ISO 11843-5:2008

Overview

ISO 11843-5:2008 - "Capability of detection - Part 5: Methodology in the linear and non-linear calibration cases" provides statistical methods for assessing detection capability when calibration functions are either linear or non-linear. The standard explains how to build a precision profile (SD or CV as a function of the net state variable), transform precision from the observable response variable to the measurand (net state variable) using the calibration function, and estimate the critical value and minimum detectable value (MDV) of the net state variable.

Key topics and technical requirements

• Precision profile construction: describes how to model the standard deviation (σ) or coefficient of variation (ρ) of the response variable Y as a function of the net state variable X.
• Transformation using calibration function: conversion of uncertainty from the response variable to the net state variable using the derivative of the calibration function (σ_X(X) = σ_Y(X) / |dY/dX|).
• Critical value and MDV definitions: critical value x_c = k · σ_X(0) and minimum detectable value x_d = x_c + k · σ_X(x_d). The coefficient k is chosen to meet specified error probabilities (α and β); for normally distributed variables k ≈ 1.65 corresponds to α = β = 5%.
• Calibration function constraints: calibration must be continuous, differentiable and monotonic (increasing or decreasing).
• Linear and non-linear cases: extends concepts from ISO 11843-2 to non-linear calibration; retains compatibility with linear methodology when applicable.
• Practical computation methods: includes differential methods, propagation of uncertainty, and model fitting procedures. Examples are provided in the standard.

Applications

ISO 11843-5 is suited to situations where detection performance must be quantified but the calibration relationship is non-linear or where ISO 11843-2 is not applicable. Typical applications:

• Environmental analysis of persistent organic pollutants (POPs) (e.g., dioxins, pesticides) using competitive ELISA.
• Biological/clinical assays (e.g., hormone-like chemicals, bacterial endotoxin tests).
• Analytical laboratories validating limit of detection and measurement capability for instruments with non-linear response.
• Regulatory and quality assurance contexts where objective, statistically based detection limits are required.

Who should use this standard

• Analytical chemists and laboratory managers
• Environmental and clinical testing laboratories
• Metrology and QA specialists
• Instrument manufacturers and validation engineers
• Statisticians working on measurement uncertainty and detection capability

Related standards

• ISO 11843-1:1997 - Terms and definitions
• ISO 11843-2:2000 - Linear calibration methodology
• ISO 11843-3, ISO 11843-4 - Other detection capability methods
• ISO 3534 (statistics vocabularies) and ISO 5725-1 (accuracy principles)

Keywords: ISO 11843-5, capability of detection, minimum detectable value, critical value, precision profile, non-linear calibration, limit of detection, ELISA, measurement uncertainty, calibration function.