Blood Tests for UTI
A complete blood count (CBC) with differential should be performed for all patients suspected of having a urinary tract infection, while blood cultures are generally not recommended except in cases of suspected urosepsis. 1
When to Order Blood Tests
Complete Blood Count (CBC) with Differential
- Order a CBC with manual differential within 12-24 hours of symptom onset for any patient with suspected UTI 1
- The manual differential is specifically important to assess band forms and other immature neutrophils 1
- An elevated WBC count (≥14,000 cells/mm³) or left shift (band neutrophils >6% or total band count ≥1,500/mm³) indicates high probability of bacterial infection and warrants careful assessment 1, 2
- Leukocytosis is associated with increased mortality in nursing home-acquired infections, making it a critical prognostic marker 1
Blood Cultures
- Blood cultures should be obtained only if urosepsis is suspected, characterized by high fever, shaking chills, hypotension, or delirium 1, 2
- For most patients with uncomplicated UTI, blood cultures have low yield and rarely influence therapy, so they are not routinely recommended 1
- Blood cultures are appropriate when bacteremia is highly suspected and paired with urine specimens for culture and antimicrobial susceptibility testing 1
Clinical Context for Blood Testing
Signs Warranting Blood Work
- Fever >100.3°F (38.8°C) 1
- Suspected pyelonephritis or upper tract involvement 1
- Recent catheter obstruction or change in catheterized patients 1
- Systemic symptoms including hypotension, altered mental status, or signs of sepsis 1, 2
Important Caveats
- Do not order urinalysis or urine cultures for asymptomatic patients, as asymptomatic bacteriuria is extremely common (15-50% in elderly, 100% in catheterized patients) and does not require treatment 1
- Blood tests should complement, not replace, urinalysis and urine culture, which remain the primary diagnostic tests for UTI 3, 2
- In the absence of fever, leukocytosis, or specific clinical manifestations of focal infection, additional blood tests may have low diagnostic yield 2
Interpretation of Results
- Even with a normal total WBC count, a left shift (elevated band forms) indicates significant bacterial infection 1
- The combination of elevated WBC and clinical symptoms has higher predictive value than either alone 1
- Leukocytosis (WBC ≥15,000-20,000 cells/mm³) in the context of UTI symptoms suggests more severe infection and potentially worse outcomes 1