In our laboratory we have tissue pieces that secrete a certain protein. After we subject them to treatment we measure the secreted protein again. The number of pieces is usually 4 or 5 in a group.

Up until now the data was analyzed by calculating the percent of difference; if following the treatment there's more than 15% reduction (average for the group) in the level of protein - the treatment is rejected.

I believe that using a paired t-test is a better a way to look at the data, but a I do not know how to translate the 15% into p-value or any other statistical threshold.

Any advice will be greatly appreciated.