A New Method for Detecting Low Concentrations of Airborne Aspergillus in HEPA-Filtered Hospital Air

Background: A rapid response during incident investigations is often critical when critical-care patients may have been exposed to microbial contaminants.  A new method using the Bi-Air (BA) filter cassette is described for rapidly detecting low concentrations of airborne Aspergillus in HEPA-filtered hospital air.  This method was used to collect airborne samples in operating rooms, intensive care units, post-operative recovery, and surgical support areas of seven hospitals. 

Objective: The first objective was to develop a method for rapidly detecting very low concentrations of airborne fungal spores using time-weighted average (TWA) sampling.  Second, concentration distributions of Aspergillus in OR's, ICU's, post-operative recovery , and surgical support areas were characterized.  The final objective was to demonstrate the routine application of the method in hospitals and patient homes.

Methods: The BA filter cassette was used to collect duplicate sample traces on a single 25 mm mixed cellulose ester (MCE) filter.  One sample trace was analyzed by microscopy at 600X magnification for the presence of Asp/Pen like spores, while quantitative PCR (qPCR) was used to rapidly identify species of concern in the archived duplicate sample when necessary.  The use of a filter cassette allowed TWA samples to be collected; which improved accuracy, reduced variability, and minimized false negatives.

Results: The BA concentrated the sample by a factor of 20.6 compared to a standard 25 mm cassette, allowing a limit of detection (LOD) of less than 1 spore/m3 to be achieved.  The geometric mean (GM) concentrations for Asp/Pen like spores ranged from 0.8 spores/m3 to 2.8 spores/m3.  Spore-equivalents ranged from 1 sp-eq/m3 to 225 sp-eq/m3 by qPCR.  The OR’s had the lowest GM concentration, while the highest GM concentration occurred in post-operative recovery. 

Conclusions: The described sampling method provided a rapid, routine and cost-effective method for pre-screening critical-care areas of healthcare facilities for the presence of Asp/Pen like spores. The method was successfully used for baseline sampling, incident investigation, and post-remediation verification sampling.  The method was also applied to baseline and incident investigations in the homes of recently released patients.