Method Verification - AOAC · PDF fileISO Technical Specification ISO/TS 21748, Guidance for...

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Transcript of Method Verification - AOAC · PDF fileISO Technical Specification ISO/TS 21748, Guidance for...

  • Method Verification

    The HowM.L. Jane Weitzel ALACC Chair

  • The What

    Released early 2008

    http://www.aoac.org/alacc_guide_2008.pdf

  • Meet ISO 17025 Requirement

  • Categories of Methods

    The six categories of chemical analytical methods are:

    1. Confirmation of Identity, a method that ensures a material is what it purports to be or confirms the detection of the target analyte.

    2. Quantifying an analyte at a low concentration. 3. Determining if an analyte is present above or

    below a specified, low concentration (often called a Limit Test). The specified concentration is close to the LOQ.

  • Categories of Methods

    4. Quantifying an analyte at a high concentration.

    5. Determining if an analyte is present above or below a specified, high concentration (often called a Limit Test). The specified concentration is substantially above the LOQ.

    6. Qualitative test.

  • Tables 2 -6

    Requirements of Method Verification for the Six Categories of Chemical Test Methods (Tables 26)

  • Category 1: Confirmation of Identity

  • ISO Technical Specification ISO/TS 21748, Guidance forthe use of repeatability, reproducibility and trueness estimates in measurement uncertainty estimation

  • ISO/TS 21748Template for Verification ISO 21748 is a thorough guide to verifying

    a method. The examples will be based on ISO 21748

  • Specification for the Method

    The results of collaborative study yield performance indicators (sR, sr) and, in some circumstances, a method bias estimate, which form a specification for the method performance.

    In adopting the method for its specified purpose, a laboratory is normally expected to demonstrate that it is meeting this specification.

  • Repeatability

    In most cases, this is achieved by studies intended to verify control of repeatability and of the laboratory component of bias, and by continued performance checks (quality control and assurance)

  • Many added benefits

    Method verification Estimate of Uncertainty Better understanding of the method

  • Equation

    y= + +B+ cixi +e

  • Equation Components

    y = + + + e + cixi

    Result = ideal result +method

    bias +lab bias +

    repeatability +

    sum of effects NOT included in collabora-

    tive study

  • What if your method was not collaboratively studied?

    The approach described by ISO 21748 is still relevant. Intermediate precision could be substituted for

    reproducibility Fewer variables would be included in intermediate

    precision study than during a reproducibility study There would be more effects included in cixi The approach provides a detailed, organized procedure for

    identifying and evaluating these effects

  • Method Bias

    method bias

    Collaborative StudyNote: the study may not include this

    component

  • Lab Bias

    lab bias

    Collaborative StudysL2

    sR2 = sL2 +sr2Thus

    sL2 = sR2 - sr2

  • Repeatability

    e

    repeatability

    Collaborative Studysr2

  • Effects NOT included cixi

    sum of effects NOT included in collaborative study

    Examine the equation and procedure.

  • Table Summary

    y e cixi

    Result Ideal result Method bias Lab bias Repeatability

    Sum of effects not included

    in collaborative

    study

    Source of Specification

    for the method

    Collaborative Study. Note:

    the study may not include

    this component

    CollaborativeStudysL2

    sR2 = sL2 +sr2

    ThussL2 = sR2 - sr2

    CollaborativeStudy

    sr2

    Examine the equation and

    procedure

  • Method Bias

    method bias

    Collaborative StudyNote: the study may not include this component.

    The study may include method bias, if for example the results are corrected for a known method bias. The procedure for assessing method bias is included in

    the ISO standard in detail in Incorporating Trueness Data.(Uncertainty associated with CRV becomes important)

  • Repeatability - Equation

    erepeatability

    Collaborative Study sr2

    Do repeatability study in lab. Calculate sw.Compare to sr2 using the F test.

  • Bias

    Often accuracy must be verifiedTerminology bias

    Can use same data from repeatability If Certified Reference Material is available

  • Lab Bias

    lab bias

    Collaborative StudysL2

    sR2 = sL2 +sr2Thus

    sL2 = sR2 - sr2

  • Bias Specification

    Follows ISO Guide 33 = m - = mean - CRV

    || < 2D

    || < 2 (sR2 - sr2 + sw2/n)sL2 = sR2 - sr2

  • n

    Choose n so that the uncertainty of the bias is not significant

    choose n such that the uncertainty sw2/n < 0.2sR

    Good rule of thumb uncertainties less than 0,2 sR lead to changes of

    under 0,02 sR in the overall uncertainty estimate.

  • cixi Look at equation to identify any source of

    uncertainty that was not included in the collaborative study.

    It is often convenient to consider each of the three factors the sample, the laboratory and the method when identifying gross uncertainties

  • Specificity

    If your sample is not identical to those included in the collaborative study or method validation, you must assess the impact of the differences.

    AOAC Food triangle can be useful

  • Useful Table 8 in ALACC Method Verification Guide Lists Parameter The difference from the validated method Required Equivalence Study

  • Useful Table 8

  • Table 8 Example

  • Uncertainty

    For a collaboratively studied method and if you follow the process from ISO 21748, you will also end up with an estimate of the uncertainty for the method.

  • Table Summary

    y e cixi

    Result Ideal result Method bias Lab bias Repeatability

    Sum of effects not included

    in collaborative

    study

    Source of Specification

    for the method

    Collaborative Study. Note:

    the study may not include

    this component

    CollaborativeStudysL2

    sR2 = sL2 +sr2

    ThussL2 = sR2 - sr2

    CollaborativeStudy

    sr2

    Examine the equation and

    procedure

  • Examples

    Method VerificationThe WhatMeet ISO 17025 RequirementCategories of MethodsCategories of MethodsTables 2 -6Category 1: Confirmation of IdentitySlide Number 8ISO Technical Specification ISO/TS 21748, Guidance forthe use of repeatability, reproducibility and trueness estimates in measurement uncertainty estimationISO/TS 21748Template for VerificationSpecification for the MethodRepeatabilityMany added benefitsEquationEquation ComponentsWhat if your method was not collaboratively studied?Method BiasLab BiasRepeatabilityEffects NOT includedTable SummaryMethod BiasRepeatability - EquationBiasLab BiasBias Specificationn cixiSpecificityUseful Table 8 in ALACC Method Verification GuideUseful Table 8Table 8 ExampleUncertaintyTable SummaryExamples