Citrobacter koseri and Nitrite Production on Urinalysis
Citrobacter koseri can produce nitrites on urinalysis, but a negative nitrite test does not rule out a C. koseri urinary tract infection due to the limited sensitivity (53%) of the nitrite test. 1
Nitrite Production by Citrobacter koseri
- Citrobacter koseri is a gram-negative bacillus that is commonly isolated from urinary tract infections (UTIs) and can produce nitrites on urinalysis 2, 3
- As a member of the Enterobacteriaceae family, C. koseri possesses nitrate reductase enzymes that convert nitrates to nitrites, which can be detected on urinalysis dipstick testing 1
- Despite its ability to produce nitrites, a negative nitrite test does not exclude C. koseri infection due to the test's limited sensitivity (53%) despite excellent specificity (98%) 1
Diagnostic Considerations for C. koseri UTIs
- A positive nitrite test strongly suggests bacterial infection and the need for antibiotic treatment in symptomatic patients 4
- When interpreting a urinalysis with suspected C. koseri infection, consider both leukocyte esterase and nitrite results, as the combination improves diagnostic accuracy (93% sensitivity, 96% specificity) 1, 5
- A definitive diagnosis of UTI requires both urinalysis suggesting infection (pyuria and/or bacteriuria) and a positive urine culture with ≥50,000 CFU/mL of a uropathogen 1
- C. koseri is frequently isolated from urinary tract infections, accounting for approximately 46.2% of all Citrobacter isolates in some studies 3
Pitfalls in Diagnosis
- Do not rule out UTI based solely on negative nitrite results, as this would miss many true infections, including some caused by C. koseri 1
- Avoid diagnosing UTI based on positive culture alone without evidence of pyuria, as this may represent asymptomatic bacteriuria rather than true infection 1
- Remember that pyuria is absent in approximately 20% of febrile infants with culture-proven pyelonephritis, highlighting the importance of urine culture 1
- C. koseri infections are often nosocomially acquired (94.6% in one study) and commonly seen in patients with significant underlying diseases 3
Clinical Implications
- C. koseri has been reported to cause serious infections beyond UTIs, including bloodstream infections and meningitis, particularly in immunocompromised patients 6, 7
- The organism shows inherent resistance to ampicillin and carbenicillin, with emerging resistance to other beta-lactams and aminoglycosides 6, 2
- In patients with anatomical abnormalities of the urinary tract (like spina bifida patients with urinary diversions), C. koseri UTIs can progress to more severe infections like perinephric abscesses 8
- For definitive diagnosis of UTI caused by C. koseri, urine culture is essential, especially when starting antibiotics 1
In conclusion, while Citrobacter koseri can produce nitrites that may be detected on urinalysis, a negative nitrite test does not rule out infection due to the test's limited sensitivity. The most reliable approach combines multiple parameters including leukocyte esterase, nitrite testing, and urine culture for definitive diagnosis.