Background: The Diabetes Control and Complications Trial (DCCT) and United Kingdom Prospective Diabetes Study (UKPDS) established the importance of hemoglobin A(1c) (Hb A(1c)) as a predictor of outcome in patients with diabetes mellitus. In 1994, the American Diabetes Association began recommending specific Hb A(1c) targets, but lack of comparability among assays limited the ability of clinicians to use these targets. The National Glycohemoglobin Standardization Program (NGSP) was implemented in 1996 to standardize Hb A(1c) results to those of the DCCT/UKPDS.
Content: The NGSP certifies manufacturers of Hb A(1c) methods as traceable to the DCCT. The certification criteria have been tightened over time and the NGSP has worked with the College of American Pathologists in tightening proficiency-testing requirements. As a result, variability of Hb A(1c) results among clinical laboratories has been considerably reduced. The IFCC has developed a reference system for Hb A(1c) that facilitates metrological traceability to a higher order. The NGSP maintains traceability to the IFCC network via ongoing sample comparisons. There has been controversy over whether to report Hb A(1c) results in IFCC or NGSP units, or as estimated average glucose. Individual countries are making this decision.
Summary: Variability among Hb A(1c) results has been greatly reduced. Not all countries will report Hb A(1c) in the same units, but there are established equations that enable conversion between different units. Hb A(1c) is now recommended for diagnosing diabetes, further accentuating the need for optimal assay performance. The NGSP will continue efforts to improve Hb A(1c) testing to ensure that clinical needs are met.