False Negatives - Because this approach simultaneously sequences DNA from many different species, more abundant species will be more common in the data. This means that rare or very small species can be missed. A species present in the environment but not detected in the eDNA data is called a false negative result. As with any detection method (including visual) there will always be some species that are missed during the survey. If the detection of rare species is a priority, then there are different approaches that can be used to increase the probability of detection.
False negatives can also occur in eDNA metabarcoding data when a particular species successfully sequenced, but there is no entry for that species in the reference data base used to decode the DNA sequences. In that case the species, while present, will be listed as ‘unknown’ in the data.
False Positives - It is also possible to get a positive detection for a species that was not actually present in the sample. This is called a false positive result and can occur when the data analysis incorrectly assigns a species to a sequence during processing. These false positive detections can be fixed during the quality control of the data.