[ash2006rf]

I have collected blood, urine and rats tissue samples from rats and I kept them in the fridge for three months, after analysed these samples to determine enzyme  using HPLC I got good results.
My question is I have another study for the same samples after about one year  that mean I have to keep them in the fridge until analysis is there any degradation will happen to these samples?

Thanks in advance 

[JGK]

Sample stabilty should be determined experimentally. this is usually dome by "spiking" blank matrix with the analyte. In bioanalysis usually at at least 3 levels. The samples are analyzed immediately (time Zero) and placed into storage and reanalyzed over time (the duration should exceed the maximum storage period of your real samples.

You should also investigate Freeze/thaw stability to show that stability is not lost when samples are removed from storage.

This is a good guide for successful validation http://www.fda.gov/downloads/Drugs/GuidanceComplianceRegulatoryInformation/Guidances/ucm070107.pdf