. The initial paper from Mahajan et al is linked here. It includes almost 5,000 babies ≤60 days of age with a temperature ≥38 C. In the original study patients were enrolled if they had blood cultures obtained as a part of the febrile workup.
How was urinary tract infection defined? Contaminant? UTI with bacteremia?
For the primary analysis UTI was defined as a growth “of ≥50 000 CFUs/mL of a known urinary pathogen from a culture obtained via catheterization or ≥1000 CFUs/mL from a culture obtained via suprapubic aspiration.” For our secondary analysis, we defined UTI as the growth of ≥10 000 CFUs/mL of a known urinary pathogen from a culture obtained via catheterization or ≥1000 CFUs/mL from a culture obtained via suprapubic aspiration.
Contaminants were skin or genitourinary flora (coag-negative Staphylococcus, Lactobacillus, and Corynebacterium) and more than two organisms.
UTI with bacteremia was defined as positive blood culture.
How did they define a positive urinalysis?
Leukocyte esterase was positive if “any amount, including a trace amount, was detected.” Nitrite results were either negative or positive. Pyuria was >5 WBC/HPF. Of note, some centers omit microscopy of the urine if the dipstick is negative. If a dipstick was negative and no microscopy done then the authors decided pyuria was negative. A positive U/A had any of the three positive. A negative U/A had all three negative.
What patients were included?
Again, these babies were from a previous study. Infants that had a urinalysis and urine cultures obtained via urethral cath or suprapubic aspiration (only 7 of these in the study) were included. They excluded the following patients:
- Bacteremia without associated UTI
- Bacteremia and concurrent UTI caused by different pathogens
- Bacteremia status was unclear
This is a large a reasonable sample of patients that is definitely similar to what you will be seeing in the ED.
Get to the results already!
OK, fine!
Overall they analyzed 4,147 infants and 289 – 7% – had UTIs (as defined by culture with colony counts ≥50,000 CFUs/mL). 27/289 (9.3%) had bacteremia. The test characteristics are as follows:
from Tzimenatos et al. PEDIATRICS, 2018
All right, let’s unpackage these results a bit. Interestingly, the sensitivity of an abnormal U/A and any UTI is 94%. Sensitivity is “ruling out” – SNOUT. So could we have 6% of febrile baby UTIs with a normal U/A? Seems that way. But the sensitivity of UTI + bacteremia is 100%. We will get blood and urine in the 29-60 day old febrile infant. If both are normal then we D/C home. These results suggest that in an (eventually) non-bacteremic UTI we will see a small proportion of babies with negative U/A but positive culture. This further highlights the importance of primary care doctor (or other) follow up within the next 24 hours.
What about urinalyses with 5-9 wbc/hpf versus more wbcs/hpf?
This is an interesting question that we discussed at a recent journal club. Essentially, the authors defined pyuria as >5 abcs/hpf in order to increase the sample size. There is no data presented on whether or not more wbcs is worse. Know however, that certain pathogens (namely Enterococcus) do not tend to have significant pyuria so the issue is more muddled than you might think. For the purposes of a clean study definition the use of the >5 abcs/hpf as pyuria is reasonable.
I don’t think you can say definitively that all infants <60 days with only 5-9 wbcs/hpf have a UTI. Conversely, the test characteristics revealed in this study show that some babies with a normal U/A with still have a UTI. So the bottom line is – don’t forget the culture!
References
Mahajan P, Kuppermann N, Mejias A, et al; Pediatric Emergency Care Applied Research Network (PECARN). Association of RNA biosignatures with bacterial infections in febrile infants aged 60 days or younger. JAMA. 2016;316(8):846–857.
