Stability of vitamin A in frozen sera.

We compared values for vitamin A measured in fresh sera with values obtained after storage at -20 degrees C for five to eight years. Loss of vitamin A from long-stored sera during the extraction step was eliminated by adding ascorbic acid to the extraction solvent, ethanol. Retinol-binding protein was also measured in the stored sera. Not only did vitamin A values remain stable during the years of storage, but also the correlation between concentrations of vitamin A and retinol-binding protein in the stored sera was typical of that observed with fresh sera.