Basic mold count testing in bulk herbs, Cannabis included, is relatively simple and typically cost less than $50. The principle of mold count testing with herbs and herbal preparations is to shake a quantity of the sample with a buffered water solution and then ‘plate’ some of that liquid on a petri dish containing a general-purpose agar media that will germinate molds and mold spores. You can plate the liquid on either a petri dish or, more commonly, on a disposable petri-like device called 3M PetriFilm®. PetriFilm is like a dehydrated petri dish formed on a card. When a watery extract of the herb or herbal preparation is plated onto the PetriFilm card, the water rehydrates the agar which germinates the spores. After 48 hours incubation, any germinated fungi or fungal spores appear as a colored colony spot which can be counted. Data is reported in Colony Forming Units per gram of sample (CFU/g). Basic mold count assays are typically non-specific—meaning that it is a metric of all active molds and spores, both harmful and harmless. In truth, almost no herbal commodities will have NO mold count. In general, acceptable limits for mold counts in raw medicinal plant material intended for processing and raw herbal material intended for internal use are 10,000 CFU/g and 1000 CFU/g, respectively.
The World Health Organization states the following, with respect to the need to test herbal medicines for microbiological contamination:
Despite once being a required quality assurance test in Oregon, mold count testing is no longer required. In other legal Cannabis states, mold counts and other microbiological contaminant checks are required testing (California, Colorado, Massachusetts, Nevada, etc.). If you think basic mold testing is important, you should ask your Cannabis producer, wholesaler or retailer about it and let them know you value it as a metric of quality and safety.
WHO Guidelines for Assessing Quality of Herbal Medicines with Reference to Contaminants and Residues. ISBN 978 92 4 159444 8. World Health Organization 2007