Patent claim negative limitations in the past, describing what an element is not instead of what it is, were generally considered improper, except in unusual cases. However, the current view of the courts is that there is nothing inherently ambiguous or uncertain about a negative limitation. In Santarus, Inc. v. Par Pharm., Inc., the court held that the written description requirement is met for negative claim limitations where the specification simply describes alternatives. Further, alternative elements need not be explicitly described as alternatives, so long as the specification conveys to a person of skill in the art that those elements are, in fact, alternative or optional. Negative limitations are only unsupported where the specification provides no examples or suggestions that such limitations are alternative or optional.
Therefore, negative claim limitations are adequately supported if the specification discloses a reason to exclude the relevant claim limitations, such as disclosing disadvantages of the presence of the excluded limitations. Actual evidence, such as a test example, is not needed. The exclusion in the specification need not rise to the level of a disclaimer. There normally would be no problem that a negative limitation per se would make a claim too broad, indefinite, etc. Under this practice, such descriptions as “metals other than mercury” would presumably be acceptable.
That said, in view of the former antipathy, it is suggested the negative statements still be avoided except where it seems the only way, or by far the clearest way, to state the limitation. For example, if one’s claim covered the combination comprising A+B+C, and the examiner cited A+B+C with a large amount of D, and if it were unobvious to eliminate D, the claim could perhaps be amended to read “comprising A+B+C and absent sufficient D to.. .” Then the claim would cover A+B+C+E, where E is the absence of excess D.
All said, the negative claim limitation must have some basis in the original disclosure, that is, the absence or negative character of the negatively claimed element must have been originally disclosed. The mere absence of a positive recitation of an element is not a basis for its exclusion in a claim.