Diagnostic Tests for Cryptococcal Meningitis
The cryptococcal antigen (CrAg) lateral flow assay is the most sensitive test for confirming cryptococcal meningitis, with 97.4% sensitivity in CSF, significantly outperforming both culture (73.7% sensitivity) and multiplex PCR panels (84.2% sensitivity). 1
Primary Diagnostic Approach
First-Line Test: Cryptococcal Antigen (CrAg) Lateral Flow Assay
- CSF CrAg testing demonstrates exceptional performance with sensitivity approaching 97-100% and specificity of 98-99.9% when compared to culture as the reference standard 2, 1
- The lateral flow assay is a point-of-care test that can be performed on CSF, serum/plasma, or fingerstick whole blood with 100% agreement between sample types 3
- CrAg remains positive even after antifungal treatment initiation, making it reliable when prior therapy may have been administered 2
Second-Line Test: CSF Culture
- CSF culture has lower sensitivity (73.7%) compared to CrAg testing but remains important for antifungal susceptibility testing 1
- Culture provides quantitative fungal burden (colony-forming units/mL), which has prognostic value 4
- Culture can be falsely negative in patients who have received prior antifungal therapy 5
Adjunctive Test: India Ink Staining
- India ink microscopy has sensitivity of 97-100% when performed with cytospin centrifugation to increase yield 6
- This test is rapid and inexpensive but requires experienced laboratory personnel for accurate interpretation 5
- Sensitivity decreases significantly without cytospin preparation 6
Multiplex PCR Panels (BioFire FilmArray ME)
Performance Characteristics
- Sensitivity for Cryptococcus is 82-84.2% using CrAg as reference standard, with specificity of 98-99.9% 4, 1
- Critical limitation: sensitivity drops dramatically at low fungal burdens, detecting only 29% of cases with 0-99 CFU/mL compared to 94% detection at ≥100 CFU/mL 4
- False negatives occur particularly in treated patients with lower CrAg titers (median reciprocal titer 128 in ME panel-negative specimens versus 1920 in ME panel-positive specimens) 1
Clinical Utility
- Negative multiplex PCR does NOT rule out cryptococcal meningitis, especially in patients with prior antifungal exposure or low fungal burden 7, 4
- May predict culture sterility during follow-up with 84% negative predictive value in previously treated patients 4
- Provides simultaneous detection of other CNS pathogens (bacterial, viral) which can be valuable in the differential diagnosis 4
Screening in HIV-Infected Patients
- Serum/plasma CrAg screening is recommended in HIV-infected persons with CD4 counts <100 cells/μL before symptoms develop 2
- Fingerstick whole blood CrAg testing enables bedside screening with 100% sensitivity and specificity compared to serum 3
- CrAg titer predicts progression to meningitis, with higher titers indicating greater risk and need for lumbar puncture even in asymptomatic patients 2
Diagnostic Algorithm
Perform lumbar puncture in any patient with suspected meningitis (immunocompromised, HIV-infected, or compatible symptoms) 5
Send CSF for CrAg lateral flow assay immediately as the primary diagnostic test 2, 1
Simultaneously send CSF for culture to confirm diagnosis and assess fungal burden 1
Consider India ink staining if rapid microscopy expertise is available 6
If multiplex PCR is performed and negative, do NOT exclude cryptococcal meningitis—proceed with CrAg testing and culture 7, 4
In treated patients with negative tests, CrAg remains most reliable, as culture and PCR may be falsely negative 1
Critical Pitfalls to Avoid
- Never rely solely on multiplex PCR panels for excluding cryptococcal meningitis, as sensitivity is inadequate at low fungal burdens 4, 1
- Do not skip CrAg testing even if other tests are ordered—it is the most sensitive available test 1
- Prior antifungal therapy reduces culture yield but CrAg remains detectable 2, 1
- Low CrAg titers in follow-up specimens may fall below the detection threshold of PCR-based assays 1