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Technical Brochure: Turbidity of Serum

Serum turbidity is usually caused by the cryoprecipitation of lipid components during freezing and thawing the serum. The more times the serum is subjected to freeze/thaw cycles, the more turbidity is noticed. Shipping the serum frozen on dry ice at approx (-60) deg C and subsequently storing frozen at (-20) deg C is thermodynamically equivalent to a freeze thaw, even if the product is not allowed to liquefy. To the matter of filterability, this attribute is affected by many variables. Total protein of the serum, lipid levels (cholesterol and triglycerides), freeze thaw cycles due to shipment of the product numerous times on dry ice, and product age from date of manufacture, all contribute to precipitation of various serum components, primarily lipids. These precipitates impact filterability of the product in the hands of the user.

Serum turbidity can be minimized by refraining from repeated freezing and thawing. If serum is not being used all at once, we suggest aliquotting into usable volumes, freezing at (-20) deg C and thawing the aliquots individually at the time of use.

In certain cases, secondary filtration of the serum is desired. However, membrane filters normally used to filter aqueous media may not be adequate to filter protein containing solutions.

Some reliable filters that Valley Biomedical would like to suggest to their customers for secondary filtering are the Whatman® Polycap™ 35 AS and the Nalgene® MF75. Both of these filters are relatively inexpensive and easily available through the Fisher Scientific Catalog.

The Whatman® Polycap™ 35 AS is sterile and nonpyrogenic. This filter has a low-extractable hydrophilic nylon membrane and a glass microfiber prefilter. This USP Class VI filter has a pore size of 0.2 Ám and a filtration area of 400 cm2. The Whatman® Polycap™ can be found in the Fisher Scientific Catalog, #09-744-61.

The MF75 Filter made by Nalgene® offers a polyethersulfone membrane with a fast flowrate, lowest protein binding, and low extractables. The 500 mL capacity filter comes with 0.2 Ám pore size and 90 mm membrane. Fisher Scientific Catalog # 09-741-05.

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