Proper diagnosis cde to use for Titer

When we send a titer to the lab, we normally use a screening diagnosis because we don't know the result, and there is not a symptom/problem code that fits. Most of the time, insurance denies. One recent patient who had his titer denied called his insurance company and was told that there is an immunity code. I did some research, and found code 279.3 - Immunity Deficiency. Would this instance be proper use of this diagnosis code?
Thank you.