In almost every instance, the serum is NOT contaminated. Occasionally thawed serum will appear cloudy. It is usually the result of normal serum components. Fibrin (a non-globular protein involved in the clotting of blood) is most common. Fibrin appears in small quantities as a fine flake or may occur as a lump of a semi-solid substance. The presence of this material does not alter the serum’s performance and may be mistaken for microbial contamination. If the turbidity is a concern, it can be removed by filtration through a 0.45µm filter. However, the sediment may clog many lab-scale filters. If necessary, allow the serum to stand. The sediments will eventually fall to the bottom of the bottle and the serum can be decanted.