The Beta-D-glucan test is a medical diagnostic tool used to help identify certain fungal infections. This blood test detects a specific substance in a patient’s bloodstream. The information it provides, when combined with other clinical findings, can guide medical decisions.
The Purpose of the Beta-D-Glucan Test
Beta-D-glucan (BDG) is a sugar molecule that forms a major part of the cell walls of many common fungi. When an invasive fungal infection (IFI) occurs, these fungal cell walls can break down, releasing BDG into the bloodstream. Detecting BDG in the blood serves as a biomarker, indicating a possible active fungal infection.
The test is frequently used for patients at increased risk of serious infections, such as those with weakened immune systems (e.g., undergoing cancer chemotherapy or organ transplantation) and critically ill patients in intensive care units. Earlier identification of an IFI through BDG detection can lead to timely treatment and improved patient outcomes. The Fungitell assay is one commercially available BDG test approved by the FDA for use on serum in the United States.
Fungi Detected by the Test
The Beta-D-glucan test can detect a wide range of medically relevant fungi because most share BDG as a cell wall component. Common examples include Candida species (candidiasis), Aspergillus species (aspergillosis), and Pneumocystis jirovecii (Pneumocystis pneumonia, PCP). Other fungi like Fusarium species, Coccidioides immitis, and Histoplasma capsulatum may also be detected.
The test does not detect all types of fungi. Some medically important fungi produce very low levels of BDG or lack it entirely. For example, the Zygomycetes class, including Mucor and Rhizopus, do not produce BDG. Similarly, Cryptococcus species, which have a capsule that can prevent BDG release, and the yeast form of Blastomyces dermatitidis produce minimal BDG, making them generally undetectable.
Interpreting Test Results
A positive Beta-D-glucan result, typically above 80 picograms per milliliter (pg/mL), indicates the possible presence of an invasive fungal infection. Conversely, a negative result, usually below 60 pg/mL, can help rule out many fungal infections, particularly Pneumocystis pneumonia, due to the test’s high negative predictive value for this specific infection. Values between 60 and 79 pg/mL are often considered indeterminate, suggesting further sampling and testing may be needed.
The BDG test is not a standalone diagnostic tool. Doctors consider results alongside a patient’s overall clinical picture, including symptoms, medical history, and findings from other diagnostic methods like imaging scans. Other laboratory tests, such as fungal cultures and molecular tests, are also incorporated to establish a comprehensive diagnosis and guide treatment.
Factors Influencing Test Accuracy
Various factors can affect the accuracy of the Beta-D-glucan test. False-positive results can occur due to contamination from medical products or procedures. This includes surgical gauze containing BDG, which can elevate levels if exposed to the patient’s bloodstream during or after surgery. Certain intravenous medications, such as some immunoglobulins and specific antibiotics like piperacillin-tazobactam, may also contain substances that react in the test, leading to a false positive.
Patients undergoing hemodialysis with older cellulose-based membranes can also show false-positive BDG levels because these membranes may release glucans. Some bacterial infections, including those caused by Staphylococcus pneumoniae and Pseudomonas aeruginosa, have been linked to elevated BDG levels, potentially leading to false positives. False-negative results can arise if the infection is caused by a fungus that does not produce significant amounts of BDG, such as Zygomycetes or Cryptococcus species. False negatives can also occur if a fungal infection is highly localized and does not release enough BDG into the bloodstream to reach detectable levels.