Is serum ß-D-glucan level helpful in diagnosis of invasive fungal infections?

While the diagnosis of invasive fungal infections still remain troublesome, early diagnosis remains crucial for favorable clinical outcome in immunocompromised patients. Visualization of fungal elements in biopsy specimens by histopathologic methods or isolation of the fungus on culture media are the gold standards of definitive diagnosis of these infections.

However, obtaining tissue samples is not always possible due to several factors, such as poor general status of the individual patient or thrombocytopenia. Blood culture results are also not very helpful and are positive in only about 50% of patients with invasive candidiasis or fusariosis and in very few patients with invasive aspergillosis. Thus, noninvasive diagnostic methods which may help in early detection of the infecting fungus are required.

Among these methods, detection of circulating 1-3-ß-D-glucan is of current interest (Obayashi et al. 1995; Odabasi et al. 2004; Verweij et al. 2000). ß-D-glucan is found in cell wall of various fungal genera. However, ß-D-glucan will be low or absent in patients with cryptococcal infection as well as those patients with infections due to zygomycetes. ß-D-glucan activates factor G of the horseshoe crab coagulation cascade by binding to the alpha subunit of factor G and activating its serine protease zymogen subunit. This reaction, combined with the proclotting enzyme of the cascade and the chromogenic substrate, make the spectrophotometric detection of ß-D-glucan possible at levels as low as 1 pg/ml.

Commercial systems to detect ß-D-glucan are now available; Fungitec-G glucan detection test (Seikagau) and Glucatell (Associates of Cape Cod).

These two assays were evaluated by Odabasi et al. for validation and determination of the cut-off value for Glucatell assay in patients (n=283) with acute myelogenous leukemia and myelodysplastic syndrome who were receiving antifungal prophylaxis.

The results of this study provided significant clues for potential use of ß-D-glucan detection in diagnosis of invasive fungal infections. Based on the data obtained in candidemic patients and healthy individuals, a cut-off value of 60 pg/ml was chosen for Glucatell assay. Using this cut-off value, at least one serum sample of each patient was found to be positive for ß-D-glucan at a median of 10 days before the clinical diagnosis in all patients with proven or probable invasive fungal infection (candidiasis, fusariosis, trichosporonosis, or aspergillosis).

Importantly, the negative predictive value of Glucatell assay was 100%. The specificity of the test, on the other hand, was higher when >1 serum sample was tested as compared to the evaluation of a single sample from each patient; 90 and >96% of specificity values were detected for single and sequential positive results, respectively.

When the two commercial systems were compared, both were found to be specific for polysaccharides containing ß-glucan sequences. By kinetic analysis, Glucatell assay was found to be 2.5 fold less reactive than Fungitec-G assay with the pachyman standard.

In conclusion, detection of ß-D-glucan by using Glucatell assay and a cut-off value of 60 pg/ml appear potentially useful in diagnosis of invasive fungal infections. The negative predictive value of the test is very high and testing serial samples and considering sequential positive results tend to further increase the specificity of the test.

Related reading

Obayashi, T., M. Yoshida, et al. (1995). "Plasma (1->3)-beta-D-glucan measurement in diagnosis of invasive deep mycosis and fungal febrile episodes." Lancet 345: 17-20.

Odabasi, Z., Mattiuzzi G., et al. (2004). "Beta-D-glucan as a diagnostic adjunct for invasive fungal infections: validation, cutoff development, and performance in patients with acute myelogenous leukemia and myelodysplastic syndrome." Clin Infect Dis 39: 199-205.

Verweij, P. E., J. Figueroa, et al. (2000). "Clinical applications of non-culture based methods for the diagnosis and management of opportunistic and endemic mycoses." Med Mycol 38: 161-171.