Improving the Quality of HbA1c Testing

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Hemoglobin A_1c (HbA_1c) is an important indicator of mean glycemia and outcome risks in patients with diabetes. HbA_1c is a specific glycated hemoglobin formed by a non-enzymatic attachment of glucose to the N-terminal valine of the hemoglobin beta chain, and thus represents a weighted average of blood glucose levels over the lifespan of the erythrocytes (~120 days).

The importance of the test was established by the results of the Diabetes Control and Complications Trial (DCCT) published in 1993, which demonstrated a direct relationship between HbA_1c levels and risks for development and progression of diabetes complications, including retinopathy, neuropathy and nephropathy, in patients with Type 1 diabetes. The results of the United Kingdom Prospective Diabetes Study (UKPDS) published in 1998 showed similar findings in patients with Type 2 diabetes.

Based on the results of the DCCT, the American Diabetes Association (ADA) began recommending specific therapeutic goals for HbA_1c in 1994. However, lack of standardization made it difficult for healthcare providers to utilize these HbA_1c targets in clinical practice. Some methods measured total glycated hemoglobin while others measured HbA1 (HbA_1c plus several other minor hemoglobins) and still others measured HbA_1c specifically. Thus, the National Glycohemoglobin Standardization Program (NGSP) was initiated in 1996 with the goal of standardizing HbA_1c results to a common reference.

The NGSP consists of a steering committee which oversees the program, an administrative core and a network of reference laboratories, which assist and certify manufacturers and clinical laboratories in harmonizing HbA_1c results to those of the DCCT and UKPDS (Figure 1).

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The NGSP laboratory network includes the Central Primary Reference Laboratory (CPRL), which analyzes HbA_1c by the HPLC method originally used in the DCCT; backup Primary Reference Laboratories (PRLs), which use the same method as the CPRL; and secondary reference laboratories (SRLs), which utilize highly precise commercial methods to measure HbA_1c. SRLs assist manufacturers and laboratories with calibration of their methods and certify their methods via sample comparisons, while the CPRL anchors the network via initial certification and subsequent monthly monitoring of the SRLs.

The administrative core analyzes all certification and monitoring data and provides regular updates of network activities to the steering committee. In addition the administrative core maintains the NGSP web site (www.ngsp.org), which includes lists of NGSP certified methods and laboratories as well as information regarding HbA_1c testing including the results of College of American Pathologists (CAP) GH-2 whole blood proficiency surveys and potential interferences with HbA_1c results.

Results from the CAP GH-2 survey performed twice yearly are used to assess the effectiveness of the NGSP. The results have shown dramatic improvement in the comparability of HbA_1c results since 1993 (Figure 2). Virtually all results are now reported as HbA_1c compared to ~50% in 1993 and overall variability among laboratories and methods has been greatly reduced.

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The NGSP criteria for certification have been tightened significantly since the inception of the program; the current criteria are the 95% confidence interval of the differences between the method and SRL must fall within ±0.75% HbA_1c (±0.70% HbA_1c for Level 1 laboratories). In addition, the NGSP has worked with the CAP in improving the HbA_1c survey. The CAP began the GH-2 survey, which utilizes fresh whole blood instead of lyophilized material (thus avoiding matrix effects that limit the usefulness of the survey), in 1996. In 2007 the CAP survey began to utilize accuracy-based grading, with target values assigned by the NGSP network, in place of peer-group grading. The acceptance criteria were originally ±15% of the target value and have been gradually tightened to the current criteria of ±7%.

IFCC

The International Federation of Clinical Chemistry (IFCC) has developed a higher order reference method that is specific for HbA_1c; IFCC results are reported in mmol/mol of hemoglobin. Although results from the two networks are highly correlated, the actual HbA_1c numbers are different. Many countries are changing their numbers for reporting patient results from DCCT/NGSP percent to IFCC mmol/mol, while the U.S. has chosen to continue reporting results on the DCCT/NGSP scale. Regardless, the relationship between these number scales-monitored twice yearly-has been established, allowing easy conversion between them.

Laboratories and clinicians need to be aware of potential interferences that can affect HbA_1c results, some of which are method-specific such as interferences from common hemoglobin variants. The NGSP regularly evaluates HbA_1c assay methods for common potential interferences and posts this information on the NGSP web site.

The ADA has now recommended that HbA_1c be used for diagnosing diabetes using a cutoff of ≥6.5% HbA_1c. This makes continued improvement in the quality of HbA_1c results more important than ever. Further tightening of both the NGSP and CAP criteria are anticipated to ensure that the quality of HbA_1c testing is sufficient to meet clinical needs.