Guidelines for Drugs of Abuse Testing vers 2012 - Brenneisen

11.1 Metrology Terms for Verification and Validation of Testing
Procedures
The SCDAT relies on the International Vocabulary of Basic and General Terms in Metrology (VIM
2010), the Guidelines for the Validation of Physicochemical Testing Procedures and for the
Determination of Measurement Uncertainty [GUM 2008] and the Guidelines and
Recommendations of the GTFCh [www.gtfch.ch; Peters 2006].
Generally speaking, instruments used for laboratory analyses must undergo maintenance on a
routine basis and be kept in good working order at all times. The manufacturers’ operating
instructions are to be heeded. In addition, laboratories must guarantee that their analyses are
performed according to the current, recognized state of the art in analytical techniques.
The following parameters are to be understood as quality criteria for the methods and tests
implemented and, as such, serve to document the suitability of analysis procedures for their
intended purpose.
11.1.1 Trueness (VIM 2.14)
Trueness describes the proximity of agreement between the average of a large number of readings
and a reference value. The trueness of the results of the immunochemical methods covered in
these guidelines is influenced by various factors:
• Biological matrix
• Interference (documented as “selectivity” )
• Cross-reactivity (documented as “specificity” )
• Differing reactivity responses as a function of antibody concentration and affinity in the case of
substance-group testing.
11.1.2 Precision (VIM 2.15)
Precision describes the random deviation of the values that are near the mean.. Precision is
usually expressed in terms of “imprecision” and is calculated as a standard deviation of the
readings obtained. High imprecision is expressed as a large standard deviation. A distinction is
made between repeatability, laboratory precision, and comparative precision
Repeatability (in a series) describes the extent of agreement of repeated measurements of one
and the same quantity, performed under the same experimental conditions. It is a measure of the
random error component in a quantitative experiment.
Laboratory precision is obtained from a determination of one and the same sample within a
laboratory, with the intentional change of one parameter (for instance, a person, an instrument,
time of analysis, internal quality control on a day- to-day basis to the next).
Comparative precision describes precision under conditions such that readings are obtained by
different individuals using different instruments, the same method, and identical sample sources in
different laboratories (external quality control).
11.1.3 Accuracy (VIM 2.13)
Accuracy (measurement accuracy) describes the extent of the proximity of a reading to the true
value of a measured quantity. It is qualified by a systematic (trueness) and a random (precision)
error.
11.1.4 Selectivity (VIM 4.123) (Interference)
Selectivity is the capability of a measuring system to discriminate and unequivocally identify
different analytes without interfering with each other or without interference due to other
endogenous or exogenous substances (metabolites, contamination, degradation products, matrix).
11.1.5 Limit of Detection (VIM 4.18)
The limit of detection (LOD) is defined as the lowest concentration of an analyte in a sample for
which the defined probability criteria are met. Neither qualitative nor quantitative results that are
below this concentration are to be reported.
The limit of detection depends on:
• the analyte being sought
• the analysis method used
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