Testing for aspergillus in the lungs typically involves a combination of imaging, blood tests, and sometimes a procedure to collect fluid or tissue directly from the airways. No single test can confirm the diagnosis on its own, so doctors layer multiple results together to distinguish between a harmless encounter with the fungus and an active infection or allergic reaction.
The specific tests you’ll need depend on which form of aspergillus-related lung disease is suspected: invasive infection (most dangerous, seen in people with weakened immune systems), a fungal ball growing in an existing lung cavity, or an allergic response called ABPA. Here’s what each testing method involves and what it can tell you.
CT Scans and Chest Imaging
A CT scan of the chest is usually the first step. Standard chest X-rays can miss early aspergillus infections, so a high-resolution CT is preferred. What doctors look for depends on the type of disease suspected.
In invasive pulmonary aspergillosis, the classic early finding is the “halo sign,” a dense nodule in the lung surrounded by a fuzzier ring. That ring represents bleeding around a core of dead tissue where the fungus has invaded blood vessels. The halo sign appears early in the infection, often before other tests turn positive, making it especially useful in patients with severely suppressed immune systems, such as those undergoing chemotherapy for leukemia.
Two to three weeks later, as the immune system begins to recover, a different pattern called the “air crescent sign” may appear. This is a crescent-shaped pocket of air forming around a chunk of dead tissue inside a cavity. It signals the body is starting to clear the infection. A fungal ball (aspergilloma) looks different still: a solid mass sitting inside a pre-existing lung cavity, often shifting position when you change posture. ABPA tends to show up as mucus plugging in the airways or areas of bronchiectasis (permanently widened airways).
Blood Tests for Aspergillus
Galactomannan Antigen Test
Galactomannan is a sugar molecule that makes up part of the aspergillus cell wall. As the fungus grows, it sheds galactomannan into the bloodstream, where a blood test can detect it. A serum result is considered positive at an index of 0.5 or higher, confirmed on repeat testing. This test is FDA-approved and most useful for monitoring patients at high risk for invasive aspergillosis, such as those with blood cancers or organ transplants.
The test works best as a screening tool when drawn regularly in high-risk patients, sometimes catching the infection days before symptoms or imaging changes appear. It can also be run on fluid collected directly from the lungs during a bronchoscopy, where it tends to be more sensitive than the blood version.
Beta-D-Glucan Assay
This blood test detects a different cell wall component shared by many types of fungi, not just aspergillus. Its sensitivity and specificity for invasive fungal infections are both around 80%, which means it’s moderately helpful but not definitive. A positive result tells your doctor a fungal infection is likely; a negative result provides some reassurance but doesn’t completely rule one out. Because it reacts to multiple fungal species, it’s typically used alongside other aspergillus-specific tests rather than on its own. False positives can occur in people receiving certain IV antibiotics, blood products like albumin, or hemodialysis with cellulose filters.
IgE and Allergy Testing for ABPA
If allergic bronchopulmonary aspergillosis is the concern, the diagnostic workup shifts toward allergy-related blood markers. The two essential findings are a positive aspergillus-specific IgE (at or above 0.35 kUA/L) and a total serum IgE level of 500 IU/mL or higher. Beyond those, doctors look for at least two supporting features: elevated aspergillus-specific IgG antibodies, a high eosinophil count (a type of white blood cell involved in allergic reactions), or characteristic imaging findings. ABPA is most common in people with asthma or cystic fibrosis, so testing is often triggered when these patients develop worsening symptoms that don’t respond to standard treatment.
Bronchoscopy and Lung Fluid Collection
When blood tests and imaging aren’t enough, or when doctors need a sample directly from the lungs, bronchoscopy with bronchoalveolar lavage (BAL) is the next step. The procedure takes roughly 30 to 90 minutes. You’ll receive a sedative through an IV and a numbing spray in your mouth and throat. A thin, flexible scope is then guided down your throat and into your airways.
During the lavage portion, a small amount of sterile saline is pushed through the scope into a specific section of the lung, then suctioned back up. That recovered fluid carries cells, proteins, and any microorganisms present in the airways. The sample is sent for multiple tests at once: fungal culture, galactomannan measurement, PCR, and microscopic examination. BAL galactomannan and BAL PCR are among the most accurate individual tests available. BAL-based PCR testing has a reported sensitivity around 90% and specificity around 96%, making it particularly valuable when the diagnosis remains uncertain after blood testing alone.
Culture and Microscopy
Fungal culture remains one of the standard diagnostic methods. Samples from sputum (coughed-up mucus), BAL fluid, or tissue can all be cultured. Aspergillus typically becomes visible within one to three days after incubation, and culture is the only method that can identify the exact species involved, which matters for choosing the right treatment and checking for drug resistance.
The main limitation is sensitivity. Cultures from respiratory specimens catch only 35% to 63% of cases of invasive aspergillosis. A negative culture doesn’t mean you’re free of infection. It may simply mean the fungus wasn’t present in the particular sample collected, or wasn’t growing vigorously enough to be detected. That’s why cultures are always interpreted alongside other test results, never in isolation.
Under the microscope, aspergillus hyphae (the thread-like filaments of the fungus) have a distinctive look: they branch at sharp angles in a Y-shaped pattern and have visible internal dividers called septa. Pathologists can often recognize aspergillus from this appearance, but microscopy alone can’t confirm the species with certainty, so culture or molecular testing is still needed.
PCR (Molecular Testing)
PCR testing detects aspergillus DNA in blood or lung fluid samples. It’s faster than culture and, when performed on BAL fluid, highly accurate. In blood samples, sensitivity averages around 80% and specificity about 79%. More importantly, serum PCR can flag an infection four to six days earlier than galactomannan or beta-D-glucan testing, giving doctors a head start on treatment.
PCR isn’t yet universally standardized across laboratories, so availability and methodology can vary depending on where you’re being treated. Current guidelines recommend that clinicians use PCR results alongside other tests rather than relying on them as a standalone diagnostic tool.
Tissue Biopsy
In some cases, especially when other tests are inconclusive, a tissue biopsy provides the most definitive answer. This can be obtained during bronchoscopy, through a CT-guided needle, or during surgery. The tissue is examined under a microscope for the characteristic branching fungal filaments and signs of tissue invasion, such as the fungus growing into blood vessel walls. Biopsy carries more risk than blood tests or BAL, so it’s typically reserved for situations where the diagnosis is unclear and the stakes are high.
Colonization vs. Active Infection
One of the trickiest aspects of aspergillus testing is that the fungus is everywhere in the environment. Aspergillus spores are in the air you breathe daily. A positive sputum culture in someone with healthy lungs may simply mean the fungus landed in the airways without causing harm. This is called colonization, and it doesn’t require treatment.
Distinguishing colonization from true infection requires context: your immune status, your symptoms, your imaging findings, and whether multiple different tests point in the same direction. A single positive sputum culture in an otherwise healthy person is rarely enough to diagnose invasive disease. But that same result in someone with a severely weakened immune system, combined with a halo sign on CT and a positive galactomannan, paints a very different picture. This is why aspergillus diagnosis almost always involves layering several types of evidence together rather than relying on any one test.