In the world of scientific measurement, accuracy and reliability are not just desired—they are imperative. This is especially true in testing and medical laboratories, where the precision of measurements can directly impact patient outcomes and scientific integrity. A key concept that underpins this precision is measurement traceability. Understanding and ensuring traceability is crucial for laboratories aiming to maintain high standards of accuracy and compliance with regulatory requirements.
What is Measurement Traceability?
Measurement traceability can be defined as 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. In simpler terms, it’s a way to ensure that measurements are consistently accurate and comparable over time and across different locations or systems.
The Unbroken Chain of Traceability
The “unbroken chain” of traceability refers to a series of calibration steps that link a measurement back to a reference standard. Each step in this chain must be performed using methods and equipment that are traceable to known standards, and the uncertainty of each measurement must be clearly stated. This chain ensures that the final measurement is not just accurate, but also reliably connected to a recognized standard.
Example in Testing and Medical Laboratories
Consider a medical laboratory that conducts blood glucose tests. The accuracy of these tests is critical for diagnosing and managing diabetes. The traceability chain in this case might start with a primary standard for glucose concentration held by a National Metrology Institute (NMI). The NMI provides a calibrated reference material to a secondary laboratory, which then uses this reference to calibrate their own measurement instruments. When the medical laboratory uses these calibrated instruments to measure a patient’s blood glucose, the results can be confidently traced back to the primary standard.
Role of National and International Bodies
The integrity of measurement traceability chains is overseen by various national and international bodies:
- National Institute of Standards and Technology (NIST) in the USA: NIST provides calibration services and standard reference materials that are traceable to SI (International System of Units) units. Laboratories in the United States rely on NIST for traceable measurements in various fields, including health and environmental monitoring.
- International Bureau of Weights and Measures (BIPM): BIPM ensures worldwide uniformity of measurements and their traceability to the SI. It coordinates international comparisons of national measurement standards and provides a database of calibration and measurement capabilities of NMIs.
- National Metrology Institutes (NMIs): Every country typically has an NMI that maintains national standards for various measurements. These institutes ensure that the measurement standards used within the country are traceable to the SI units, either directly or through equivalence established by BIPM.
Importance in Testing and Medical Laboratories
- Ensuring Accuracy: Traceability ensures that measurements are consistently accurate, critical in fields like healthcare, where patient safety and treatment efficacy depend on precise measurements.
- Regulatory Compliance: Many industries are legally required to maintain traceable measurements. In healthcare, for example, regulatory bodies may require laboratories to demonstrate traceability to ensure quality control.
- Global Standardization: In an increasingly connected world, traceability to SI units allows for the standardization of measurements globally, facilitating international trade and cooperation in science and technology.
Examples in Testing Laboratories
- Mass:
- Unit: Kilogram (kg)
- Traceability with NMI: A testing laboratory calibrates its balance using weights traceable to the national standard maintained by the NMI, which in turn is traceable to the international prototype of the kilogram.
- Traceability without NMI: The laboratory could use a balance that has been calibrated by a manufacturer whose calibration process is traceable to international standards, ensuring indirect traceability.
- Temperature:
- Unit: Celsius (°C)
- Traceability with NMI: A laboratory thermometer is calibrated against a standard thermometer provided by the NMI, which aligns with the International Temperature Scale of 1990 (ITS-90), a standard for temperature measurement adopted internationally.
- Traceability without NMI: Use of digital thermometers that are factory-calibrated against international standards, providing a level of confidence in their measurements.
- Volume:
- Unit:Liter (L)
- Traceability with NMI: Pipettes and volumetric flasks are calibrated against volume standards maintained by the NMI, ensuring precise liquid measurement.
- Traceability without NMI: Utilizing pre-calibrated, certified volumetric instruments supplied by accredited manufacturers can provide traceability.
Examples in Medical Laboratories
- Concentration:
- Unit: Moles per liter (mol/L)
- Traceability with NMI: Blood analyzers are calibrated using solutions with known molar concentrations provided by the NMI.
- Traceability without NMI: Calibration using standard solutions from certified suppliers whose calibration standards are internationally recognized.
- Pressure:
- Unit:Millimeters of mercury (mmHg)
- Traceability with NMI: Blood pressure cuffs (sphygmomanometers) are calibrated using a reference standard traceable to the NMI.
- Traceability without NMI: Use of electronic blood pressure monitors that are factory-calibrated according to international standards.
- Hematology (Blood cell count):
- Unit: Cells per microliter (cells/µL)
- Traceability with NMI:Hematologyanalyzers are calibrated using control materials with a known number of cells per unit volume, traceable to standards set by the NMI.
- Traceability without NMI: Employing pre-calibrated instruments or calibration services from internationally accredited suppliers.
Example of Broken Chain of Traceability in a Testing Laboratory
1. Measurement of Air Quality Using a Gas Chromatograph:
Step 1: International and National Standards Not Followed
The gas chromatograph should be calibrated against standards traceable to the SI units maintained by the National Metrology Institute (NMI). However, in this case, the laboratory uses a calibration gas mixture from an uncertified supplier without verification of its traceability to national or international standards.
Step 2: In-house Calibration
The testing laboratory calibrates the gas chromatograph using the unverified calibration gas mixture. Since the source of the calibration gas is not traceable to the SI units or NMI standards, the calibration process breaks the chain of traceability.
Step 3: Air Quality Measurement
The laboratory conducts air quality measurements with the improperly calibrated gas chromatograph. The results, now lacking traceability to recognized standards, may be inaccurate or non-compliant with regulatory requirements, leading to potential misinterpretations and consequential public health or environmental risk assessments.
Example of Broken Chain of Traceability in a Medical Laboratory
1. Measurement of Hemoglobin in Blood Samples:
Step 1: Non-Standard Calibration Practices
Ideally, hemoglobin analyzers should be calibrated using reference materials that are traceable back to recognized international standards. However, in this scenario, the medical laboratory uses a calibration standard from a non-accredited source without verification of its traceability.
Step 2: Internal Calibration Process
The laboratory calibrates its hemoglobinanalyzer with the non-traceable standard. This step breaks the traceability chain, as the calibration standard’s accuracy and reliability are not assured.
Step 3: Patient Diagnosis
The laboratory uses the incorrectly calibrated analyzer for patient diagnostics. Due to the broken chain of traceability, the accuracy of the hemoglobin measurements is questionable. Misleading results can lead to incorrect diagnoses or treatment plans, potentially impacting patient health outcomes.
In both examples, the failure to maintain an unbroken chain of traceability results from using calibration standards or methods that are not linked to recognized international or national standards. This breach in protocol can lead to significant inaccuracies in measurements, undermining the reliability of the testing processes in various fields, including environmental monitoring and healthcare. It highlights the critical importance of adhering to established traceability chains for ensuring the validity and reliability of laboratory measurements
Conclusion
Measurement traceability is a fundamental aspect of ensuring accuracy and reliability in testing and medical laboratories. The unbroken chain of calibrations, overseen by organizations like NIST, BIPM, and NMIs, establishes confidence in measurement results, supporting critical decisions in healthcare, research, and industry. As technology advances and global collaboration increases, the role of measurement traceability will continue to be a cornerstone of scientific integrity and public trust.
About the Author
Dr. Sambhu Chakraborty is a distinguished consultant in quality accreditation for laboratories and hospitals. With a leadership portfolio that includes directorial roles in two laboratory organizations and a consulting firm, as well as chairman of IOL ( An ILAC stakeholder organisation), Dr. Chakraborty is a respected voice in the field. For further engagement or inquiries, Dr. Chakraborty can be contacted through email at director@iaqmconsultants.com and info@sambhuchakraborty.com. Additional resources and contact information are available on his websites,https://www.quality-pathshala.com and https://www.sambhuchakraborty.com, or via WhatsApp at +919830051583