SEATTLE – according to investigators at the University of Texas Health Science Center, Houston.
The researchers found that urine testing negative for nitrites with a specific gravity above 1.015 in children up to 2 months old had a sensitivity of 53% for ruling out UTIs, but that urine with a specific gravity below that mark had a sensitivity of just 14%. The finding “should be taken into account when interpreting nitrite results ... in this high-risk population,” they concluded.
Bacteria in the bladder convert nitrates to nitrites, so positive results are pretty much pathognomonic for UTIs, with a specificity of nearly 100%, according to the researchers.
Negative results, however, don’t reliably rule out infection, and are even less reliable in infants because they urinate frequently, which means they usually flush out bacteria before they have enough time to make the conversion, which takes several hours, they said.
The lead investigator, MD, an assistant professor of pediatrics at the University of Texas McGovern Medical School in Houston, said he had a hunch that negative results might be more reliable when newborns urinate less frequently and have more concentrated urine.
He and his team reviewed data collected on 413 infants up to 2 months old who were admitted for fever workup and treated for UTIs both in the hospital and after discharge. Nitrite results were stratified by urine concentration. A specific gravity of 1.015 was used as the cutoff between concentrated and dilute urine, which was “midway between the parameters reported” in every urinalysis, Dr. Parlar-Chun said.
Although the sensitivity of concentrated urine was only 53%, “it’s a stark difference from” the 14% in dilute urine, he said.“You should take a look at specific gravity to interpret nitrites. If urine is concentrated, you have [more confidence] that you don’t have a UTI if you’re negative. It’s better than taking [nitrites] at face value.”
The subjects were 31 days old, on average, and 62% were boys; 112 had a specific gravity above 1.015, and 301 below.
There was no external funding, and Dr. Parlar-Chun didn’t have any disclosures.