What is the significance of a positive nitrate test in asymptomatic urine?

From the Guidelines

Asymptomatic bacteriuria with positive nitrates in the urine generally does not require treatment in most patients, except for pregnant women and those undergoing urologic procedures where mucosal bleeding is anticipated. The presence of nitrates in urine indicates bacterial conversion of nitrates to nitrites, suggesting bacterial colonization, typically by gram-negative organisms like E. coli 1. However, without symptoms such as dysuria, frequency, urgency, or suprapubic pain, treating asymptomatic bacteriuria in non-pregnant, non-urologic procedure patients can lead to unnecessary antibiotic exposure, increased risk of adverse effects, and development of antibiotic resistance.

Key considerations for treatment include:

• Pregnant women should receive antibiotic therapy directed at the cultured organism and follow-up monitoring 1.
• Patients undergoing urologic procedures where mucosal bleeding is anticipated should be screened for asymptomatic bacteriuria and treated if positive 1.
• Non-pregnant, non-urologic procedure patients do not require treatment for asymptomatic bacteriuria, as the harms of screening and treatment outweigh the benefits 1.

It is essential to note that urine culture is the gold standard for detecting asymptomatic bacteriuria, but it is expensive for routine screening in populations with a low prevalence of the condition 1. Regular monitoring may be appropriate in some cases, especially for recurrent UTI patients, but intervention should be based on symptom development rather than laboratory findings alone. Treatment should only be initiated based on clinical judgment and patient-specific factors, rather than solely on the presence of positive nitrates in the urine.

From the Research

Significance of a Positive Nitrate Test

• A positive nitrate test is highly specific for urinary infection, with only 1% of 300 uninfected urine specimens giving a positive result 2
• The positive predictive value (PPV) of the nitrite test was high (96%) and the specificity was 94% for all samples 3
• A positive nitrite test or a negative nitrite test with a positive leucocyte-esterase (LE) test confirmed UTI, whereas a negative nitrite together with a negative LE test did not rule out infection 3

Asymptomatic Urine

• The dipstrip test for urinary nitrite is fairly unreliable in symptomatic urinary infections, but yielded positive results in 83 of 100 outpatients with largely asymptomatic urinary infection 2
• Asymptomatic bacteriuria (ASB) affects 2% to 7% of pregnant women, and treatment of ASB results in a reduction in the incidence of low birth weight and preterm birth 4
• The sensitivity for nitrite was 80.9%, specificity 57.9% and positive predictive value 41.4% in a study of 78 patients with UTI during pregnancy 5

Clinical Implications

• The finding of urinary nitrite is highly specific for urinary infection, but the test may not be reliable in all cases, especially in symptomatic urinary infections 2
• A positive nitrate test in asymptomatic urine may indicate the presence of a urinary tract infection, and further testing or treatment may be necessary 3, 2
• The use of overnight, first morning urine specimens may improve the sensitivity of the nitrite test 2