Can fetal bovine serum still be used if there are white flocculent precipitates after thawing

The previous batches did not show this issue, indicating either vial-to-vial variability or fluctuations during transport and storage. Keep records of the lot number, arrival temperature, and photographs. When contacting the supplier, explain the thawing method and the appearance of the precipitate, and request the endotoxin, total protein, hemoglobin, and other specifications from the COA for that lot. If the specifications show no significant deviation and small-scale cell tests show no obvious differences, the product can be aliquoted and used. However, it is recommended to centrifuge at low speed or let it stand to collect the supernatant before each use, avoid repeated freeze-thaw cycles, and note the condition of this vial in the experimental records.

Whether filtration affects the cells depends on the type of precipitate. It is not recommended to directly use a 0.22 μm filter, as flocculent material can easily clog the membrane, causing slow filtration or loss of negative pressure. You can let it stand overnight at 4°C and then collect the supernatant, or perform sterile low-speed centrifugation and then aliquot the supernatant. A 0.22 μm pore size mainly retains bacteria and cannot retain mycoplasma; filtration cannot replace sterility assessment. HepG2 and RAW264.7 are relatively sensitive to serum lot, so it is recommended to perform a 3–5 day growth curve comparison to see whether the doubling time and morphology are close to those of the previous same lot.

Don't rush to discard it, and don't filter it directly either. If the supernatant is basically clear after the white flocculent material settles, it is more likely fibrinogen or lipoprotein precipitation rather than typical contamination; contamination is often accompanied by uniform turbidity, yellowing of color, or a rapid drop in pH. You can take a small amount of the flocculent material and inoculate it onto standard bacterial culture medium or perform a smear microscopy, while also taking about 10 mL and centrifuging it (500×g, 5 minutes) to observe the sediment characteristics. Then use the supernatant for small-scale test culture with HepG2 and RAW264.7 cells, observing attachment, proliferation, and morphology. If there are no obvious abnormalities after 3-4 days of test culture, this bottle can continue to be used.