Cryptococcal antigen (CrAg) is a protein-sugar molecule shed from the outer capsule of Cryptococcus, a fungus that causes serious infections in people with weakened immune systems. When doctors order a “CrAg test,” they’re looking for this molecule in your blood or spinal fluid as evidence that the fungus is present in your body. The test is one of the most sensitive tools available for diagnosing cryptococcal disease, and it can detect infection an average of 22 days before symptoms appear.
What the Test Actually Detects
The capsule surrounding Cryptococcus is made almost entirely of a large sugar molecule called glucuronoxylomannan, which accounts for about 90% of the capsule’s mass. This molecule is what CrAg tests are designed to pick up. The fungus continuously sheds fragments of its capsule into surrounding body fluids, so when those fragments show up in a blood draw or spinal fluid sample, it signals active infection.
The capsule isn’t just a passive shell. It actively helps the fungus survive by blocking immune cells from engulfing it, disrupting the chemical signals your immune system uses to coordinate its response, and interfering with the movement of white blood cells toward the infection site. This makes the capsule both a key weapon for the fungus and a reliable marker for detecting it.
How the Test Works
Three types of CrAg tests have been used over the past 35 years, but the newer lateral flow assay (LFA) has largely replaced the older methods in many settings. It works like a home pregnancy test: a dipstick strip coated with antibodies that bind to the cryptococcal capsule molecule. You apply a drop of blood, serum, or spinal fluid, and a visible line appears if the antigen is present. It requires little to no lab equipment, which makes it especially valuable in resource-limited settings where cryptococcal disease is most common.
The two older methods, latex agglutination and enzyme immunoassay, require more lab infrastructure and have a notable limitation: they are less sensitive at detecting Cryptococcus gattii, a species that can infect people with healthy immune systems. The LFA was specifically designed to detect all Cryptococcus serotypes and performs equally well or better for C. gattii infections. One study found serum CrAg was positive in 97.3% of patients with C. gattii disease compared to 77.1% of those with C. neoformans disease when using the LFA.
Accuracy in Blood vs. Spinal Fluid
CrAg testing is highly accurate regardless of sample type. A large systematic review found that serum (blood) testing had a sensitivity of 99.7% and specificity of 94.1% for detecting cryptococcal meningitis. Spinal fluid testing showed 98.8% sensitivity and 99.3% specificity. When both were performed on the same patients, there was no statistically significant difference in accuracy between the two.
The slightly lower specificity of serum testing means that blood-based CrAg results occasionally come back positive in people who don’t have meningitis. This can happen because the antigen circulates in blood during early or localized infections that haven’t yet reached the brain. A positive blood test in someone without symptoms typically prompts a lumbar puncture to check the spinal fluid directly.
False Positives
False positive results are uncommon but do occur. Infections with Trichosporon (another fungus) or certain bacteria like Stomatococcus and Capnocytophaga can trigger low-titer positive results because their surface molecules resemble the cryptococcal capsule enough to cross-react with the test antibodies. The LFA can also cross-react with Aspergillus infections. Insufficient sample dilution is another documented cause of false positives, particularly at low titer levels.
Who Gets Screened and Why
The World Health Organization recommends CrAg screening for all people living with HIV whose CD4 count falls to 100 cells per microliter or below, regardless of whether they’re already on antiretroviral therapy. At these levels of immune suppression, cryptococcal infection can silently take hold weeks before causing noticeable illness. Screening catches it early enough to start preventive antifungal treatment before meningitis develops.
Some evidence suggests the screening threshold could reasonably extend to people with CD4 counts of 200 cells per microliter or below, though current guidelines haven’t formally adopted this wider range. The key insight behind screening is timing: detectable CrAg appears in blood a median of 22 days before symptoms of meningitis, and roughly 11% of people have it circulating more than 100 days before they get sick. That window is long enough to intervene meaningfully.
What CrAg Titers Mean
CrAg tests can report results as a titer, a number reflecting how much antigen is present. Higher titers in spinal fluid generally correlate with a greater burden of fungus, and higher fungal burden is associated with worse outcomes. In one study, 10-week mortality was 31% in patients with the lowest spinal fluid fungal counts and 54% in those with the highest.
However, the relationship between antigen levels and prognosis isn’t perfectly straightforward. The amount of antigen shed per organism varies from person to person, influenced by individual immune responses. Two patients with similar fungal loads can have meaningfully different CrAg titers. For this reason, clinical guidelines note that serial CrAg titers are not precise enough to guide day-to-day treatment decisions during acute illness. A titer that remains elevated doesn’t necessarily mean treatment is failing, and a persistently positive CrAg result alone is not a reason to continue antifungal therapy indefinitely.
Titers do play a role in longer-term management. Guidelines suggest that people living with HIV may be able to stop maintenance antifungal therapy once their CD4 count recovers above 100 cells per microliter and their CrAg titer is at or below 1:512 and not rising. If the CD4 count drops again or the titer starts climbing, restarting treatment is recommended.