Aims To investigate how clinical microbiology laboratories should report and interpret mixed mould isolates including Aspergillus species from clinical samples and the criteria for susceptibility testing of the isolates.

Methods Retrospectively collected data from our laboratory information system of moulds isolated between January 2005 and December 2007. Patient case notes were also reviewed.

Results A total of 502 isolates (from 273 patients) were found. 20 patients with clinical diagnosis of a probable fungal infection had mixed Aspergillus species.

Conclusions In most instances, the isolation of Aspergillus species from non-sterile sites does not represent clinical disease, but only colonisation/contamination. However, for high-risk patients including transplant recipients, a positive culture is associated with invasive disease. Our tertiary centre routinely reports single fungal isolates and mixed cultures with appropriate comments, and those considered significant will also have susceptibility testing carried out. The correlation of culture results with clinical features can differentiate between invasive disease and contamination.