What does “NIST Traceable” really mean?

Oct 27, 2017

NIST Traceability Pipette Calibration

When you work in a life science lab, you’ll probably encounter the phrase “NIST traceable” at some point or other, especially if you work in a regulated environment such as diagnostics, pharmaceutical manufacturing, forensics, food safety, or a reference lab. But what does “NIST traceable” really mean?

A few years back we wrote a blog post called “The ABCs of ISOs: What ISO Standards in the Life Science Industry Are All About,” where we talked about the traceability of measurement results to NIST standards, commonly referred to as “NIST traceability,” as part of ISO standards; but “NIST traceability” is such an important topic that it’s worthwhile to focus on it for a whole blog post.

The concept of traceability—origins

The modern story of traceability begins all the way back in 1875. Representatives from seventeen different countries got together in Paris and signed a treaty, the Treaty of the Meter, organizing how countries could work together to ensure measurement uniformity and concordance. The issue was originally one of trust in commerce—if a merchant in one country wanted to sell a kg of apples to a merchant in another country, how can both sides be sure that what they call a kg is, in fact, the same amount?

The Treaty of the Meter—amended slightly in 1921 and expanded to presently include fifty-five member countries—not only provides assurances that measurements across borders are the same, but also ensures that measurements within a country are the same within certain pre-defined acceptable tolerances. This treaty is the foundation of the International System of Units (SI).

The concept of traceability—how it works

The main concept at the heart of the treaty is one of connection, although it’s commonly referred to in the industry as traceability.  The idea is that somewhere, there’s a single standard that defines a specific measurement—say a one meter (or, metre, if you’re European) platinum bar housed in Paris. Traceability means that you can certify that your one meter length is the same length as the standard in Paris because you have a list of certified measurements that can trace back to that standard in an unbroken chain of connected measurements. The process looks something like this:

  • My meter stick is accurate because it was compared to the meter stick at a local calibration laboratory
  • Which in turn is accurate because it was compared to the meter stick at a primary standards laboratory
  • Which in turn is accurate because it was compared to the meter stick at the national standards body—in the US, this organization is the National Institute of Standards and Technology, or NIST
  • Which in turn is accurate because it was compared to the meter stick in Paris, to which everyone calibrates their meter sticks.

NIST traceability

This system of connection and traceability back to a single accurate measurement standard is part of what ensures measurement uniformity.  These days, however, most measurements are traced back to universal physical constants (for instance the speed of light, which together with the vibration frequency of an atom of Cesium 133, defines the length of a meter) rather than physical objects such as the one meter bar in Paris.  The lone exception is the definition of mass, which is still traced back to a physical artifact, the “prototype” kilogram in Paris.  Effective in 2018 all base quantities, including mass, will be traceable to universal physical constants.

If you’d like to learn more about NIST reference standards, there’s a nice article called “Measurements Matter” at the NIST website. It’s written for the general public, making it an entertaining and readily accessible piece.

The concept of traceability—it’s about measurement results

One important aspect of traceability is that it only applies to measurement results—not a lab, an assay, or an instrument, although the data that comes out of that instrument could be “NIST traceable” if certain conditions are met. There’s a nice blog post about “NIST traceability” and forensic sciences that stresses this point.

What makes a measurement result “NIST traceable”?

In order to claim that a measurement result is traceable to NIST standards, the following conditions need to be met:

What? You need a well-defined measurement where both the item being measured and the quantity that is measured are clearly described and defined.

How? You need a comprehensive description of the system you are using to make the measurement, and the working standard used to perform that measurement. This is critical not only for replicating the measurement, but also to help assess and/or limit the uncertainties in the measurement.

How sure? You need to document the uncertainties in the measurement so it’s clear how much trust a user can place in the measurement result. NIST has guidelines on expressing and evaluating measurement uncertainty here.

How good is your measurement system? You need a quality assurance program that verifies that your measurement system or working standard is performing as expected.

How good is your reference standard? You need a quality assurance program that verifies that your reference standard is performing as expected at the time it was used as a reference standard.

Proof? Lastly, everything mentioned above needs to be clearly documented. In addition, in order to meet many ISO standards, not only do the measurement results need to be “NIST traceable,” but the competency of the scientists performing the measurements needs to be demonstrated (see our blog post, “The ABCs of ISOs: What ISO Standards in the Life Science Industry Are All About,” for more on how ISO standards and “NIST traceability” intersect).

Wrapping up

“NIST traceability,” the traceability of measurement results to NIST standards, is critical for quality assurance in life science labs, which can be a life or death matter for certain measurements, such as when manufacturing pharmaceuticals. Knowing exactly what the phrase means can help scientists that need to obtain traceable measurement results understand exactly what should be done to maintain high-quality, trustworthy results and keep the traceability chain intact.

Additional Resources

About the Author

A. Bjoern Carle

A. Bjoern Carle, PhD, is a Product Manager at Artel, where he supports product development, scientific marketing, regulatory affairs and technical training efforts. Dr. Carle earned his B.S. at the University of Stuttgart (Germany), his M.S. and PhD at the University of Cincinnati, and performed post-doctoral research at Harvard Medical School / Massachusetts General Hospital.