A concentration above the highest calibrator creates a separate analytical question: can the laboratory dilute the sample and accurately recover its original concentration? Cui and colleagues tested that step while developing an LC–MS/MS assay for dexmedetomidine, dezocine, and midazolam in rat plasma.
The calibration ranges topped out at 100 ng/mL for dexmedetomidine and dezocine and 200 ng/mL for midazolam. The researchers prepared six replicates containing 500, 500, and 1,000 ng/mL, respectively. They diluted the replicates tenfold with blank rat plasma, bringing the working concentrations to 50, 50, and 100 ng/mL. For all three analytes, reported precision was below 15% relative standard deviation and accuracy fell between 85% and 115% of the theoretical value.
The experiment supports the tenfold dilution under the tested conditions. For a forensic result above the calibration range, ask which matrix and dilution factor the reporting lab validated, whether the diluted reading landed within the calibration range, and how the lab documented its back-calculation. A rat plasma validation supplies evidence for its own matrix. A result in human whole blood needs matrix-specific support.
Cui et al., Journal of Analytical Methods in Chemistry (2018)