Measurement uncertainty in quantifying delta-9-tetrahydrocannabinol (THC) in blood using SPE and LC/MS/MS

A method for the quantitative analysis of delta-9-tetrahydrocannabinol (THC, the main active ingredient of cannabis) in whole blood using solid phase extraction and LC/MS/MS has been developed. A bottom-up approach with method validation data was used to evaluate and estimate the measurement uncertainty (MU) of the analytical method. The sources of uncertainty were identified using a cause and effect diagram. The contribution of each uncertainty component was estimated and were combined to derive the overall uncertainty of the analytical method. The combined uncertainty was estimated to be 0.131 μg/L (<7%). At a 99.7% confidence level, the expanded uncertainty was 0.393 μg/L for a THC concentration of 2 μg/L in a whole blood sample. The calculations not only enable the laboratory to quantify the uncertainty associated with a quantitative result, but can also be used to identify the sources of uncertainty and determine if the analytical method can be improved. An open access Measurement Uncertainty Calculator (MUCalc) software has been developed using the method described in this paper.