1. Blood should be allowed to clot at 37°C for 30 to 60 min. After this time the clot should be solid (see Notes 7 and 8).
2. Leave at 4°C overnight to allow retraction to take place.
3. If necessary, gently detach the clot from the wall of the container using a blunt instrument such as a sealed Pasteur pipet, or a plastic rod, and remove the serum.
4. More serum may be recovered from the clot by centrifugation at 2500g for 30 min or by gently pressing the clot with a flat instrument, although this has to be conducted with care if hemolysis is to be avoided.
5. The serum from steps 3 and 4 can be combined and centrifuged at 2500g to remove any cell and clot debris.
6. The serum should be stored in suitable aliquots at -20°C or at 4°C with the addition of a suitable antibacterial agent such as 0.02% (w/v) sodium azide.
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