Are deposits / precipitates in serum harmful?

On occasion, turbidity and flocculent deposits may be present after prolonged freezing / refrigeration of serum. These deposits have no affect on the biological performance of the serum and are cosmetic.

Multiple freeze-thaw cycles increase the chance for precipitates being present in the serum. Fibrinogen is the clot-forming protein (Fibrin) and appear as white flakes. Fibrin could present itself after thawing or heat inactivating the serum. Fibrin does not alter the performance of the serum and poses no threat to cultures. Commonly, Fibrin is mistaken for mycoplasmas contaminations in lab settings and is not harmful.