TABLE 2.
Nonmolecular tests used for diagnosis of the most common invasive fungal infectionsa
| Microorganism | Diagnostic test | Optimal specimen type | Sensitivity (%) | Specificity (%) | Reasons for false-positive results | Reasons for false-negative results | Comments |
|---|---|---|---|---|---|---|---|
| Candida spp. | Cultures | Blood | 50–60 | 95 | None | None | Gold standard. May take up to 3 days for a positive result. |
| Beta-glucan assay | Serum | 77.6–81.3 | 87.1–92.4 (for patients not infected with fungal pathogens) | Other fungal infection, dialysis filters made from cellulose, bacteremia? | Hemolyzed samples, higher cutoff values | A cutoff of 80 pg/ml is associated with higher accuracy; used as screening test for various fungal infections. | |
| CAGTA assay | Serum | 77–89 | 91–100 | Unknown | Unknown | CAGTA assay is not affected by Candida colonization or intake of antifungal. | |
| Aspergillus spp. | Histopathology | Various, depending on the infection site | 100 | 100 | Fusarium and Scedosporidium spp. have similar microscopic appearances | Formation of pseudoseptations by the organism | Most accurate test is tissue biopsy. Used as a last resort in undiagnosed cases. |
| Culture | Various, depending on the infection site | 30–68 | 72–100 | Aspergillus sp. colonization | Slow-sporulating organisms, hematopoietic stem transplant recipients | Gold standard, but with low sensitivity. PPV largely depends on the population tested. | |
| Galactomannan assay | Serum, BAL fluid, or CSF | 71 for serum, 90 for BAL fluid | 89 for serum, 94 for BAL | Histoplasma sp. and Fusarium sp. infections, fungal colonization | Steroid treatment | Optimal diagnostic cutoff is not yet established. GM levels could be used to monitor the response to treatment. | |
| Beta-glucan assay | Serum | 55–95 | 77–96 | Other fungal infection, Gram-negative bacteremia, dialysis | Hemolyzed samples, higher cutoff values | A cutoff of 80 pg/ml is associated with higher accuracy; screening test for various fungal infections. | |
| Lateral-flow device antigen detection | Serum and BAL fluid | 48–100 | 100 | Unknown | Unknown | Interpretation is subjective; perhaps has a better performance than that of GM assay. | |
| Pneumocystis spp. | Histopathology | Expectorated sputum, induced sputum, or BAL fluid | 33–100, depending on stain and specimen used | 100 | Uncommon | Varies by specimen type and stain | Methenamine silver stain on BAL fluid is the current gold standard. Toluidine blue stain on induced sputum may be the most cost-effective method. Pneumocystis does not grow easily in culture. |
| Beta-glucan assay | Serum | 94.8 | 86.3 | Other fungal infection, bacteremia, dialysis | Uncommon | Excellent screening test for high-risk patients, not useful for monitoring response to treatment. | |
| Cryptococcus spp. | Cultures | CSF | >95 | 100 | Uncommon | Uncommon | Gold standard, but takes 3–7 days for a positive result. |
| Histopathology | Mostly CSF | 75 | 100 | Uncommon | Low levels of microorganism | India ink stain often used as a screening test. | |
| Cryptococcal antigen test (LA, EIA, or LFD) | CSF or serum | 97 for CSF, 87 for serum | 93–100 | Trichosporon sp., Capnocytophaga sp., or Stomatococcus sp. invasive infections | Uncommon | Most accurate test when performed on CSF. The three methods are comparable, although LA gives more false-positive results. LFD is best for rapid point-of-care diagnosis. | |
| Histoplasma capsulatum | Culture | Tissue, BAL fluid, or other bodily fluids | 85 for disseminated and acute pulmonary infections | 100 | Uncommon | Low fungal levels on specimen | Gold standard, but takes 2–4 weeks to grow. |
| Histopathology | Tissue or BAL fluid | 76 for disseminated infection | 100 | Uncommon | Low fungal levels | Unacceptably low sensitivity, which is even lower for pulmonary infection. | |
| Antibody tests (CF or ID) | Serum | 75 for disseminated infection, 67 for acute pulmonary infection | 100 | Uncommon | Low fungal levels | Best performance with combination of the two methods. One study showed unacceptably low sensitivity for solid organ transplant patients. | |
| Antigen test | Urine and serum | 88–92 | 100 for patients without fungal infection | High cross-reactivity in cases of Blastomyces sp. or Coccidioides sp. infection | Uncommon | Most accurate test overall, but shows cross-reactivity with other dimorphic fungi. | |
| Blastomyces dermatitidis | Culture | Sputum, BAL fluid, tissue | 86 for sputum, 92 for BAL fluid | 100 | Uncommon | Low levels | Gold standard, grows better on fungal isolator cultures, takes a long time to grow. |
| Histopathology | Varies based on infection site | 46 for sputum, 90 for tissue | 100 | Uncommon | Incorrect specimen | Broad-based budding. | |
| Antibody test (CF and ID), new EIA | Serum | 57 for CF, 65–80 for ID, 88 for novel EIA | 37 for CF, 100 for ID, 100 for EIA | Cross-reactivity with other dimorphic fungi | Low levels of circulating antibodies | ID method is clearly preferable due to higher performance. EIA has high specificity even in cases of histoplasmosis. | |
| Antigen test | Mostly urine | 93 | 99 for patients free of fungal infection | High cross-reactivity with other dimorphic fungi (e.g., 96% with histoplasmosis) | Uncommon | Most accurate test, but with cross-reactivity issues | |
| Coccidioides spp. | Culture | Sputum or tissue | 90 | 100 | Uncommon | Low fungal levels | Grows better than all endemic fungi. Culture is used mainly for hospitalized patients. Can grow within a week, but identification can take longer. |
| Histopathology | Sputum or tissue | 31–42 | 100 | Uncommon | Low fungal levels on specimen | Spherule detection. | |
| Antibody assays (CF, TP, and novel EIA) | Serum | 95 | 99 | False-positive results have been reported for asymptomatic individuals | Uncommon | Most commonly used test. EIA seems to have the best accuracy but is still not widely tested. A single positive IgM result must be interpreted based on pretest probability. |
a
BAL, bronchoalveolar lavage; CAGTA, Candida albicans germ tube antibody; CF, complement fixation; CSF, cerebrospinal fluid; EIA, enzyme immunoassay; GM, galactomannan; ID, immunodiffusion; LA, latex agglutination; LFD, lateral-flow device; PPV, positive predictive value; TP, tube precipitin.