Hi - I also had a rash from Keytruda on my chest. By the third infusion I started noticing I was getting colitis - mucus with stools which then over a series of days turned into some bad diarrhea which I ended up having for weeks. This is just my story by the way. I went to the hospital and stayed for five weeks - losing over 30 pounds as I could hardly eat my stomach and intestines were so inflamed. But I was one of those people who developed a bad reaction to the drug. Even though I went through all that - the cancer was killed and it is now five years later. It was worth it to me for such a great outcome. I believe the Keytruda killed off anything else that was brewing in my body. Does that help - it was my experience. That you are experiencing the rash - that alone tells me the Keytruda is working.