A Potential Explanation of a Positive Serum β-Glucan Assay in Mucormycosis

A Potential Explanation of a Positive Serum β-Glucan Assay in Mucormycosis To the Editor—I read with interest the nice study by Angebault et al [1] that gives a " real world " picture of the subop-timal performance of the serum β-glucan assay. Unexpectedly, the authors found that serum β-glucan test was positive in 3 of the 6 cases of mucormycosis in their series with no apparent cause of false-positive test results (eg, prior administration of intravenous immunoglobulin, recent surgery, etc). This was an unantic-ipated finding because prior biochemical studies detected unusually low amounts of glucans in the cell wall of Rhizopus [2]. In addition to the possibility of an undetected mixed infection with another glucan-producing fungus (for example , concomitant growth of Aspergillus or Fusarium species was found in 46% of high-risk patents with mucormycosis in one of our earlier series [3]), I would like to offer another explanation: the cell wall of some Mucorales does contain glucan at a concentration above the threshold of detection by the β-glucan assay. We have previously shown that induction of interleukin-23 producing dendritic cells by Rhizopus oryzae was dectin-1 dependent and was mediated by β-glucan [4], a finding that corroborates with a study that identified β-glucan synthetase, the enzyme responsible for synthesis of β-glu-can, in R oryzae [5]. It is unclear whether these experimental observations extend to other R oryzae strains, other Rhizopus species, or other Mucorales. Because 3 of the 6 cases reported by Angebault et al [1] were caused by Rhizopus species, it would be of interest to see whether there was a Mucorales-specific positivity (and its magnitude) in the small number of mucormy-cosis cases with a positive serum β-glucan assay in their work.