Blood Tests for Meningitis
In all patients with suspected meningitis, blood cultures, pneumococcal and meningococcal PCR (EDTA sample), glucose, lactate, procalcitonin (if available), full blood count, urea, creatinine, electrolytes, liver function tests, and clotting screen should be obtained immediately, ideally within the first hour of hospital arrival. 1
Essential Blood Tests - Immediate Priority
The UK Joint Specialist Societies guideline provides the most comprehensive framework for blood testing in suspected meningitis 1:
Microbiological Tests (Highest Priority)
Blood cultures: Must be obtained prior to antibiotics whenever possible, or within 1 hour of hospital arrival if antibiotics were given in the community 1
Pneumococcal and meningococcal PCR: Requires EDTA sample 1
- Critical when cultures are negative, especially after antibiotic pretreatment
- Has significant additive value for pathogen identification 2
Biochemical Markers
Glucose: Essential for calculating CSF/serum glucose ratio 1
- The CSF/serum glucose ratio is highly discriminative (AUC 0.843) for bacterial meningitis 3
Lactate: Recommended as a prognostic marker 1
Procalcitonin: Grade 2C recommendation if available 1
Routine Laboratory Tests
Full blood count: Assess for leukocytosis and thrombocytopenia
- Neutrophil-to-lymphocyte ratio (NLR) >8.87 predicts bacterial meningitis and mortality 3
Urea, creatinine, electrolytes: Assess renal function and guide antibiotic dosing
Liver function tests: Baseline assessment and antibiotic monitoring
Clotting screen: Essential before lumbar puncture 1
Storage Sample
- Serum storage: For serological testing if no cause identified 1
- Convalescent sample should be sent 4-6 weeks later (discuss with microbiologist)
Timing Considerations - Critical for Outcomes
Blood cultures must be obtained within 1 hour of hospital arrival 1. This timing is non-negotiable as delays in diagnosis and treatment directly impact mortality and morbidity. The guideline emphasizes that stabilization, investigation, and treatment should occur concurrently 1.
Clinical Context Matters:
For patients with meningitis without shock/severe sepsis:
- Obtain blood tests immediately
- Perform LP within 1 hour if safe
- Start antibiotics after LP (within first hour total)
For patients with predominantly sepsis or rapidly evolving rash:
- Obtain blood cultures immediately
- Start antibiotics immediately after blood cultures
- Defer LP until patient stabilized 1
Diagnostic Performance of Key Markers
Recent research validates specific biomarkers 3, 4, 5:
- Serum procalcitonin: Most discriminative blood marker (AUC 0.99) 4
- NLR: Good discriminative ability and mortality predictor 3
- CRP: Useful but less specific than procalcitonin 6
- In children, CRP <2 mg/dL at least 12 hours after fever onset makes bacterial meningitis far less likely 6
Common Pitfalls to Avoid
Do NOT delay blood cultures waiting for antibiotics to be prepared - obtain cultures first, then give antibiotics immediately after 1
Do NOT skip blood cultures if antibiotics were given pre-hospital - they remain valuable despite reduced yield 1, 2
Do NOT delay LP to await clotting results unless there is clinical suspicion of coagulopathy 1
Do NOT forget concurrent serum glucose when performing LP - the CSF/serum glucose ratio is more valuable than CSF glucose alone 1, 3
Do NOT rely solely on standard inflammatory markers (WBC, CRP) - they lack sufficient specificity 2, 7
Integration with CSF Analysis
Blood tests complement but do not replace CSF analysis. The combination of serum procalcitonin >0.25 ng/mL and CSF lactate >33 mg/dL achieves the highest predictive power for bacterial meningitis 5. Blood glucose must be drawn concurrently with CSF for accurate CSF/serum glucose ratio calculation, which is one of the most discriminative parameters (AUC 0.843) 3.