This is about laboratory test results, right?
During the mapping, I’d suggest to map to the most granular concepts exist. You can go with Presumptive positive and Uncertain since ‘presumptive negative’ doesn’t make a lot of sense to me.
On the phenotyping side, it depends on the granularity you’d like to achieve. Distinguishing between strong positive/weak positive/presumptive positive and positive lies somewhere in the area of specific studies in molecular biology what is not a main purpose of OMOP CDM. Most of the observational studies should be satisfied with positive/negative result, the rest if kind of junk.
But you still need to decide, whether ‘presumptive positive’ will be treated as ‘positive’ or omitted. It depends on method, analyte/pathogen, study aims, current practice in research area and investigator’s opinion.
For instance, according to FDA/CDC presumptive positive COVID-19 test result (please don’t confuse with ‘presumptive lab evidence’) is strong enough to consider the case as laboratory confirmed.