DNA sequencing provides such detailed information that a positive result is generally considered conclusive. If the report indicates that a source of DNA was present in your sample, we can be confident enough in this conclusion there’s little point in estimating an exact probability.
Of course false positives may occur in principle with any test. In this context, possible causes include:
- Database incompleteness. If a novel organism from your sample is not present in the database, but a close relative is present, our process may falsely label the DNA as coming from the relative instead of the unknown organism itself. This is generally an issue for unknown species, and not a problem for well-studied species like known pathogens or parasites.
- Physical contamination. Although we and our clients work hard to prevent contamination while collecting and processing the sample, it seems inevitable that errors must occur at some low frequency in both stages. One benefit of our perspective as we review all the clients’ reports from each batch is that cross-contamination should be apparent. We find no evidence of this. In fact, many times when we report the presence of a parasite or pathogen it’s the only sample in that batch of samples that even contained that subject, so cross-contamination wouldn’t even be possible. With that said, biological contamination is probably the most likely explanation for any false positives that occur.
Its useful to limit our interpretation to what the positive result really says. Its a statement about the DNA present in the sample shipped to us. It may contain DNA from sources that are not alive in your tank; for example, we often find DNA from common food sources like mysis and brine shrimp. It may contain DNA from sources outside the tank, that were not introduced deliberately, including DNA from land plants (presumably introduced via pollen). But the identity of the source of each DNA sequence shown in the report is generally pretty well known, however it got there.