Serum storage - (May/31/2011 )

Hello,
I have serum of patients that I want to analyse later by ELISA.
They were first stored at -80°C. Then I thawed them and aliquot them in small tubes that I store at -20°C.
For each assay I will only use a very small quantity (1 or 2µL) and my aliquots are 30µL so I don't want to use a new aliquot each time because I would waste a lot. So my question is how many time can I thaw and freeze again these aliquots without affecting the serum composition?
Thanks for your help.

the composition remains the same however you may degrade your protein of interest. Best suggestion I can give is to do an experiment with one sample and run it through several freeze thaw cycles.