How to unify the operation of BCA assay kits with different incubation temperatures and times, resulting in significant differences in OD values?

Membrane proteins and secreted proteins differ in supernatant composition, so a single fixed condition cannot be used without validation. Take 3–5 representative samples each and test them under room temperature for 2 hours and 37°C for 30 minutes, respectively, then compare the calculated concentrations and recovery rates. If the deviation between the two conditions exceeds 15%, choose the condition with stronger interference resistance and a recovery rate closer to 100%, and complete all samples within the same system. If membrane protein samples have turbidity or precipitation, centrifuge first and take the supernatant before measurement. Insufficient room temperature incubation time often has a greater impact than slight temperature fluctuations, because when the reaction is incomplete, OD continues to rise over time.

It is recommended to incubate at room temperature for 2 hours on a daily basis. Although 37 ℃ is fast, after adding a large amount of samples, there may be temperature gradients in the orifice plate of the water bath or oven, and the temperature difference between the edge and the middle hole may not be small. The time difference between reading the plate before and after may also introduce errors. Incubate at room temperature for 2 hours, approaching the reaction plateau period, insensitive to time fluctuations, suitable for large quantities of samples. The room temperature can be extended to 2.5 hours below 22 ℃, and slightly shortened above 26 ℃, but the same batch of samples and standard curves must be read at the same temperature and time point. Don't read in advance just because the colors look similar, as OD is unstable before BCA reaches the finish line.

BCA color development is a temperature- and time-dependent copper reduction reaction. If readings are taken at room temperature before 2 hours, especially under 1.5 hours, many wells have not yet reached endpoint, resulting in lower OD values, with greater deviation at high-concentration points. The slope of the standard curve also changes with the degree of endpoint completion. If samples and standards are not timed simultaneously on the same plate, calculated concentrations will drift. If time is limited, 37°C for 30 minutes can be used, but plate sealing film, strict temperature control, and simultaneous transfer and timing of all wells are required. Do not mix standard curves generated at 37°C and room temperature, as the difference in slope between the two is normal and not a reagent issue.

Unified SOP is more important than struggling with which conditions are superior. The working solution should be prepared and used immediately. Timing should start after adding the last well, and the sample addition sequence should be consistent with the plate reading sequence to avoid a significant difference in actual incubation time between the first and last wells. A large number of samples can be divided into batches, with each batch having its own standard curve. Do not mix different batches of standard curves. Cover the orifice plate and avoid light, centrifuge at low speed or tap lightly to remove bubbles before reading. For membrane protein extract and secretion protein supernatant, it is recommended to make matching standards or standard additions for each type of substrate, as detergents, salts, and lipids have different degrees of color interference.