After five years of continuous, post-registration use, a trademark registration can become “incontestable” under 15 U.S.C. § 1065. An “incontestable” trademark registration is immune from challenge on certain grounds, such as that the mark is merely “descriptive” and lacks secondary meaning. Although the validity of the registration may not be challenged on a “descriptiveness” basis, the mark’s strength (which affects the scope of protection) can be attacked when considering the “likelihood of confusion” prong of a trademark infringement analysis. Earlier this week, the Tenth Circuit examined this very issue when it unanimously affirmed a lower court’s summary judgment of non-infringement. In Water Pick, Inc. v. Med-Systems, Inc., the Court analyzed the use of similar marks used by competitors in the nasal irrigation market. The senior user, Med-Systems, owned several incontestable federal registrations for SINUCLEANSE. After negotiations broke down to buy Med-Systems, Water Pik developed its own nasal-irrigation line under the name SINUSENSE. The Tenth Circuit provided a nice analysis of the problems associated with Med-Systems’ expert’s survey. However, the more interesting issue (which should serve as a reminder to all litigators) was how the Court analyzed the conceptual strength of the senior SINUCLEANSE mark as part of its evaluation of a “likelihood of confusion.” A particularly strong mark can give rise to a “likelihood of confusion” while a weak or “descriptive” mark suggests that use of a similar, junior mark even on similar goods is unlikely to cause confusion. Determining whether a mark is descriptive and hence, weak, can be difficult. The Tenth Circuit set forth it’s test as “’[d]escriptive terms are those which directly convey to the buyer the ingredients, qualities, or characteristics of the product.’” In the end, the Tenth Circuit found the mark to be “descriptive” of the product’s characteristics or qualities as “Sinu” is a widely used prefix by third parties, including several sinus-irrigation products, and “cleanse” communicates the “nature and purpose of Med-Systems’ products….” The lesson to be learned is that even if a trademark registration is “incontestable,” in certain Circuits (like the Tenth), it may be possible to defend against a charge of infringement by arguing that the senior mark is “descriptive” as part of the “likelihood of confusion” analysis. Often, this is an overlooked, possible defense.