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Toward Standardization of Cardiac Troponin I Measurements Part II: Assessing Commutability of Candidate Reference Materials and Harmonization of Cardiac Troponin I Assays

¹ Department of Pathology, University of Maryland School of Medicine, Baltimore, MD.
² Department of Laboratory Medicine and Pathology, Hennepin County Medical Center, and University of Minnesota School of Medicine, Minneapolis, MN.
³ Denver Health Medical Center, Denver, CO.
⁴ Analytical Chemistry Division, National Institute of Standards and Technology, Gaithersburg, MD.
⁵ Dipartimento di Scienze Cliniche "Luigi Sacco", Università degli Studi di Milano, Milano, Italy.
⁶ Department of Pathology and Laboratory Medicine, University of California at San Francisco, San Francisco, CA.
⁷ Departments of Pathology, Cell Biology, Neurobiology and Anatomy, Loyola University Medical Center, Maywood, IL.
⁸ Member of the American Association for Clinical Chemistry Cardiac Troponin I Standardization Committee.
⁹ Chair, American Association for Clinical Chemistry Cardiac Troponin I Standardization Committee

^aAddress correspondence to this author at: University of Maryland Medical Center, 22 S. Greene Street, Baltimore, MD 21201. Fax 410-328-5880; e-mail rchristenson{at}umm.edu.

Background: Cardiac tropoin I (cTnI) measurements show an 20- to 40-fold difference between assays, and better standardization and harmonization are needed. Toward this goal, the AACC cTnI Standardization Committee collaborated with the National Institute of Standards and Technology (NIST) in an earlier study to select 2 candidate reference materials (cRMs).

Methods: Two troponin cRMs, a troponin C-troponin I-troponin T (CIT) complex from human heart tissue and a CIT complex from recombinant technology, were supplied to NIST for assessment of composition and purity, and cTnI value assignment. These cRMs and 6 cTnI-positive human serum pools were shipped to manufacturers of 15 cTnI assays. Commutability of the materials was examined by determining the numerical relationship for the cRM preparations between each manufacturer-specified field method and each of the other 14 field methods. These relationships were then compared with the corresponding numerical relationships for the human serum pools. Harmonization of methods was accomplished by determining regression parameters relative to the analytical system yielding values closest to the median for each serum pool. These regression parameters were used to recalculate pool values to harmonize the assays. Interassay CVs before and after harmonization were determined.

Results: Characterization of the CIT and CI cRMs showed that these materials were of specified composition. The proportion of cTnI methods that demonstrated commutability for the CIT cRM was 45%; for the CI cRM, 39% of methods demonstrated commutability. Interassay cTnI variability for the field methods ranged from 82% to 97%, median 88%. After harmonization, variability of the serum pools for the cTnI methods was decreased to between 9.0% and 23%, median 15.5%.

Conclusions: The proportion of methods demonstrating commutability was too low for use as a common calibrator for the cTnI field methods. However a simple strategy using serum pools can improve harmonization of field cTnI methods by more than 5-fold. The CIT cRM was selected by the AACC cTnI standardization committee, and a new lot has been classified as the cTnI certified reference material Standard Reference Material 2921 by NIST.

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