Blood collected into glass or plastic containers will clot rapidly. However, the maximum amount of serum is obtained by incubating at 37°C for an hour, and then leaving the blood at 4°C overnight to allow the clot to retract. The serum is recovered by centrifugation (1000 g for 10 min). It is important to be sure that there are no red cells present in the serum because freezing of red cells will cause haemolysis.
Serum may be stored frozen at -20°C or below. Alternatively, it may be kept at 4°C as a slurry in 50% saturated ammonium sulfate, or may be lyophilized and stored at 4°C (see Section 9.7).
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