NKDEP Creatinine Standardization Program: Commutability Study

In 2006, NKDEP’s Laboratory Working Group (LWG) conducted a commutability study of creatinine reference materials with the National Institute of Standards and Technology’s (NIST) Standard Reference Material (SRM) 967 to ensure commutability with native serum samples for most commonly used creatinine methods.

Reference materials are typically used to establish or validate the calibration traceability of a routine measurement procedure to a reference measurement procedure of higher order. When a reference material is intended to be measured by a routine clinical method, commutability* must be validated for all routine methods for which the reference material is intended for use.

Results
Fresh, frozen serum creatinine reference materials from NIST, SRM 967 (and from the College of American Pathologists’ Accuracy Calibration Verification/Linearity Survey LN24 used in 2006) were found to be commutable with a panel of native clinical serum samples for the manufacturers’ reagents and instruments listed below. Reference materials and clinical samples were measured by the stated analytical system and by a LC/IDMS reference measurement procedure at NIST. Commutability was evaluated using criteria described in Clinical and Laboratory Standards Institute (CLSI) document EP14-A2.¹

The list provided above documents commutability of the NIST SRM 967 and CAP LN24 creatinine reference materials with native clinical serum samples for various reagent-instrument combinations determined for specific reagents used on these specific instrument systems in 2006. It may not apply to different reagent-instrument combinations.

Because these creatinine reference materials met the CLSI EP14-A2 criteria for commutability with all reagent-instrument combinations tested, it is expected that these reference materials be commutable with native clinical serum samples when the same reagent is used on very similar instruments using the same methodological principle.

Nevertheless, caution should be exercised in extrapolating the above findings to different reagent-instrument combinations, even when the instruments and reagents are produced by the same in vitro diagnostic manufacturer. It is recommended that clinical laboratories consult with the manufacturer(s) of their reagent and instrument to determine the applicability of the established commutability characteristics for these reference materials to particular configurations of creatinine reagents and instrument systems.

*Definition of Commutability
Commutability has been defined as the equivalence of the mathematical relationships between the results of different measurement procedures for a reference material and for representative samples from healthy and diseased individuals.² This definition is based on the more technical definition in ISO document 17511.³ Commutability does not imply accuracy or trueness of results, only that results for a reference material had the same mathematical relationship between two methods that was observed for native clinical samples measured by the same two methods.

References

CLSI Document EP14-A2. Evaluation of Matrix Effects; Approved Guideline. 2nd ed. Wayne, PA: Clinical and Laboratory Standards Institute; 2005.
Miller WG, Myers GL, Rej R. Why commutability matters. Clinical Chemistry. 2006;52(4):553–554.
ISO International Standard 17511:2003(E). In vitro diagnostic medical devices – Measurement of quantities in biological samples – Metrological traceability of values assigned to calibrators and control materials. 1st ed. Geneva, Switzerland: International Organization for Standards; 2003.