In automated chemistry, what is the difference between primary and secondary dilution and when is each used?

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The idea being tested is how dilutions are used to get results into the instrument’s reliable measurement range. The initial step, called the primary dilution, lowers the sample concentration so the assay signal falls within its validated dynamic range and can be measured accurately. If this first dilution still doesn’t put the analyte within range or if the sample has interfering matrix effects or is extremely concentrated, a second dilution—the secondary dilution—is performed to further reduce the concentration. These dilutions are typically validated to ensure accuracy, and the final result is back-calculated using the known dilution factors. Other options get it wrong by mislabeling which dilution is first, or by suggesting dilutions are only for low concentrations, or by not tying the secondary dilution to high concentrations or interference.