There are several ways to amend an issued patent. These procedures are helpful in case new prior art is discovered or a mistake is made.
The most straightforward procedure is the correction of a clerical error. The patentee simply writes a letter to the Patent Office requesting the correction and explaining the origin of the mistake. The corrected patent is then read as though it was always in the revised form. The correction of clerical errors may be less likely to be permitted if it would negatively affect the rights of others by, for example, broadening the claim scope.
Disclaimer is a procedure allowing a patentee to narrow the claims of the patent by disclaiming particular subject matter. A patentee may disclaim any subject matter that was included by mistake, accident or inadvertence without any willful intent to mislead or defraud the public. There is no time limit on disclaimer. For example, a patentee may disclaim subject matter if it is determined that the claims are overly broad. A patentee can disclaim a whole claim or only a portion of a claim. Generally, a disclaimer will not affect court proceedings initiated before the filing of the disclaimer.
Re-issue is the only method to obtain broadening amendments for an issued patent. A patent may be reissued if a patentee claimed more or less than s/he had a right to claim as new, and (i) the error arose from inadvertence or accident; (ii) the error was without deceptive intention, and (iii) the application for re-issue is filed within four years of the date of patent issuance. For a broadening amendment to be valid, the claims must be directed to what the patentee was attempting to protect in the original application. Further, the broadening amendments must be reasonably inferable from the original specification.
Unlike the other methods for amending issued patents discussed above, which are only available to the patentee, either a third party or the patentee may request re-examination to narrow the patent scope. The requesting party must provide prior art, explain its pertinency and how it raises a substantial new question of patentability of a claim. The possible consequences of re-examination include (i) upholding of the claim in question as patentable; (ii) cancellation of the claim as invalid; or (iii) amendment of the claim by the patentee to be patentable over the prior art.
The post-issuance amendments to a patent discussed above can be significant tools for accurately protecting the invention, particularly if new prior art is discovered, and should be kept in mind during the entire term of a patent.