Testing for a fungal lung infection typically involves a combination of imaging, blood tests, and sometimes sampling fluid or tissue directly from the lungs. No single test catches every type of fungal infection, so doctors layer multiple approaches depending on your symptoms, immune status, and which fungus is suspected. The process usually starts with the least invasive options and escalates from there.
Chest Imaging: The First Step
A chest X-ray is often the starting point, but a CT scan provides far more detail and is the preferred imaging tool when a fungal infection is on the table. CT scans can reveal patterns that point toward specific fungi. A “halo sign,” where a patch of hazy ground-glass opacity surrounds a denser nodule, suggests invasive aspergillosis, a serious infection most common in people with weakened immune systems. A “reversed halo sign,” essentially the opposite pattern, leans more toward mucormycosis. Cavities forming inside existing lung lesions also raise suspicion for fungal disease, particularly in moderately immunocompromised patients.
None of these patterns are absolute proof of a fungal infection on their own. Bacterial pneumonia, tuberculosis, and even cancer can mimic some of these findings. But in someone with the right risk factors, certain CT patterns are strong enough clues to guide the next round of testing and, in some cases, to start antifungal treatment before confirmatory results come back.
Blood Tests That Detect Fungal Markers
Two widely used blood tests look for molecules shed by fungi into the bloodstream. Each has strengths and limitations, and they work best when used together.
The beta-D-glucan (BDG) test detects a sugar molecule found in the cell walls of many fungi. It casts a wide net: it can flag infections from Aspergillus, Candida, Pneumocystis, and others. In studies of patients with blood cancers, the BDG test showed a sensitivity of about 65% and specificity of 83%. That means it catches roughly two-thirds of true infections but occasionally produces false positives, especially in patients receiving certain medications or blood products.
The galactomannan (GM) test is more targeted, primarily detecting Aspergillus. It’s more specific at around 93%, meaning fewer false alarms, but less sensitive at about 48%, so it misses more true cases. When both tests are positive simultaneously, specificity jumps to 97%, making a confirmed result highly reliable. When either test is positive, sensitivity climbs to nearly 80%, improving the chance of catching an infection early. This is why many clinicians order both tests together rather than relying on one alone.
Antigen Tests for Specific Fungi
If histoplasmosis is suspected, often because you live in or have traveled through the Ohio and Mississippi River valleys or Central America, a urine antigen test is one of the fastest and most accurate options. Newer versions of the Histoplasma urine antigen test achieve sensitivity above 90% and specificity above 90% in patients with progressive disease. A serum (blood) version performs similarly well, with sensitivity reported as high as 96% in some studies of patients living with HIV. Urine testing is particularly useful because it’s completely non-invasive and results come back quickly.
For coccidioidomycosis (Valley fever), common in the southwestern United States, doctors rely on antibody blood tests that detect your immune system’s response to the fungus rather than the fungus itself. Cryptococcal infections, which primarily affect people with advanced immune suppression, have their own rapid antigen test that works on both blood and spinal fluid.
Bronchoscopy and Lung Fluid Sampling
When blood tests and imaging aren’t conclusive, the next step is often a procedure called bronchoalveolar lavage, or BAL. During a bronchoscopy, a thin, flexible scope is passed through your nose or mouth into the airways. A small amount of sterile fluid is squirted into the affected area of the lung and then suctioned back out, carrying cells, organisms, and debris with it.
That fluid goes through several analyses. Direct microscopy, where a technician examines the sample under a microscope right away, can provide an early diagnosis in roughly 74% of fungal and mycobacterial infections. This rapid look can sometimes guide treatment within hours. The fluid is also sent for culture, where it’s placed on growth media and monitored for fungal growth. Cultures remain the gold standard for identifying the exact species, but they’re slow. Fungal cultures typically require a minimum of two weeks to produce results, and some slow-growing organisms take even longer.
In one study of immunosuppressed patients with suspected pneumonia, the overall diagnostic yield of this combined approach was about 52%, with fungi, bacteria, and Pneumocystis jirovecii being the most commonly identified pathogens. The yield improves when multiple testing methods are applied to the same sample.
PCR and Molecular Testing
Molecular tests that detect fungal DNA are increasingly available and can dramatically speed up diagnosis. Real-time PCR testing on lung fluid samples has shown sensitivity of about 82% and specificity of 98% for pulmonary aspergillosis, significantly outperforming traditional culture. A newer approach called metagenomic next-generation sequencing (mNGS) performs comparably to PCR and can identify a broader range of organisms in a single test, including fungi that clinicians might not have specifically suspected.
Both PCR and mNGS significantly outperform culture alone, which is limited by low sensitivity. The main barriers to wider use of molecular testing are cost and availability. Not every hospital lab offers these assays, and insurance coverage varies. But when accessible, they can shave days or weeks off the time to diagnosis compared to waiting for a culture to grow.
Tissue Biopsy and Histopathology
In cases where less invasive methods haven’t provided an answer, a lung biopsy may be necessary. A small piece of tissue is removed, either through a bronchoscope, a CT-guided needle, or occasionally surgery, and examined under a microscope by a pathologist.
Standard tissue staining with hematoxylin and eosin (the routine stain used for most biopsies) can sometimes reveal fungal organisms, but specialized stains are far more reliable. Grocott’s Methenamine Silver (GMS) stain and Periodic Acid-Schiff (PAS) stain are the two workhorses for highlighting fungi in tissue. GMS is especially useful for identifying Histoplasma, particularly in areas of the lung where tissue has started to die off. These stains make fungal cell walls stand out clearly against surrounding tissue, allowing pathologists to assess the shape, size, and branching patterns that distinguish one fungus from another.
Even with special stains, some fungi have overlapping appearances under the microscope. In those cases, the tissue sample is also sent for culture or PCR to pin down the exact species. And in certain conditions, like allergic reactions to Aspergillus in the airways, the fungus itself is often invisible even on GMS stain, making clinical context and other test results essential for diagnosis.
How Testing Differs by Immune Status
Your immune system plays a major role in determining which tests are ordered and how aggressively doctors pursue a diagnosis. In people with healthy immune systems, a fungal lung infection is often mild and self-limiting. Testing might be limited to a chest X-ray and a blood antibody test, with the diagnosis sometimes made retrospectively after symptoms have already resolved.
For immunocompromised patients, including those undergoing chemotherapy, organ transplant recipients on anti-rejection drugs, or people with advanced HIV, the stakes are much higher. Fungal infections in these groups can progress rapidly and become life-threatening. Testing is more aggressive from the start: CT scans are ordered early, blood biomarkers like BDG and galactomannan are checked routinely (sometimes even before symptoms appear, as surveillance), and bronchoscopy with BAL is pursued sooner rather than later. In neutropenic patients, those with dangerously low white blood cell counts, imaging findings like the halo sign can be enough to start antifungal treatment immediately while waiting for confirmatory test results.
The specific fungus suspected also shapes the testing strategy. For someone returning from Arizona with a persistent cough, Valley fever antibody testing is the logical first move. For a leukemia patient with a new lung nodule during chemotherapy, galactomannan testing and CT imaging take priority. The combination of your symptoms, geography, and immune status determines which tests are most likely to give a clear answer.