Should I order a lactate level if I suspect meningitis?

Lactate Testing in Suspected Meningitis

Yes, you should obtain a lactate level when concerned for meningitis, as CSF lactate has excellent diagnostic accuracy for distinguishing bacterial from viral meningitis with 96-100% sensitivity and 100% specificity using a cutoff of >4.2 mmol/L. 1

Diagnostic Value of CSF Lactate in Meningitis

High Diagnostic Accuracy

• CSF lactate measurement provides superior diagnostic performance compared to other CSF parameters:
  • Sensitivity: 96-100%
  • Specificity: 100%
  • Positive predictive value: 100%
  • Negative predictive value: 97-99%
  • Using cutoff values of >3.5-4.2 mmol/L 1, 2

Comparison with Other CSF Parameters

• CSF lactate outperforms traditional markers in distinguishing bacterial from viral meningitis:
  • More reliable than CSF/blood glucose ratio 1
  • Higher diagnostic accuracy than CSF protein levels 2
  • More definitive than CSF white cell count alone 3

Clinical Implementation

When evaluating a patient with suspected meningitis:

1. Obtain CSF lactate alongside standard tests:

   • Blood cultures (within 1 hour of arrival) 1
   • CSF for cell count, protein, glucose
   • CSF Gram stain and culture

2. Interpret lactate results:

   • 4.2 mmol/L: Strongly suggestive of bacterial meningitis 1

   • <2.5 mmol/L: Strongly suggestive of viral meningitis 3
   • 2.5-4.2 mmol/L: Requires clinical correlation

Clinical Decision Making Algorithm

For Patients with Suspected Meningitis and No Signs of Shock:

1. Perform lumbar puncture within 1 hour of arrival if safe to do so 1
2. Include CSF lactate measurement with standard CSF tests
3. If CSF lactate >4.2 mmol/L, treat empirically for bacterial meningitis regardless of Gram stain results
4. If CSF lactate <2.5 mmol/L and other CSF parameters are consistent with viral etiology, consider withholding antibiotics

For Patients with Suspected Meningitis with Shock or Rapidly Evolving Rash:

1. Start antibiotics immediately after blood cultures 1
2. Begin fluid resuscitation with 500 mL crystalloid bolus 1
3. Obtain serum lactate as part of sepsis evaluation (target <2 mmol/L) 1
4. Defer lumbar puncture until patient is stabilized 1

Important Caveats and Pitfalls

• False elevations of CSF lactate can occur with:

  • Cerebral hypoxia/ischemia
  • Anaerobic glycolysis
  • Vascular compromise
  • CSF leukocyte metabolism 1

• Pretreatment with antibiotics:

  • May affect CSF lactate levels, but elevated levels may still indicate bacterial etiology 4
  • LP should still be performed within 4 hours of antibiotic administration 1

• Contraindications to immediate LP:

  • Focal neurological signs
  • Papilledema
  • Continuous/uncontrolled seizures
  • GCS ≤12 1

• Special populations:

  • In neurosurgical patients, CSF lactate >4.0 mmol/L has 88% sensitivity and 98% specificity for bacterial meningitis 1

Conclusion

CSF lactate measurement is a rapid, inexpensive, and highly accurate test for differentiating bacterial from viral meningitis 5. While it should be interpreted alongside other clinical and laboratory findings, its high sensitivity and specificity make it a valuable addition to the diagnostic workup of suspected meningitis.