Is the appearance of flocculent sediment in thawed fetal bovine serum abnormal or normal?

When processing, I prefer low-speed centrifugation rather than 0.22 μm filtration. Centrifugation can be performed at 4°C, approximately 500–1000 × g, for 10 minutes, and the supernatant collected. A 0.22 μm filter will retain macromolecular aggregates, lipoproteins, and possibly some extracellular vesicles, which may affect primary cells; moreover, flocculent material can easily clog the membrane, making filtration more troublesome instead. After centrifugation, mix gently to avoid lipid layering. Ensure that the centrifuge tubes are sterile, and perform the procedure in a biosafety cabinet.

White flocculent precipitate does not necessarily indicate deterioration. When fetal bovine serum is thawed at low temperature, lipoproteins, fibrinogen, or cryoprecipitable proteins tend to precipitate out, and a precipitate that does not disperse upon shaking is also common, especially after standing overnight at 4°C. First check whether the color has noticeably darkened and whether there is any off odor. If the serum itself is clear with only a small amount of flocculent material, it can usually still be used. Since primary macrophages are sensitive, it is not recommended to filter and use it directly; it is best to first take the mixed supernatant for a small-scale adherence or functional test to confirm that there is no difference from the previous lot.

For primary macrophages, batch management deserves more attention than the precipitate itself. It is recommended to thaw the same bottle of serum thoroughly at 4°C before aliquoting, mix gently by inversion to make the components as uniform as possible, then aliquot and record the precipitate condition. If a particular tube already has obvious precipitate, do not use only the supernatant from that tube, otherwise there may be differences between tubes. After aliquoting, perform small-scale comparative testing on primary cells, looking at attachment, morphology, and yield; if there is little difference from the previous batch, it can continue to be used, but if there are obvious changes, use it only for ordinary cell lines.