Understanding Spore Trap Results

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When enumerating airborne fungal spores, most labs, for practical reasons, analyze a percentage of the sample trace (often 15-25%) instead of 100%. A few labs claim that they analyze 100% of the sample trace. Obviously analyzing 100% of the sample trace sounds better than analyzing say 15 or 25%. In fact, some people argue that analyzing less than 100% of the sample trace is incorrect, inaccurate and gives skewed results. However, it is important to note that the accuracy of spore traps is affected by so many factors such that the contribution to inaccuracy/accuracy by the percentage of the sample trace analyzed in the lab is highly likely insignificant if at least 15% of the sample is analyzed. Even if 100% of the sample trace is analyzed, the results of spore traps would still be skewed in one way or another. Perhaps the most important of these factors is the sample volume collected. The volume of air collected for analysis is in most cases 75L or 150L. However, the final spore count is reported as the number of spores per cubic meter of air. That means ideally we should collect and analyze 1000L of air because by collecting 75 or 150L of air, the samples are only 7.5% and 15% respectively of what should have been analyzed. Unfortunately the spore traps (Air-O-Cell, Allergenco, etc.,) are not designed to collect large volumes of air especially in a dusty environment. In a dusty environment spore traps get overloaded with dust making it difficult for the analyst to see the spores.

The other factor that skews results is the fact that fungal spores are not uniformly distributed in the air.  That means if the spores are not uniformly distributed in the air, even the position and the height at which we place the sampler would skew the results.

Another factor that skew results is the fact that the spore concentration in a building varies not only with the season but also with the day and time of the day. In one study it was found that the spore of Aspergillus fumigatus can increase temporary to a significantly high concentration and then decline to “normal” level within 30 min only. This means the 5 or 10 minutes samples that are collected using spore traps are only a measure of spore concentrations at the time of sampling.

Spore trap- notice the non-unform distribution of Asp/Pen spores

Asp/Pen- notice the non-uniform distribution of spores

Is Analyzing 25% of Spore Trap Sample Trace Incorrect and Inaccurate?

The simple answer to this question is NO. First, spore traps are not meant to determine the actual spore count in the air. They are meant to give an idea of the types of spores present in the air and the concentrations of these spores relative to each other. Analyzing spore traps by direct microscopy is a complex process and there are limitations. Depending on the experience of the analyst there are counting and identification errors. Because of eye fatigue, counting errors are likely to be higher for a sample trace analyzed at 100% than one analyzed at 25%.

At MBL we use the ASTM Standard “Standard Test Method for Categorization and Quantification of Airborne Fungal Structures in an Inertial Impaction Sample by Optical Microscopy (Designation: D 7391 – 09)“. This is the only standard currently available for spore trap analysis. The standard does not specify the minimum percentage of the sample trace to count and for good reasons. Practically, not every sample can be analyzed at 100%. The density of particulates (spores and other particulates) on the sample trace and the size of particulates determine how easy and fast the sample can be analyzed. Analyzing 100% of a highly loaded sample is not only difficult but could take one to several hours and will have little or no added value to the results because counting errors will be made during the analysis.

When Is It Possible to Analyze 100% of Sample Trace

The only time it is practically possible to analyze 100% of sample trace is when you have relatively few scattered spores and other particulates on the slide. With heavily loaded samples it’s very difficult for the analyst to keep track of what they have and haven’t counted under the microscope. As mentioned earlier, the analyst is likely to be fatigued and count many spores twice or miss counting some of them altogether. Compare the 2 images below which are showing spores and other particulates in just a single field of view of a microscope. Again, even for samples with low particle density, does analyzing 100% of sample trace add any value to the results? We would like to hear from you…

Spore trap- mixture of spores

Heavily loaded sample with tiny spores- Difficult to count 100%

Spore trap- Pithomyces spores

Big scattered spores- Easy to count 100%

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Dr Jackson Kung'u
Dr. Jackson Kung’u is a Microbiologist who has specialized in the field of mycology (the study of moulds and yeasts). He is a member of the Mycological Society of America. He graduated from the University of Kent at Canterbury, UK, with a Masters degree in Fungal Technology and a PhD in Microbiology. He has published several research papers in international scientific journals. Jackson has analyzed thousands of mould samples from across Canada. He also regularly teaches a course on how to recognize mould, perform effective sampling and interpret laboratory results. Jackson provides how-to advice on mould and bacteria issues. Get more information about indoor mould and bacteria at www.drjacksonkungu.com.
Dr Jackson Kung'u

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