To composite or not to composite? No, I am not trying to put a Shakespearean twist on this blog, but rather answer a question often asked when sending in environmental samples.
So, when is it appropriate to composite and how would you do so?
First, it is important to understand what compositing is. Compositing is collecting two to five samples and having them tested as one at a reduced cost. Compositing should not be done when conducting quantitative analysis in which the result is given as a number.
If a presumptive positive is the result of the composite it is important to either isolate all areas sampled and treat them as though they were each positive for the target organisms or have each of the individual samples run to identify which sample was positive. This leads to an important discussion to have with the lab conducting your analysis to learn when the samples are being combined.
If the lab combines the samples before enrichment and incubation it is likely not possible to test the individual samples if the composite result is presumptive positive without going back to the original sites and resampling them. By the time the resampling is conducted it is highly unlikely the micro flora is the same as during the original sampling event.
If the samples are enriched and incubated as individuals and then combined after incubation, the individual samples can be kept and run if a presumptive positive is the composite result. Because there is an associated cost to running the composite samples as individuals after a presumptive positive result, it is best to only utilize compositing when you do not receive routine positive environmental results.
In the information above I have mentioned multiple times “individual sample”. It is a best practice to use one sampling device per site as opposed to using one sampling device to collect all sites in a composite. If you were to use one device for all samples, you will not be able to track which site is positive if the composite is presumptive positive. There is also a slim chance to remain aseptic if the sampling device is opened and moved throughout the facility multiple times.
Remember that when compositing, only one target organism can be selected for analysis. That means if you have 10 total samples with seven of them being samples for Salmonella and the other three for Listeria you would need a minimum of three composites such as a composite of four Salmonella, a composite of three Salmonella and a composite of three Listeria.
When compositing is done correctly, it can be a great cost savings while still yielding accurate and useful results.
For more information on how Cherney approaches compositing, we encourage you to contact our team at firstname.lastname@example.org, calling us at 920-406-8300 or dropping a message below.