From the Research
Nitrite testing has a sensitivity of around 50-80% for detecting E. coli urinary tract infections, as shown in a study from 2006 1. This means that while a positive nitrite test strongly suggests the presence of E. coli, a negative test does not rule out infection. E. coli is the most common cause of UTIs and produces the nitrate reductase enzyme that creates the positive nitrite reaction on dipstick testing. For optimal sensitivity, morning urine samples should be tested, as they allow sufficient time (at least 4 hours) for bacteria to convert nitrates to nitrites in the bladder. The test is more reliable in cases with higher bacterial counts (>10,000 CFU/mL). False negatives can occur with insufficient bladder incubation time, low urinary nitrate levels (from dietary factors), or UTIs caused by organisms that don't produce nitrate reductase. Some key points to consider when interpreting nitrite test results include:
- The positive predictive value (PPV) and specificity of the nitrite test are high, but the sensitivity is moderate 1.
- A negative nitrite test with a positive leucocyte-esterase (LE) test can still indicate a UTI, especially if the patient has symptoms 1.
- The sensitivity of the nitrite test can be improved by using morning urine samples and allowing sufficient bladder incubation time 2.
- Urine culture is still the gold standard for diagnosing UTIs, especially in cases with negative nitrite results but persistent symptoms 1, 3.
- Local antibiotic resistance patterns should be considered when selecting empiric antibiotic therapy for UTIs 4, 3. Therefore, while nitrite testing is a useful screening tool, definitive diagnosis of E. coli UTI requires urine culture, especially in cases with negative nitrite results but persistent symptoms.