Meningitis - Acute, Bacterial Diagnostics

Last updated: 29 July 2025

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Laboratory Tests and Ancillaries

Lumbar Puncture (LP) and Cerebrospinal Fluid (CSF) Examination



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Lumbar puncture and cerebrospinal fluid examination are diagnostic mainstays. Perform LP promptly, then send the CSF specimen for examination to facilitate initiation of appropriate therapy. If signs of increased ICP are present, perform neuroimaging (please see Head CT Scan) and consider treating the condition first to decrease the risk of brain herniation. The presence of the following should prompt delay in performing LP: Signs of severe sepsis or rapidly evolving rash, respiratory or cardiac compromise, treatment with an anticoagulant, or antiplatelet, with known thrombocytopenia or clotting abnormality, and infection at the lumbar puncture site.  

Cerebrospinal fluid examination includes cell count and differential, glucose concentration, protein concentration, Gram stain, culture and sensitivity and other appropriate tests.  

Findings Suggestive of Bacterial Meningitis  

Bacterial meningitis is suspected in the presence of elevated opening pressure (>100-200 mmHg), increased white blood cell count >1,000 μL with neutrophilic pleocytosis, decreased glucose levels (hypoglycorrachia) <40 mg/dL, CSF/serum glucose ratio ≤0.4, elevated protein levels (>200 mg/dL) indicating injury to the blood-brain barrier (BBB), and elevated lactate levels.  

Gram Stain  

Gram stain is an inexpensive and important tool for diagnosis; however, sensitivity varies by age group, type of pathogen, and antibiotic used previously. A reliable Gram stain should have 105 CFU/mL of bacteria.  

Culture and Sensitivity



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Cerebrospinal fluid culture is the gold standard in the diagnosis of bacterial meningitis, but results take time.  

Antigen/Antibody Tests  

Antigen/antibody tests use serum containing bacterial antibodies or commercially available antisera directed against capsular polysaccharides of meningeal pathogens. Examples of antigen/antibody tests are pneumococcal bacterial antigen test (BAT), latex agglutination test, cryptococcal antigen latex agglutination system (CALAS), and immunochromatographic antigen test. The bacterial antigen test is reserved for patients whose initial CSF Gram stain is negative or CSF culture is negative after 48 hours of incubation. Pneumococcal BAT has a sensitivity for pneumococcal meningitis between 67-100% and a specificity of >95%. The latex agglutination test is a rapid diagnostic tool to identify the causative pathogen. Sensitivity varies for each causative pathogen: 78-100% for H influenzae, 22-93% for N meningitidis, and 59-100% for S pneumoniae. Its sensitivity decreases when empiric treatment is started before LP. Rapid immunochromatographic antigen tests (eg S pneumoniae BinaxNOW®) have 99-100% sensitivity and specificity for pneumococcal meningitis.  

Other Tests (As Needed)  

Other tests that could be done are acid fast bacilli (AFB) smear and tuberculosis culture, India ink, polymerase chain reaction (PCR), tissue culture, Limulus lysate test, counter immunoelectrophoresis, and metagenomic next-generation sequencing (mNGS). Multiplex polymerase chain reaction is a rapid non-culture-based method with a sensitivity of 90% and a specificity of 97%.  

Blood Cultures (BCs)  

Blood cultures are used to identify causative organisms and establish susceptibility patterns when CSF cultures are negative or not available. The samples should be obtained before instituting empiric antibiotic therapy.  

Other Tests  

Other tests that could be done are blood glucose, complete blood count (CBC), fibrin degradation products (FDP), C-reactive protein (CRP), serum procalcitonin, urine osmolality, sodium plasma osmolality, and human immunodeficiency virus (HIV) test. 

Imaging

Head Computed Tomography (CT) Scan



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A head computed tomography scan is not routinely done prior to lumbar puncture unless the patient has signs of increased intracranial pressure or suspected space-occupying lesions. This is recommended prior to lumbar puncture in immunocompromised patients (human immunodeficiency virus [HIV], acquired immune deficiency syndrome), patients on immunosuppressive therapy, after transplantation, those with a history of CNS disease and/or those with new-onset seizure within 1 week prior to presentation. The samples for blood cultures should be sent out and empirical antimicrobial therapy should be initiated prior to performing a head CT scan.  

Magnetic Resonance Imaging (MRI)  

Magnetic resonance imaging (MRI) with gadolinium enhancement and diffusion-weighted imaging may be used for patients with meningitis secondary to medical devices (eg CSF shunt, CSF drain, intrathecal drug therapy).