Regarding false negatives, there are two main reasons for our system to miss a biomedical concept: on the one hand, it can happen that the concept is not captured in the Metathesaurus at all; on the other hand, it could be that the concept is captured but not as expressed in the text, be it because it is misspelled, abbreviated in a way that the Metathesaurus does not contemplate, or formulated in any other non-standard way.
false negatives are caused by a poor lack of the Metathesaurus and the lexical variability of clinical narrative.