What should I do if the total CV of duplicate wells exceeds 15% when measuring cell supernatant with TNF-α ELISA?

Focus on troubleshooting plate washing and sample addition. Inconsistent residual liquid volumes in the plate washer can amplify color development differences in low-concentration wells; for manual plate washing, standardize the blotting method, force, and paper padding. For sample addition, multi-channel pipettes are recommended; bring reagents to room temperature in advance and mix well to avoid bubbles; do not place replicate wells side by side and close together—preferably arrange them in a staggered layout to reduce edge effects caused by differences in sample addition time. Samples and standards should be on the same plate and run in the same batch; do not add them in separate batches. Follow the instructions for the number of plate washes and do not increase them, otherwise low-value wells may become more unstable.

Mature imported pre-coated plates such as those from R&D Systems, Thermo/Invitrogen, and BioLegend typically have intra-assay CVs stated in their instructions as mostly within 10%, commonly around 5–8%. However, those are manufacturer QC conditions; when applied to your samples and procedures, you still need to validate them yourself and cannot directly assume they will necessarily stay under 10%. For 80 samples, it is recommended to buy detachable strip plates and detach them as needed to reduce waste. Before ordering, confirm whether they are pre-coated and whether they are suitable for cell culture supernatants; some kits have only been optimized for serum/plasma and may show matrix differences with supernatants. You can first buy a small-size trial pack for comparison.

Switching to an imported kit will not automatically solve the CV issue. It is recommended to perform validation upon arrival: select low, medium, and high concentration supernatants, set 3-4 replicate wells for each, and calculate intra-assay CV and dilution linearity; if conditions permit, perform spike recovery to assess matrix effects. If the CV of low-concentration wells remains high, the problem is most likely in the operation or dilution strategy rather than the kit itself. Before running 80 samples, aliquot uniformly and avoid repeated freeze-thaw cycles; it is recommended to centrifuge before freezing to remove cell debris. Record the plate washing, sample addition, and plate tapping methods in the operation log to facilitate troubleshooting of CV abnormalities.

A single freeze-thaw cycle usually does not directly cause such a large well-to-well CV in TNF-α quantification from cell supernatant; it is more often due to poor operational consistency. After freeze-thawing, samples must be centrifuged to remove cell debris and precipitate, and care should be taken to avoid aspirating particles when collecting the supernatant. A common cause of high CV in low-concentration wells is that the sample concentration is close to the lower limit of quantification of the kit. It is recommended to first perform a dilution pre-experiment and adjust the sample to the middle range of the standard curve before measurement. It is best to run 3 replicate wells and take the median or mean, which can reduce the impact of a single outlier well. A satisfactory standard curve does not mean the operation is problem-free; the sample-side pipetting and dilution steps are often more critical.