The first step in obtaining the best clinical results for PCR-based testing for viruses, parasites, and bacteria is high quality samples. Collection and handling of stool specimens is an important factor that directly relates to obtaining the best results in any clinical assay.
For sample collection, fresh stool samples should be placed in a labeled, sterile, leak-proof and preservative-free container. Alternatively, the collected specimen can be placed in a container with transport medium.
Raw Stool Specimens
Raw stool samples should be transported cold and stored immediately at 2–4°C upon arrival. It is best to test the stool sample as soon as they are received. Otherwise, the specimens should be frozen at -80°C until testing. It is advised to prepare multiple aliquots prior to freezing when possible. Avoiding multiple freeze-thaw cycles will help avoid sample degradation (for example, C. difficile is known to rapidly degrade with multiple freeze-thaw cycles). Ideally stool samples stored at -80°C should be tested within 30 days.
Stool Specimens in Transport Media
For samples collected using a holding/transport medium, add the stool directly to the storage vessel containing the medium. Specimens contained in a holding medium can be transported to the proper laboratory at room temperature and then stored at 2–4°C for less than 3 days prior to processing. If specimens are not going to be tested within 3 days, store frozen at -80°C. Again, making aliquots is recommended to avoid multiple freeze-thaw cycles. Note that stool samples stored in a fixative, such as formalin, may not be compatible with molecular based methods.
Cary-Blair medium is popular for stool transport. There are many commercially available Cary-Blair medium kits that are designed for the collection and transport of fecal samples. Cary Blair medium typically has a low-nutrient content in a phosphate buffer system (including sodium thioglycollate) that inhibits or eliminates massive bacterial growth while maintaining a stable pH range. This prevents possible compromise to species included in the panel. Low oxidation/reduction potential assures bacterial survival for a longer period of time.
Conclusion
Following these collection and storage guidelines will help ensure the integrity of your fecal samples and ultimately provide reliable results.
For additional guidance on specimen collection, refer to the Centers for Disease Control and Prevention Guidelines for Specimen Collection.