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 suboptimal 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 unanticipated 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-glucan, 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 mucormycosis cases with a positive serum β-glucan assay in their work.