10 days after starting meds for the first time (Abacavir+Sustiva) I developed a slightly raised rash which spread across my entire body. No itching, blistering or pain. I also experienced mild swelling around the lips & mild inflammation of mucous membranes in the mouth and throat.

My doctor was unable to say which drug had caused the rash, and switched me to Truvada+Kaletra.

I found it frustrating to have two treatment options wiped out in one go. I am having a hard time with the Kaletra, and would like to go back to Sustiva.

Is there a safe way to test to see whether Abacavir or Sustiva was the cause of my problem?

Good question. I generally don't prescribe abacavir until I have the results of a HLA B5701 test available. I don't prescribe it if the patient is HLA B5701 positive. If negative a true hypersensitivity reaction is very unlikely but remotely still possible. Once you are doing well on your current regimen (undetectable viral levels) you could consider adding Sustiva for several months (dose adjustment of Kaletra often necessary) to see if you develop a rash. If no rash then could perhaps switch back to a Sustiva based regimen. KH