Oops, Sorry Malcolm, I should have explained more in details. Few months ago, I prepared 0.2 M DTT using DTT powder and dissolved them in buffered saline of pH 7.3-7.4. I read this from the Red Cross Immunohematolgy methods and procedure book. After preparations, I treated three cell screen with the 0.2M DTT and ran the cells against anti-k antisera. It clearly showed that the k antigens have been denatured as the reactions were negative with anti-k. However, when I tested the treated cells with a known patient with anti-k, the reactions were positive ( the reaction did decreased from 3 to 1+-weak positive). So last week, I thought well maybe the pH of the buffered saline wasn't right, so I experimented by modifying the pH of the saline with phosphate buffer. Unfortunately, it still didn't work at all, worst were no reductions of reactivity and anti-k control came out positive. Any thoughts? Emergency room