In medical school, Patterson shared three or four classes with Leonard McCoy (whom she called "Lenny"). She was brilliant and always correct about diagnosis, but her bedside manner was bad enough that McCoy recommended that she go into surgery where her people skills would be less important. A few years later Patterson wrote to McCoy saying that she had moved into private research and was much happier for it.
On Stardate 2231.2, Patterson published the article "Searching for Answers: Hope for Humans with Blood-Based Hyperplasia" in Starfleet medical journal issue 421. This article dealt with xenopolycythemia along with several other diseases.
In 2268, Doctor McCoy discovered that he had xenopolycythemia after performing a routine physical on himself. He remembered Patterson's article and attempted to track her down to see if she knew of a cure. The computer was able to tell him that two years earlier, Patterson traveled to from Earth to Altair VI. At McCoy's request, Pavel Chekov was able to discover that after taking six months off, Patterson took a job with a medical chemical company in the Tellan system, which was a division of the Carter Winston Group. Then in 2268 she had suddenly resigned and along with several other top research scientists, had traveled to Deep Space Station R-5 and then traveled on the USS Sphinx to the Lantaru sector and simply vanished.
Shortly after this, McCoy encountered Karen Patterson when the Enterprise visited a secret cloaked science station in the Lantaru sector. The scientists there were working on a project run by Bendes Ketteract (and Section 31) to construct and harness Omega molecules. When McCoy told her about his disease, Patterson replied that she had been thinking about the problem on and off for years, but there was no known cure. She was willing to leave the station and work on the problem while McCoy was ill, but she was killed a short time later when the station exploded immediately after they succeeded in building the molecules. (TOS - Star Trek: Section 31 novel: Cloak)