Biomedical Engineering Reference
In-Depth Information
also assist and guide the laboratories on the procedures to be followed before chemical
analysis: from sampling to acceptance of sample, rules to be followed to ensure the
sample traceability.
All guidelines specify that in the case of analysis aimed at medicolegal purposes,
the analytical steps envisaged are basically two: the
screening tests
and the
con fi rmatory tests
.
Screening tests allow to quickly analyze several samples, even by untrained per-
sonnel, in an economic, efficient and standardized way to prevent a negative sam-
ples. These tests identify samples in which the concentration is below or above a
specific threshold value, defined cutoff (a reference value, expressed in concentra-
tion, above which the result of an analytical test is considered positive and below
negative). These methods, therefore, should aim to limit the error of false negative
below a certain threshold.
Confirmatory method means methods that provide full or complementary infor-
mation enabling the substance to be unequivocally identified and if necessary
quantified at the level of interest.
7.2
Validation
Validation involves documenting, through the use of specific laboratory investiga-
tions, that the performance characteristics of the method are suitable and reliable for
the intended analytical applications. The acceptability of analytical data is related to
the criteria used for the validation. Reliable analytical data are a prerequisite for
correct interpretation of toxicological findings in the evaluation of scientific studies,
as well as in daily routine work. Unreliable analytical data might not only be con-
tested in court, but could also lead to unjustified legal consequences for the defen-
dant or to wrong treatment of the patient. Therefore, new analytical methods to be
used in forensic and/or clinical toxicology require careful method development and
thorough validation of the final method.
The fundamental parameters for this validation include accuracy, precision,
selectivity, sensitivity, reproducibility, and stability [
58
]. Other parameters are lim-
its of detection (LODs) and quantification (LOQs), linear dynamic range (LDR)
[
56
]. In the case of LC-MS/MS based procedures, appropriate steps should be taken
to ensure the lack of matrix effects throughout the application of the method, as
outlined by several authors [
55,
60-
64
] .
7.2.1
Accuracy
Accuracy means the closeness of agreement between a test result and the accepted
reference value. It is determined by determining trueness and precision [
57
] . In a
strict sense, the accuracy of a method is affected by systematic (bias) as well as
random (precision) error components, but the term is often used to describe only the
systematic error component, i.e., in the sense of bias. In this sense sometimes
