The phrase "undetectable levels" means that something cannot be measured or identified because it is present in such small amounts that it cannot be detected or noticed. Full definition
At later time points, qPCR assays resulted in undetectable levels of these parasites. (journals.plos.org)
SNTF is a protein that is present at undetectable levels in healthy human brains, but is produced under conditions where nerve cells are traumatized and begin to die. (sciencedaily.com)
By keeping the virus at undetectable levels, researchers believe they can slow the spread of drug - resistant varieties. (scientificamerican.com)