It is time critical to diagnose ABM and begin empiric antimicrobial therapy based on the known/most likely pathogen. The diagnosis of ABM depends on clinical features and the CSF profile. The CSF Gram stain and CSF lactic acid (LA) levels provide the most rapid, reliable and cost-effective tests to diagnose ABM. CSF LA levels are also the best way to diagnose partially treated acute bacterial meningitis (PTABM). In those cases of AME/AVE due to viruses with a CSF profile mimicking ABM, for example, HSE, unelevated CSF LA levels rapidly/reliably rule out ABM as a diagnostic possibility.