Fungitell Positive: Accuracy and False Positive Causes

A positive Fungitell result means that the blood sample contained (1→3)-β-D-glucan (BDG) at or above 80 pg/mL, a threshold the manufacturer considers consistent with invasive fungal infection. But “consistent with” is doing heavy lifting in that sentence. The test detects a sugar molecule found in the cell walls of many fungi, not any single species, and a long list of non-fungal exposures can push the number above that cutoff. Whether a positive Fungitell result changes your diagnosis or your treatment depends heavily on clinical context, the specific infection being considered, and ruling out the surprisingly common causes of false-positive readings.

How the Test Detects Fungal Infection

Fungitell is built around a biological cascade derived from the blood cells of the horseshoe crab. The reagent uses a modified version of Limulus amebocyte lysate (LAL), specifically engineered so that one of its activation pathways, the factor C pathway, is eliminated. What remains is the factor G pathway, which reacts specifically to (1→3)-β-D-glucan. That modification is what makes the test selective for BDG rather than responding to bacterial endotoxins or other polysaccharides, including beta-glucans with different chemical linkages.¹

Because BDG is a structural component in the cell walls of a wide range of fungi, including Candida, Aspergillus, and Pneumocystis, the test is intentionally broad. It does not tell you which fungus is present. A positive result is a signal that some fungal process may be underway, and further workup is needed to identify the culprit. Two notable exceptions are Cryptococcus and the Mucorales group (causing mucormycosis), which produce little or no detectable BDG, so the test is not useful for those infections.

How Accurate Is a Positive Result

The short answer is that it depends on which fungal infection you are looking for. Fungitell’s diagnostic performance varies dramatically across different clinical scenarios, and the numbers are less reassuring than many clinicians expect.

For invasive candidiasis in ICU patients, the picture is mixed. One prospective study found a sensitivity of about 74% and a specificity of just 45% at the standard 80 pg/mL cutoff, meaning more than half of patients without candidiasis still tested positive.² A head-to-head comparison of three BDG assays found broadly similar results for Fungitell: at the 80 pg/mL cutoff, sensitivity was 77% and specificity was 51%. Even at an optimized lower cutoff of 50 pg/mL, specificity only reached 51% while sensitivity improved to 85%. Across all three assays tested, positive predictive values for an estimated candidiasis prevalence of 20% hovered around 30 to 35%, while negative predictive values were stronger, ranging from 85 to 95%.³

Those numbers carry a practical implication that is easy to miss. A positive predictive value of around 30% means roughly two out of three positive results in the ICU setting do not reflect actual invasive candidiasis. The test’s real strength for candidiasis lies on the other side: a negative result is fairly reliable for ruling the infection out.

For invasive aspergillosis, the BDG assay showed strong sensitivity of 81% but only 82% specificity when compared against galactomannan testing (the Platelia Aspergillus assay), which had 97% specificity but a much lower sensitivity of 49%.⁴ The BDG test catches more true cases but generates more false alarms, while galactomannan rarely gives a false positive but misses about half of actual infections. Many institutions use both tests together for this reason.

Where the Test Shines: Pneumocystis Pneumonia

The strongest diagnostic performance for Fungitell is in Pneumocystis jirovecii pneumonia (PCP), the opportunistic lung infection seen mainly in immunocompromised patients. An earlier systematic review and meta-analysis reported a pooled sensitivity of 96% and specificity of 84% for PCP, concluding that BDG can serve as a screening tool for this infection.⁵

A more recent and larger meta-analysis, incorporating 26 studies and over 5,000 patients, found somewhat more conservative numbers: sensitivity of about 84% and specificity of roughly 76% at the standard 80 pg/mL cutoff. The clinical takeaway, though, was compelling. When the pre-test probability of PCP is below 20% and the BDG comes back under 80 pg/mL, the chance of the patient actually having PCP drops below 5%. At a higher cutoff of 400 pg/mL, sensitivity fell to about 64%, but specificity jumped to nearly 94%. In a patient with a roughly 50% pre-test probability, a BDG above 400 pg/mL pushed the post-test probability above 90%.⁶

The practical message: a very high BDG level in a patient with risk factors for PCP is a strong positive signal. A normal BDG in a low-risk patient is reassuringly negative. Values between 80 and 400 pg/mL sit in a gray zone where clinical judgment, imaging, and sometimes bronchoscopy need to pick up where the lab result leaves off.

The Long List of Things That Cause False Positives

If you are trying to figure out whether a positive Fungitell result is real, the false-positive problem is probably the most important section of this article. BDG can be elevated in the absence of any fungal infection for a surprisingly wide range of reasons, and in critically ill patients receiving multiple treatments, one or more of these causes is often present.

Intravenous Antibiotics and Antifungals

Many common IV medications are themselves contaminated with BDG. One study tested 44 intravenous antimicrobials and found that colistin, ertapenem, cefazolin, trimethoprim-sulfamethoxazole, cefotaxime, cefepime, and ampicillin-sulbactam all tested positive for BDG at reconstituted-vial concentrations.⁷ A broader investigation found that 25 of the antimicrobials tested, including 20 antibiotics and all tested antifungals, contained enough BDG to trigger a positive result, with concentrations ranging from 9 to over 2,800 pg/mL.⁸

Penicillin G is a well-documented offender. In one case, continuous high-dose IV penicillin for bone infection drove the BDG level up to 81 pg/mL, which crossed the positive threshold. The level dropped back to 38 pg/mL within 14 hours of stopping the drug, and the amount of BDG found in the drug vial closely matched what would have been needed to produce the observed serum level.⁹ The contamination comes from the manufacturing process, not from the drugs themselves having any fungal component.

Blood Products and IVIG

Intravenous immunoglobulin (IVIG) is a recognized cause of elevated BDG. The problem creates a genuine clinical trap: patients receiving IVIG are often immunocompromised, which is exactly the group most at risk for invasive fungal infections and most likely to get a BDG test ordered. One case report described a patient with an autoimmune disorder and hematologic malignancy who developed respiratory failure and had a positive BDG drawn after IVIG administration, leading to diagnostic uncertainty about whether the result reflected Pneumocystis pneumonia or was simply an artifact of the infusion.¹⁰ Albumin infusions carry a similar risk.

Hemodialysis, Bacteremia, Gauze, and Other Exposures

Cellulose-based hemodialysis membranes are a classic source of false-positive BDG. Patients on dialysis frequently show elevated levels that have nothing to do with fungal infection. Bacteremia is another culprit: in a study comparing BDG and galactomannan in patients with bloodstream infections, 37% of patients with bacteremia had false-positive BDG results, compared with just 2% for the galactomannan test.¹¹ Surgical gauze packing, certain IV tubing, and even some brands of cotton swabs have been reported to introduce BDG into the sample or the patient. One study of patients with bacteremia and no fungal infection found a false-positive rate of about 9%, and nearly every false-positive patient had at least one alternative explanation such as hemodialysis, colitis, albumin therapy, or gauze exposure.¹²

Things That Do Not Cause False Positives

One concern clinicians and patients sometimes raise is whether eating mushrooms, bread, or other foods rich in beta-glucan could spike BDG levels. A controlled study gave healthy participants a standardized oral BDG challenge and found no change in blood BDG levels over time.¹³ Dietary glucan simply does not cross the gut barrier into the bloodstream in amounts that matter. You do not need to fast or avoid certain foods before a Fungitell test.

Lab Handling and Interference

The assay is sensitive to sample quality. High levels of hemoglobin, bilirubin, or triglycerides in the blood sample can interfere with the reading. The thresholds at which these substances caused 20% interference were 588 mg/dL for hemoglobin (indicating severe hemolysis), 72 mg/dL for bilirubin, and 466 mg/dL for triglycerides.¹⁴ In practice, this means a severely jaundiced or hemolyzed specimen could give an unreliable result. If a sample is visibly discolored or lipemic, the lab may flag the result or request a redraw.

Using Positive Results to Guide Treatment

Beyond diagnosis, serial BDG levels can help clinicians decide how long to continue antifungal therapy. During a multistate outbreak of fungal meningitis, researchers tracked BDG levels in cerebrospinal fluid (CSF) over time. Patients whose CSF BDG levels declined during treatment generally improved clinically. All three patients whose BDG stayed persistently elevated had poor outcomes, including stroke, meningitis relapse, or new disease.¹⁵ In another case, serial serum BDG monitoring was used to determine when to stop antifungal therapy for Aspergillus bone infection, with the patient completing nine months of treatment guided by declining BDG values and remaining infection-free for nine months after stopping.¹⁶

Perhaps the most impactful clinical application involves knowing when to stop antifungals in ICU patients started empirically. A randomized controlled trial tested a strategy of using BDG results (along with other biomarkers) to decide whether to discontinue empirical antifungal treatment in critically ill patients with suspected invasive Candida infection. In the biomarker-guided group, 54% of patients had antifungals stopped early, compared with just 2% in the routine care group. The median treatment duration was cut roughly in half, from 13 days to 6 days, with no increase in subsequent proven Candida infections, ICU stay, or mortality.¹⁷ That finding matters because unnecessary antifungal use drives drug resistance and adds cost.

Testing Beyond Serum

While most Fungitell testing is done on serum, the assay has also been applied to cerebrospinal fluid for diagnosing fungal meningitis. During the 2012 outbreak linked to contaminated methylprednisolone injections, CSF BDG testing proved remarkably accurate. Using the standard 80 pg/mL cutoff, sensitivity was 96% and specificity was 95% for proven meningitis. A slightly lower cutoff of 66 pg/mL achieved 100% sensitivity with 94% specificity.¹⁸ CSF testing avoids many of the false-positive pitfalls that plague serum testing, since the cerebrospinal fluid is not exposed to IV medications, dialysis membranes, or gauze in the way blood is.

How Fungitell Compares to Molecular Testing

PCR-based tests that detect fungal DNA in the blood are increasingly available and offer species-level identification, something BDG testing cannot do. However, BDG testing still holds some advantages. In a comparison between a multiplex Candida PCR and BDG testing for candidemia, the BDG assay was more sensitive overall (58% versus 33%). The gap was especially large in ICU patients, where BDG sensitivity was 60% compared with 28% for PCR. For early diagnosis of candidemia, BDG performed better, while PCR sensitivity actually declined the further the sample was drawn from the date of the positive blood culture.¹⁹ In hematology-oncology patients, though, the two tests performed similarly. The upshot is that BDG and molecular testing answer somewhat different questions and are often most useful when combined.

The Rapid Single-Sample Version

The original Fungitell assay is a batch test, meaning the lab typically collects multiple samples and runs them together, which can delay turnaround time. A newer version called Fungitell STAT processes a single sample and returns results faster. In validation testing, the STAT assay showed strong agreement with the original: when indeterminate samples from the predicate test were excluded, positive percent agreement was 99% and negative percent agreement was 98%. The STAT assay uses a different reporting scale, a beta-glucan index (BGI) rather than a raw pg/mL value, with a BGI of 1.2 or above considered positive and below 0.74 considered negative.²⁰ The indeterminate zone between those values is where interpretation gets murkiest, just as with the original assay.

Children and Baseline BDG Levels

Pediatric patients add another layer of complexity. Preliminary data found that healthy children have higher baseline BDG levels than adults.²¹ The standard 80 pg/mL cutoff was developed and validated largely in adult populations, so applying it to children may produce more false positives. Pediatric-specific thresholds have been discussed but are not yet widely established. If your child has a positive Fungitell result, the treating team should be aware that the adult cutoff may not apply cleanly.