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ISO 28218:2010 — Performance Criteria for Radiobioassay in Radiation Protection
Radiation protection — Performance criteria for radiobioassay
Introduction to ISO 28218:2010
ISO 28218:2010 provides comprehensive performance criteria for radiobioassay service laboratories that measure internal contamination from radioactive materials. Developed by ISO/TC 85 (Nuclear energy, nuclear technologies, and radiological protection), this standard supersedes ISO 12790-1:2001 and complements ISO 20553. It establishes a consensus framework for statistical definitions including decision threshold, detection limit, relative bias, and repeatability — following the principles of ISO 11929 — and replaces the older minimum detectable amount (MDA) concept with the more rigorous detection limit (y#).
For radiation protection professionals, ISO 28218 is the essential bridge between raw measurement data and defensible dose assessments. It standardizes how “detectable” is defined — a critical legal and safety foundation for internal dosimetry programmes.
Statistical Framework: Decision Threshold and Detection Limit
The standard introduces rigorous statistical criteria for quantifying radioactivity in biological samples. The decision threshold (y*) defines the critical value above which a measurement is considered to indicate the presence of radioactive material (controlling Type I error). The detection limit (y#) represents the smallest true activity that can be reliably detected (controlling both Type I and Type II errors). These concepts are fundamental for distinguishing genuine contamination from background fluctuations. Performance criteria for relative bias and repeatability ensure measurement accuracy and precision are maintained within acceptable bounds.
| Performance Measure | Symbol | Definition | Typical Acceptance Criterion |
|---|---|---|---|
| Decision threshold | y* | Critical value for deciding presence of activity | α = 0.05 (Type I error) |
| Detection limit | y# | Smallest detectable true activity value | β = 0.05 (Type II error) |
| Relative bias | Brel | Systematic error relative to expected value | ≤ ±20% for most nuclides |
| Repeatability | r | Relative standard deviation of replicate measurements | ≤ 10% at detection limit level |
| Minimum testing level | MTL | Minimum activity for performance testing participation | Defined per radionuclide in Clause 9 |
In Vivo and In Vitro Radiobioassay Requirements
ISO 28218 provides separate detailed clauses for in vivo (whole-body or organ counting) and in vitro (urine, faecal, or biological sample analysis) radiobioassay. For in vivo measurements, requirements cover radionuclide identification, quantification accuracy, phantom calibration, and reporting formats. For in vitro analysis, the standard specifies analytical methodology validation, detection limit achievement, sample traceability, and quality control procedures. Both sections define responsibilities for the customer (e.g., providing accurate exposure history and measurement objectives) and the service laboratory (maintaining calibrated instrumentation and validated methods).
The customer’s responsibilities under ISO 28218 are often underestimated. Incomplete exposure history or incorrect measurement objectives can render otherwise excellent laboratory data unsuitable for dose assessment. Always document the purpose of the bioassay programme before sample collection.
Quality Assurance and Performance Testing
Clause 8 establishes comprehensive quality assurance requirements including documented QA plans, control charting, certified reference materials (CRMs), traceability chains, and inter-laboratory comparisons. Clause 9 defines performance testing programmes with specific minimum testing levels (MTLs) for both in vivo and in vitro methods. The standard provides detailed protocols for blind testing, evaluation criteria, and corrective actions when performance criteria are not met. Annexes A and B offer practical guidance on detection limit models and application examples for different measurement scenarios.
Implementing the full QA framework of ISO 28218 — including regular inter-laboratory comparisons — is the most effective way to demonstrate regulatory compliance and defend bioassay data in legal or occupational health contexts.
Engineering Design Insights
From a laboratory design perspective, ISO 28218 requires careful consideration of detector selection (HPGe vs. NaI for in vivo; alpha spectrometry vs. liquid scintillation for in vitro), shielding configuration to achieve required detection limits, sample preparation facilities, and data management systems that support traceability and chain-of-custody documentation. The replacement of MDA with the decision threshold/detection limit framework represents a fundamental improvement in statistical rigour — laboratories transitioning from older standards must update their uncertainty calculation algorithms accordingly.
Q1: What is the difference between decision threshold and detection limit?
A: The decision threshold (y*) is the value above which a measurement is considered to indicate real activity (controlling false positives). The detection limit (y#) is the smallest true activity that can be reliably detected (controlling both false positives and false negatives).
A: The decision threshold (y*) is the value above which a measurement is considered to indicate real activity (controlling false positives). The detection limit (y#) is the smallest true activity that can be reliably detected (controlling both false positives and false negatives).
Q2: Which radionuclides are covered by the performance testing programme?
A: The standard covers common fission and activation products relevant to occupational exposure, including but not limited to H-3, C-14, Sr-90, Cs-137, Pu-239, and natural uranium/thorium series nuclides.
A: The standard covers common fission and activation products relevant to occupational exposure, including but not limited to H-3, C-14, Sr-90, Cs-137, Pu-239, and natural uranium/thorium series nuclides.
Q3: How should a laboratory validate its detection limit claims?
A: By performing replicate measurements of blank and spiked samples at levels near the claimed detection limit, then calculating the decision threshold and detection limit per the statistical formulas in Clause 5.
A: By performing replicate measurements of blank and spiked samples at levels near the claimed detection limit, then calculating the decision threshold and detection limit per the statistical formulas in Clause 5.
Q4: Does ISO 28218 apply to environmental radioactivity measurements?
A: It is specifically intended for occupational radiobioassay (internal exposure monitoring). Environmental monitoring follows different standards, though some statistical concepts may be transferable.
A: It is specifically intended for occupational radiobioassay (internal exposure monitoring). Environmental monitoring follows different standards, though some statistical concepts may be transferable.
