LoD is the lowest detectable amount of the test target that a test can reliably distinguish from zero.
Low LoD is good = test needs LESS virus to be present in the sample for a positive result.
LoD impact on how much a COVID-19 test will tell us – some tests need more virus to be present than others to show a positive result.
Nucleic Acid Amplification Techniques (NAATs) such as PCR and LAMP can identify the virus in a sample that has a thousand viral RNA copies per millilitre (ml) or less. At those levels, test subjects will likely not be infectious yet.
For the sorts of visually-read lateral flow devices used in mass testing, that number is far greater and this is the principal reason they perform so poorly as a screening tool. Visually-read LFDs require the equivalent of hundreds of thousands to millions of copies per ml to show a positive result.
NAATs usually quote LoDs in copies per ml of sample. Other tests tend to use a different measure called TCID50 and comparisons between the two are difficult – but not impossible so comparisons between LoDs of the different tests can be estimated.