SI Units and Laboratory Accreditation:

What Regulatory Bodies Expect

Article By Industries Needs.

In testing and calibration, precision is not a luxury; it is the currency of technical credibility. Whether a laboratory measures trace chemical residues in food, validates the dimensions of aerospace components, or analyzes diagnostic blood samples, its outputs carry economic, regulatory, and life-safety implications.

When regulatory bodies—such as the U.S. Food and Drug Administration (FDA), Environmental Protection Agency (EPA), European Medicines Agency (EMA), and National Metrology Institutes (NMIs)—assess a laboratory, they look beyond operational efficiency. They demand evidence that measurement results are scientifically valid, internationally comparable, and legally defensible.

At the core of this assurance lie two interconnected frameworks: the International System of Units (SI) and laboratory accreditation (primarily through standards like ISO/IEC 17025 and ISO 15189). Together, they establish metrological traceability—the unbroken, documented chain of calibrations linking a routine bench measurement to the fundamental physics of the universe.

1. The Foundation: The SI System and Metrological Traceability

The Revised SI: Constant Physics as the Reference

In 2019, the International System of Units underwent a historic revision. The International Bureau of Weights and Measures (BIPM) redefined the seven base units—the meter (m), kilogram (kg), second (s), ampere (A), kelvin (K), mole (mol), and candela (cd)—tying them directly to fundamental physical constants (such as Planck's constant $h$, the elementary charge $e$, and the Boltzmann constant $k$) rather than physical artifacts like the International Prototype Kilogram.

For accredited laboratories, this conceptual shift reinforces a practical mandate: every reported numerical value must be expressed in SI units or recognized non-SI units accepted for use with the SI (such as liters, hours, or electronvolts).

What is Metrological Traceability?

As defined by the International Vocabulary of Metrology (VIM), metrological traceability is the property of a measurement result whereby the result can be related to a reference through a documented, unbroken chain of calibrations, each contributing to the measurement uncertainty.

Regulatory bodies expect every measurement link in this chain to satisfy six specific criteria:

• An unbroken chain of comparisons: Originating at the primary SI reference level and terminating at the working laboratory instrument.
  
  
• Documented measurement uncertainty: Calculated at each level according to international consensus models.
  
  
• A documented procedure: Executed by competent staff using validated methods.
  
  
• Technical competence: Demonstrated through accreditation to ISO/IEC 17025 or ISO 15189.
  
  
• SI reference points: Explicit linkage back to National Metrology Institutes (e.g., NIST in the US, PTB in Germany, NPL in the UK).
  
  
• Calibration intervals: Periodically reviewed and adjusted based on drift and instrument usage.
  
  

2. The Regulatory Framework: International Standards

Regulatory bodies do not invent measurement standards in isolation; they enforce international consensus standards managed by bodies such as the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC).

Standard	Scope & Primary Focus	Key Metrological Requirements
ISO/IEC 17025:2017	Testing and calibration laboratories across all industrial, scientific, and technical disciplines.	Clause 6.5 (Metrological Traceability), Clause 7.6 (Evaluation of Measurement Uncertainty), Clause 7.8 (Reporting Results).
ISO 15189:2022	Medical and clinical diagnostic laboratories.	Clause 6.5 (Equipment and Traceability), focus on patient sample handling, reference intervals, and clinical utility.
GLP (Good Laboratory Practice)	Non-clinical health and environmental safety studies (FDA, OECD).	Focuses on data integrity, clear instrument calibration logs, and certified reference standards (CRMs).

The Role of Accreditation Bodies and Mutual Recognition

Accreditation Bodies (ABs)—such as A2LA, ANAB, UKAS, or NABL—are assessed by the International Laboratory Accreditation Cooperation (ILAC). Through the ILAC Mutual Recognition Arrangement (MRA), testing data generated by an ISO/IEC 17025-accredited laboratory in one country is recognized by regulatory authorities across over 100 member economies.

When a regulator audits a laboratory, they look for the accreditation logo alongside explicit SI-derived units on official test certificates.

3. What Regulatory Bodies and Assessors Expect

When technical assessors perform on-site audits or document evaluations, they focus on specific operational areas to verify compliance with SI unit reporting and metrological standards.

A. Expression of Units and Format Consistency

Regulatory auditors expect strict adherence to standardized notation:

• Standard SI Symbols: Use of $kg$ for kilograms, $m/s$ or $m \cdot s^{-1}$ for meters per second, $\mu g/mL$ for micrograms per milliliter.
  
  
• Elimination of Non-Standard Terms: Terms like "ppm" (parts per million) or "ppb" (parts per billion) are discouraged by regulators because they can be ambiguous (mass/mass vs. volume/volume). Auditors prefer explicit SI ratios, such as $mg/kg$ or $\mu g/L$.
  
  
• Correct Prefixes: Proper application of SI prefixes (kilo-, milli-, micro-, nano-) without stacking multiple prefixes (e.g., using $pF$ rather than $uuF$).
  
  

B. Proof of Calibration and Traceability Hierarchies

Laboratories must present clear evidence that their equipment is calibrated using reference standards tied to SI units.

An auditor will reject a calibration certificate if:

• The calibrating vendor is not accredited to ISO/IEC 17025 for that specific scope.
  
  
• The certificate lacks a statement of measurement uncertainty.
  
  
• The calibration chain relies on uncertified, in-house references without validated comparison procedures.
  
  

C. Certified Reference Materials (CRMs) and ISO 17034

In chemical, biological, and environmental analysis, physical instrument calibration alone is insufficient. Laboratories rely on Certified Reference Materials (CRMs) to establish SI traceability for concentration and purity measurements (typically expressed in $mol/kg$ or $g/kg$).

Regulators expect CRMs to be sourced from Reference Material Producers (RMPs) accredited to ISO 17034. Certificates of Analysis (CoAs) for these materials must state the metrological origin, expiration date, storage conditions, and the expanded uncertainty associated with the certified value.

D. Evaluation and Reporting of Measurement Uncertainty

A measurement result expressed solely as a single number (e.g., "$25.4\text{ mg/L}$") is incomplete under regulatory review. ISO/IEC 17025 Clause 7.6 requires laboratories to evaluate measurement uncertainty using the principles outlined in the GUM (Guide to the Expression of Uncertainty in Measurement).

The result must be reported as:

$$Y = y \pm U$$

Where:

• $y$ is the measured value in SI units.
  
  
• $U$ is the expanded measurement uncertainty ($U = k \cdot u_c$).
  
  
• $k$ is the coverage factor (typically $k = 2$ for a $95\%$ confidence interval).
  
  
• $u_c$ is the combined standard uncertainty.
  
  

Example of an Audit-Ready Test Report Entry:

• Parameter: Cadmium concentration in water
  
  
• Result: $2.14 \ \mu g/L$
  
  
• Expanded Uncertainty: $\pm 0.08 \ \mu g/L \ (k = 2\text{, } 95\%\text{ confidence level})$
  
  
• Traceability: Traceable to NIST SRM 3108 via accredited calibration curve.
  
  

4. Technical Challenges and Specialty Scenarios

While mechanical and physical measurements (length, mass, time) map directly to SI units, certain fields present unique traceability challenges that require regulatory adaptation.

A. Biological and Clinical Measurements

In medical testing, many analytes (e.g., antibodies, hormones, enzymes) are complex biological mixtures that cannot be fully characterized in pure SI base units ($mol$ or $kg$).

In such cases, regulators and standards like ISO 15189 accept higher-order reference materials, such as:

• WHO International Units (IU): Arbitrary biological activity units defined by the World Health Organization.
  
  
• International Reference Materials: Established by consensus expert panels when SI traceability is technically impossible.
  
  

However, if an analyte's molecular structure is fully known (e.g., glucose, cholesterol, steroid hormones), regulators expect laboratories to transition to SI units ($mmol/L$).

B. Qualitative and Ordinal Testing

Certain testing regimes yield non-numerical outcomes (e.g., positive/negative, present/absent, color matching). Regulatory bodies expect laboratories performing qualitative tests to:

• Validate the limit of detection (LOD) using SI-traceable standards.
  
  
• Calibrate the underlying detection hardware (e.g., spectrometers, optical sensors, thermal cyclers) against SI units.
  
  

5. Common Audit Non-Conformities and Pitfalls

During accreditation assessments and regulatory audits, non-conformities related to SI units and traceability are among the most frequently cited.

1. In-House Calibrations Without Validated Procedures: Performing internal calibrations of balances, pipettes, or thermometers without documented procedures, trained personnel, or calculated measurement uncertainties.
   
   
2. "Certificate of Conformance" vs. "Calibration Certificate": Confusing an equipment manufacturer's Certificate of Conformance (which states an instrument met factory specifications when built) with a true ISO/IEC 17025 Calibration Certificate containing measurement data, uncertainties, and SI traceability logs.
   
   
3. Ignoring Environmental Factors: Failing to account for environmental influences (ambient temperature, humidity, atmospheric pressure) on sensitive SI-linked measurements, such as micro-gram mass determinations or volumetric calibrations.
   
   

6. Building a Compliant Traceability System: A Roadmap

To meet the expectations of accreditation bodies and regulatory agencies, laboratories should maintain a structured compliance routine:

• Step 1: Map the Traceability Hierarchy
  Document every piece of measuring equipment, identifying its calibration standard, the vendor providing the calibration, their ISO/IEC 17025 accreditation status, and the highest-level NMI reference point.
  
  
• Step 2: Enforce Standardized SI Reporting
  Standardize report templates, LIMS (Laboratory Information Management System) outputs, and analytical procedures to ensure units are consistently formatted in SI-compliant notation.
  
  
• Step 3: Establish Methodical Uncertainty Budgets
  Develop uncertainty budgets for every accredited test method, taking into account repeatability, intermediate precision, calibrator uncertainties, environmental drift, and sample matrix variations.
  
  
• Step 4: Vet External Service Providers (ESPs)
  Maintain a vetted list of calibration laboratories and reference material producers. Confirm annually that their scope of accreditation covers the specific parameters, ranges, and measurement uncertainties required by your facility.
  
  
• Step 5: Participate in Proficiency Testing (PT)
  Regularly enroll in interlaboratory comparisons and Proficiency Testing programs organized by accredited PT providers (ISO/IEC 17043). PT results provide objective third-party proof that a laboratory's SI-traceable measurements produce valid results compared to global peer networks.
  
  

Summary Perspective

For modern laboratories, alignment with SI units and accreditation standards is not merely a bureaucratic requirement—it is the core mechanism that ensures global data comparability. When an analytical laboratory establishes an unbroken chain of metrological traceability, it guarantees that a measurement made in its facility today will hold its integrity when reviewed by regulators across the world tomorrow.

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