What does a negative Cryptococcal Antigen Lateral Flow Assay (CALA) result mean in a patient with suspected Cryptococcal Meningitis?

Reasons for Negative CALA in Cryptococcal Meningitis

Primary Explanation

A negative Cryptococcal Antigen Lateral Flow Assay (CALA) does not rule out cryptococcal meningitis, as false-negative results occur in approximately 1-2% of cases, and can result from early infection, low fungal burden, acapsular strains, or technical factors. 1, 2

Key Causes of False-Negative CALA Results

Early Cryptococcal Meningoencephalitis

• Patients can present with neurologic symptoms and positive serum cryptococcal antigen but negative CSF CrAg, representing early cryptococcal meningoencephalitis before full CSF dissemination. 3
• These patients have lower CSF pleocytosis (16% with ≥5 white cells/μL) and lower opening pressures (16% with >200 mmH₂O) compared to confirmed CSF CrAg-positive meningitis. 3
• Despite negative CSF CrAg, 9% of these patients had Cryptococcus isolated on CSF culture or PCR, confirming infection. 3
• In-hospital mortality remains high (32%) even with negative CSF CrAg, similar to confirmed cryptococcal meningitis (31%). 3

Acapsular or Poorly Encapsulated Strains

• Acapsular Cryptococcus neoformans strains produce minimal or undetectable capsular polysaccharide antigen, leading to negative latex agglutination and direct microscopy despite viable organisms in CSF. 4
• These strains may develop capsules when cultured in sterile CSF or normal serum, but remain undetectable by antigen testing in vivo. 4
• Culture remains positive even when antigen tests are negative, emphasizing the importance of fungal culture. 4

Technical and Pre-Analytical Factors

• False-positive results can occur with CryptoPS LFA due to interferent material in serum, though false-negatives are less common. 5
• Heat pretreatment of serum (100°C for 5 minutes) can occasionally convert true-positive results to false-negatives, particularly in patients with isolated antigenemia or early meningitis. 5

Low Fungal Burden or Antigen Concentration

• The sensitivity of CSF CrAg is 98.8% (96.2-99.6%), meaning approximately 1-2% of true cases will be missed. 2
• Patients with very low plasma CrAg titers (≤1:80) may have minimal CSF antigen despite symptomatic disease. 6

Diagnostic Approach When CALA is Negative

Immediate Actions

• Obtain CSF fungal culture, which remains the gold standard and can detect cryptococcal meningitis even when antigen tests are negative. 1, 4
• Check serum cryptococcal antigen, as it has 99.7% sensitivity and may be positive when CSF CrAg is negative in early meningoencephalitis. 3, 2
• Measure CSF opening pressure, as elevated pressure (>20 cm H₂O) occurs in 75% of cryptococcal meningitis cases. 1

Additional Testing

• Perform India ink stain or direct microscopy, though sensitivity is limited and negative results do not exclude disease. 1
• Consider repeat lumbar puncture after 24-48 hours if clinical suspicion remains high and initial testing is negative. 7
• Send blood cultures, as up to 75% of HIV-associated cryptococcal meningitis cases have positive blood cultures. 1

Clinical Context Matters

• In HIV-infected patients with CD4+ counts <50 cells/μL presenting with headache, fever, and altered mental status, maintain high suspicion even with negative CALA. 1
• Evaluate for other causes of meningitis, as 11% of patients with symptomatic cryptococcal antigenemia and negative CSF CrAg had tuberculous meningitis. 3

Treatment Implications

When to Treat Despite Negative CALA

• Initiate antifungal therapy if serum CrAg is positive with neurologic symptoms, even if CSF CrAg is negative, as this represents early cryptococcal meningoencephalitis requiring full meningitis treatment. 3
• Fluconazole monotherapy is inadequate for symptomatic cryptococcal antigenemia with negative CSF studies; use amphotericin B-based induction therapy. 3
• The standard regimen is amphotericin B deoxycholate 0.7 mg/kg daily plus flucytosine 100 mg/kg daily for ≥2 weeks. 1

Critical Pitfall

• Do not rely solely on antigen testing to exclude cryptococcal meningitis; negative results require correlation with clinical presentation, CSF parameters, and culture results. 1, 4
• The specificity of CSF CrAg is 99.3%, meaning positive results are highly reliable, but the 1-2% false-negative rate is clinically significant given the high mortality of untreated disease. 2