1. Overview
Cryptococcal infection is caused primarily by Cryptococcus neoformans and Cryptococcus gattii, encapsulated opportunistic yeasts capable of producing severe and potentially fatal disease, particularly in immunocompromised individuals. The organism is commonly found in soil and pigeon droppings, and infection is acquired through inhalation of spores.
A defining feature of Cryptococcus species is the presence of a polysaccharide capsule, which sheds cryptococcal antigen (CrAg) into biological fluids during infection. Detection of this capsular antigen in serum or cerebrospinal fluid (CSF) serves as a key diagnostic biomarker for cryptococcosis.
2. Symptoms
Clinical manifestations vary depending on the organ system involved and the degree of immune suppression.
- Pulmonary involvement: Cough, chest pain, fever, pneumonia, and dyspnea
- Central nervous system (meningitis): Headache, fever, neck stiffness, confusion, cranial nerve palsies, and vision loss
- Skin: Papules, nodules, ulcers, and plaques
- Disseminated disease: Eye involvement with vision loss, bone pain, arthritis, and multi-organ involvement
- Constitutional symptoms: Fatigue, weight loss, and night sweats
3. Causes
Cryptococcosis results from infection with Cryptococcus species, particularly C. neoformans, an encapsulated spherical yeast. After inhalation, the organism may cause primary pulmonary infection and can subsequently disseminate, especially to the central nervous system.
Shedding of the capsular polysaccharide antigen into blood, CSF, or urine forms the basis for antigen detection assays used in diagnosis.
4. Risk Factors
Individuals at highest risk include:
- Patients with advanced HIV/AIDS, especially with CD4 counts <100 cells/c.mm
- Patients with hematological malignancies
- Organ transplant recipients
- Patients receiving immunosuppressive or prolonged steroid therapy
Testing is also indicated in patients with suspected meningitis, disseminated cryptococcosis, or those requiring monitoring of treatment response.
5. Prevention and Diagnostic Approach
The cryptococcal antigen test detects fungal capsular polysaccharide antigens using specific antibodies and can be performed on serum, CSF, or urine.
Testing methods include:
- Latex agglutination assay
- Enzyme immunoassay (EIA/ELISA)
- Lateral flow immunoassay (LFA/LFIA)
- Fluorescent immunoassay (FIA)
The lateral flow assay is particularly useful as a rapid, point-of-care test and employs a dipstick “sandwich” format with visible results.
Sample collection:
- Serum: Collect 3.0 ml blood in a plain red-capped tube; separate serum promptly
- CSF: Collect 3.0 ml via aseptic lumbar puncture into a sterile container; sample should be collected prior to antimicrobial therapy for highest diagnostic sensitivity
- Urine: Collect 10.0 ml in a clean container
Samples should be transported to the laboratory immediately. In case of delay, samples should not be refrigerated; they should be kept at room temperature or at 37°C if available.
Diagnosis
- Culture remains the gold standard for diagnosis, but requires 3 days to 1 month for growth
- India ink preparation of CSF demonstrates encapsulated yeasts but has limited sensitivity (50–80%)
- Antigen detection provides the most rapid and sensitive diagnostic approach and may remain positive for several months after infection
Interpretation of Results
- Negative: No antigen detected; suggests no active infection or low antigen burden
- Positive: Antigen detected; indicates active or disseminated cryptococcal infection
- Rising titer: Persistent infection or inadequate therapy
- Falling titer: Response to treatment and clinical improvement
Clinical Significance
Cryptococcal antigen testing is a highly sensitive and rapid diagnostic tool, especially valuable for early detection of cryptococcal meningitis, often before the onset of severe symptoms. Antigen titers correlate with fungal burden, disease severity, and prognosis, and serial testing aids in monitoring response to antifungal therapy.
The test is particularly useful for risk stratification in HIV-infected patients, point-of-care screening in low-resource settings, and predicting outcomes, as higher antigen titers are associated with increased mortality.
Limitations
False-negative results may occur due to the prozone effect (very high antigen levels), low fungal burden, or immune complex formation. False-positive results can occur in certain patient groups. The test cannot differentiate between Cryptococcus species and has limited sensitivity in isolated pulmonary nodules. Therefore, clinical correlation is essential.
