How to manage a patient with urinary stones and a nitrite positive urine test potentially due to blood contamination rather than a true urinary tract infection (UTI)?

Nitrite-Positive Urine in Stone Patients: Distinguishing True UTI from Blood Contamination

In patients with urinary stones and nitrite-positive urine, you should treat this as a true UTI rather than a false positive from blood contamination, because nitrite testing has 98% specificity and blood does not cause false-positive nitrite results.

Why Nitrite Positivity Indicates True Infection

• The nitrite test has excellent specificity (98%), meaning false positives are extremely rare—when nitrite is positive, infection is almost certainly present 1, 2
• Blood in urine does not produce false-positive nitrite results; nitrites are generated only when bacteria (specifically nitrate-reducing organisms like E. coli) are present in urine for sufficient time 1, 2
• The presence of urinary stones does not invalidate nitrite testing—the test remains highly specific for bacterial infection regardless of stone presence 1

Clinical Decision Algorithm

When you encounter nitrite-positive urine in a stone patient:

1. Confirm pyuria is present (≥10 WBCs/HPF or positive leukocyte esterase) to support the diagnosis of true UTI rather than asymptomatic bacteriuria 1

2. Obtain urine culture before starting antibiotics to identify the pathogen and guide definitive therapy, as this is essential for proper management 3

3. Start empiric antibiotics immediately if the patient has fever, systemic symptoms, or signs of urosepsis, as infection stones can develop from untreated UTIs 4

4. Assess for infection stones (struvite/staghorn calculi), which occur following UTIs with urease-producing organisms and require complete stone removal plus infection eradication 3, 4

Important Caveats About Nitrite Testing

• Nitrite has low sensitivity (53%), so a negative nitrite does NOT rule out UTI—many true infections will be nitrite-negative 1, 2
• Nitrite-negative infections occur with: non-nitrite-producing organisms (Staphylococcus saprophyticus, Enterococcus), frequent voiding (insufficient time for nitrate conversion), or dilute urine 2, 5
• The combination of leukocyte esterase OR nitrite positive achieves 93% sensitivity for UTI detection 1

Management of Stones with Confirmed UTI

• Defer elective stone procedures if purulent urine is encountered—establish drainage with stent or nephrostomy, continue antibiotics, and obtain culture 3
• Remove all stone fragments when infection stones are suspected, as residual fragments perpetuate recurrent UTI and stone growth 3
• Administer antimicrobial prophylaxis based on prior culture results and local antibiogram before any stone intervention 3

Common Pitfall to Avoid

Do not assume nitrite positivity is a "false positive" from hematuria—this is not how the nitrite test works biochemically. Blood does not contain or produce nitrites; only bacterial conversion of urinary nitrate to nitrite causes a positive result 1, 2. The high specificity means you should trust a positive nitrite result and investigate for true infection.