Hi  – serum zinc is a reflection of what is in the blood which means that it is easily influenced by what someone ate or supplemented with the day or so prior to testing.

There is some debate however in terms of the accuracy between plasma and RBC zinc testing. I still have not come across sufficient studies that utilize large enough sample sizes to prove that plasma is superior to RBC testing, plus it isn’t as readily available, so until that time comes, RBC zinc is what I recommend.

Hope that helps.