Q-172. The Effect of the Number of Counted Traverses on the Estimation of the Total Spore Count Sampled on a Non-Culturable Slit Impactor

G. Marchand, Y. Cloutier, C. Pépin, D. Drolet;
Québec Occupational Hlth. and Safety Res. Inst., Montréal, QC, CANADA.

Even though they are ubiquitous and we are always exposed to them, molds can cause health problems in some individuals. Analytical methods based on laboratory culture methods allow only a partial evaluation of the mycological flora present. The property of spores to induce health effects even when they are no longer cultivable requires methods that establish the total mycological flora in the environments. A method widely used at the present time consists of collecting the spores by impaction on a cover slip coated with adhesive in a cassette. The spores deposited in trace form are then counted by an optical method. Some laboratories, in order to minimize the analytical time, count a small percentage of the trace, down to 15%. The objective of this project is to evaluate the sampling error on the final results depending on the percentage of the counted trace. For this project, 39 samples were 100% counted and the result recorded for each traverse independently. Those results were then used in a Monte Carlo simulation to calculate the error produce by a partial and random count of the trace. The Monte Carlo simulation reproduced the counts of 10 000 analysts. The averages of the error for each percentage count were used to evaluated the minimum number of traverses that should be count to achieved a maximum sub-sampling error of 10%. The results of this project demonstrate that the number of traverses counted by laboratories has a non-negligible impact on the final result. The 15% trace currently counted by some laboratories does not allow a maximum error of 10% to be reached in the majority of cases.