Opinion
VIEWPOINT
David B. Sacks, MB, ChB, FRCPath Department of Laboratory Medicine, National Institutes of Health, Bethesda, Maryland. W. Garry John, PhD, FRCPath Department of Clinical Biochemistry, Norfolk and Norwich University Hospital, and Norwich Medical School, Norwich, United Kingdom.
Corresponding Author: David B. Sacks, MB, ChB, FRCPath, National Institutes of Health, Bldg. 10, Room 2C306, 10 Center Dr, Bethesda, MD 20892 ([email protected] .gov).
Interpretation of Hemoglobin A1c Values Since better glucose control reduces microvascular complications in patients with diabetes, reliable evaluation of diabetes control is essential. Hemoglobin A1c (HbA1c) values reflect average glucose over approximately 120 days (the average erythrocyte lifespan) and are a better assessment of glucose control than blood glucose measurement, which provides information at only one point of time. Although HbA1c is universally accepted as a means for monitoring diabetes control, its measurement is challenging. Concerns have been expressed about deficiencies of HbA1c analysis—most notably, lack of accuracy and inability to use HbA1c in subsets of individuals (eg, patients with hemolytic anemia or acute blood loss). This Viewpoint addresses some important considerations relevant to laboratory determination of HbA1c. HbA1c is formed by the nonenzymatic attachment of glucose (termed glycation) to hemoglobin in the circulation. HbA1c is integral to monitoring the response of patients with diabetes to therapy, and many organizations have developed HbA1c target values. Moreover, large randomized clinical trials, particularly the DCCT (Diabetes Complications and Control Trial), 1 documented the value of HbA 1c in predicting the development of microvascular complications in patients with diabetes. More recently, several organizations (including the American Diabetes Association2 and World Health Organization) endorsed HbA1c for the diagnosis of diabetes. HbA1c measurements became commercially available in 1978.3 Initially, the test had some limitations, particularly a lack of standardization that resulted in wide variation of values among laboratories. For example, in 1983, measurement of the same blood sample could yield a result of 4.5% in one laboratory and 8.0% in another laboratory.3 Major efforts were undertaken to standardize results among laboratories. A national standardization effort, termed NGSP (National Glycohemoglobin Standardization Program), was initiated in the United States in 1993 shortly after publication of the DCCT. Other countries, notably Japan and Sweden,3 also implemented standardization schemes; all 3 standardization schemes use high-performance liquid chromatography as the reference method. Prior to global standardization (see below), the NGSP was the most widely used HbA1c standardization system in the world. The goal of the NGSP is to harmonize HbA1c results so that values reported by individual clinical laboratories are comparable with those reported in the DCCT (in the DCCT, all HbA1c measurements were performed in a central laboratory using a precise high-performance liquid chromatography method, thus avoiding standardization concerns between laboratories). Using a network of 10 laboratories in the United States, Europe, and Japan, the NGSP works with manufacturers of HbA1c
testing methods and certifies those methods that meet stringent accuracy criteria.4 In addition, the NGSP is part of the process by which laboratories that measure patient samples are evaluated to ensure they report accurate results. This is mandatory in the United States under the Clinical Laboratory Improvement Amendments of 1988 (CLIA’88). The NGSP has significantly reduced the variability in HbA1c testing.5 In 1995, the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) established a working group to standardize HbA1c, anchored on a metrologically sound international reference measurement system.6 The IFCC working group developed a reference method based on enzymatic cleavage of intact hemoglobin to obtain hexapeptides of HbA1c and nonglycated hemoglobin.6 The hexapeptides are separated by high-performance liquid chromatography and quantified by electrospray ionization–mass spectrometry or capillary electrophoresis. A network of 14 reference laboratories using the reference procedure assign HbA1c target values to reference materials, reference panels of blood samples, and control materials necessary for the reference measurement system.7 Results obtained with the new standardization system can be related to the previous harmonization initiatives and therefore, to the DCCT. The NGSP and other networks produce HbA1c values different from the IFCC method. This is attributable to the lack of specificity of the original high-performance liquid chromatography methods. Nevertheless, there is a linear correlation with the IFCC method, allowing reliable linear regression equations to be devised. These published equations7 can be used to calculate DCCT values from IFCC results and vice versa. Ongoing network comparisons demonstrate a stable relationship between NGSP and IFCC networks.7 Two important factors must be considered. The IFCC reference methods are not designed for routine testing of patient samples, but rather for setting standards for use by manufacturers producing routine methods. Moreover, the IFCC recommends that laboratories report results in SI units, namely mmol/mol rather than percent. Using the derived equation, an HbA 1c value of 7% would be 53 mmol/mol. Several countries in Europe and Asia have adopted SI units for HbA1c. Measurement accuracy reflects the closeness of a result to the true value and repeatability (or precision) is closeness of agreement of repeated measurements coefficient of variation. How precise and accurate should HbA1c measurements be? Recent guidelines recommend intralaboratory and interlaboratory coefficients of variation of less than 2% of the measured value and less than 3.5%, respectively.8 This is similar to the coefficient of variation of less than 2.9%, recommended for glucose measurement.8 At an HbA1c
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Copyright 2014 American Medical Association. All rights reserved.
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Opinion Viewpoint
concentration of 7.0% (which is the therapeutic target for the majority of patients with diabetes), 95% of the more than 3200 laboratories that participated in the 2014 College of American Pathologists survey use methods with between-laboratory coefficients of variation less than 3.5%.4 In general, blood tests should have sufficient accuracy to detect clinically relevant differences. The National Institute for Clinical Excellence recommends that treatment regimens for patients with diabetes should be evaluated based on a measured change in HbA1c of at least 0.5%. By 2010, 80% of laboratories used an HbA 1c method that could accurately distinguish a change of less than 0.5% HbA1c.5 Evaluation of HbA1c every 3 to 6 months (as recommended)2 will reduce measurement uncertainty. Methods to measure HbA1c continue to improve and variability will likely be reduced further. Aside from technical issues influencing HbA1c measurement, some clinical conditions may affect the ability of HbA1c to reflect temporally averaged blood glucose control. Formation of HbA1c is dependent on the hemoglobin circulating being predominantly HbA. There is a widely held misconception that HbA1c cannot be measured accurately in individuals with mutated Hb (also known as variant Hb). The prevalence of hemoglobinopathies (non-HbA) varies among countries, with HbS or HbC trait present in 10% of the 26 million US African Americans.9 More than 95% of instruments can accurately measure HbA1c in patients with these Hb variants; however, it is always advisable to discuss this with the laboratory. Similarly, by selecting the correct instrument, accurate HbA1c results can be obtained in patients with HbAE or HbAD.4,9 Some hemoglobinopathies alter erythrocyte survival, which will influence all HbA1c measurements. ARTICLE INFORMATION Conflict of Interest Disclosures: Both authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest and none were reported. Funding/Support: This work is supported in part by the Intramural Program of the National Institutes of Health. Role of the Sponsor: The Intramural Program of the National Institutes of Health had no role in the preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication. REFERENCES 1. The Diabetes Control and Complications Trial Research Group. The effect of intensive treatment of diabetes on the development and progression of
2272
Hemolytic anemia, from whatever cause, lowers HbA1c values because of reduced red blood cell survival. However, irondeficiency anemia can lead to an inappropriate increase in HbA1c of 1% to 1.5%, which declines after successful iron replacement. Thalassemia is an inherited disorder with decreased synthesis of one of the globin chains. Disease severity determines whether HbA1c reflects average glycemia. Mild disease may not alter erythrocyte lifespan, but for patients with more severe disease (especially if requiring transfusion), HbA1c measurements will not be reliable. Other patient factors that may influence HbA1c concentrations are age and race. The molecular mechanisms are poorly understood and the clinical relevance, if any, remains to be established. In addition to hemoglobin, other proteins in the blood are glycated. Albumin is the most abundant protein in plasma and tests to quantify glycated albumin (ie, fructosamine and glycated albumin) have been developed. However, these tests have not been evaluated in large clinical trials and no evidence validates their ability to predict complications. Importantly, no therapeutic targets for these tests have been widely adopted. Factors other than glucose (eg, change in blood albumin levels) also can alter glycated albumin concentrations. Another marker of long-term glycemia, 1,5anhydroglucitol, has similar problems, with limited evaluation in clinical studies, exclusion from use in certain patients (notably in renal impairment), and limited availability in clinical laboratories. In conclusion, HbA1c remains the only test that can predict the microvascular complications of diabetes and for which there are generally accepted therapeutic targets. HbA1c can be measured accurately in the majority of patients and provides valuable information to help guide treatment decisions.
long-term complications in insulin-dependent diabetes mellitus. N Engl J Med. 1993;329(14):977986. 2. American Diabetes Association. Standards of medical care in diabetes—2014. Diabetes Care. 2014;37(suppl 1):S14-S80.
6. Hoelzel W, Weykamp C, Jeppsson JO, et al; IFCC Working Group on HbA1c Standardization. IFCC reference system for measurement of hemoglobin A1c in human blood and the national standardization schemes in the United States, Japan, and Sweden: a method-comparison study. Clin Chem. 2004;50(1):166-174.
3. Berg AH, Sacks DB. Haemoglobin A1c analysis in the management of patients with diabetes: from chaos to harmony. J Clin Pathol. 2008;61(9):983987.
7. Weykamp C, John WG, Mosca A, et al. The IFCC Reference Measurement System for HbA1c: a 6-year progress report. Clin Chem. 2008;54(2):240-248.
4. National Glycohemoglobin Standardization Program. Harmonizing hemoglobin A1c testing. http://www.ngsp.org. Accessed May 20, 2014.
8. Sacks DB, Arnold M, Bakris GL, et al. Guidelines and recommendations for laboratory analysis in the diagnosis and management of diabetes mellitus. Clin Chem. 2011;57(6):e1-e47.
5. Little RR, Rohlfing CL, Sacks DB; National Glycohemoglobin Standardization Program (NGSP) Steering Committee. Status of hemoglobin A1c measurement and goals for improvement: from chaos to order for improving diabetes care. Clin Chem. 2011;57(2):205-214.
9. Bry L, Chen PC, Sacks DB. Effects of hemoglobin variants and chemically modified derivatives on assays for glycohemoglobin. Clin Chem. 2001;47(2): 153-163.
JAMA June 11, 2014 Volume 311, Number 22
Copyright 2014 American Medical Association. All rights reserved.
Downloaded From: http://jama.jamanetwork.com/ by a Western Oregon University User on 05/24/2015
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VIEWPOINT
David B. Sacks, MB, ChB, FRCPath Department of Laboratory Medicine, National Institutes of Health, Bethesda, Maryland. W. Garry John, PhD, FRCPath Department of Clinical Biochemistry, Norfolk and Norwich University Hospital, and Norwich Medical School, Norwich, United Kingdom.
Corresponding Author: David B. Sacks, MB, ChB, FRCPath, National Institutes of Health, Bldg. 10, Room 2C306, 10 Center Dr, Bethesda, MD 20892 ([email protected] .gov).
Interpretation of Hemoglobin A1c Values Since better glucose control reduces microvascular complications in patients with diabetes, reliable evaluation of diabetes control is essential. Hemoglobin A1c (HbA1c) values reflect average glucose over approximately 120 days (the average erythrocyte lifespan) and are a better assessment of glucose control than blood glucose measurement, which provides information at only one point of time. Although HbA1c is universally accepted as a means for monitoring diabetes control, its measurement is challenging. Concerns have been expressed about deficiencies of HbA1c analysis—most notably, lack of accuracy and inability to use HbA1c in subsets of individuals (eg, patients with hemolytic anemia or acute blood loss). This Viewpoint addresses some important considerations relevant to laboratory determination of HbA1c. HbA1c is formed by the nonenzymatic attachment of glucose (termed glycation) to hemoglobin in the circulation. HbA1c is integral to monitoring the response of patients with diabetes to therapy, and many organizations have developed HbA1c target values. Moreover, large randomized clinical trials, particularly the DCCT (Diabetes Complications and Control Trial), 1 documented the value of HbA 1c in predicting the development of microvascular complications in patients with diabetes. More recently, several organizations (including the American Diabetes Association2 and World Health Organization) endorsed HbA1c for the diagnosis of diabetes. HbA1c measurements became commercially available in 1978.3 Initially, the test had some limitations, particularly a lack of standardization that resulted in wide variation of values among laboratories. For example, in 1983, measurement of the same blood sample could yield a result of 4.5% in one laboratory and 8.0% in another laboratory.3 Major efforts were undertaken to standardize results among laboratories. A national standardization effort, termed NGSP (National Glycohemoglobin Standardization Program), was initiated in the United States in 1993 shortly after publication of the DCCT. Other countries, notably Japan and Sweden,3 also implemented standardization schemes; all 3 standardization schemes use high-performance liquid chromatography as the reference method. Prior to global standardization (see below), the NGSP was the most widely used HbA1c standardization system in the world. The goal of the NGSP is to harmonize HbA1c results so that values reported by individual clinical laboratories are comparable with those reported in the DCCT (in the DCCT, all HbA1c measurements were performed in a central laboratory using a precise high-performance liquid chromatography method, thus avoiding standardization concerns between laboratories). Using a network of 10 laboratories in the United States, Europe, and Japan, the NGSP works with manufacturers of HbA1c
testing methods and certifies those methods that meet stringent accuracy criteria.4 In addition, the NGSP is part of the process by which laboratories that measure patient samples are evaluated to ensure they report accurate results. This is mandatory in the United States under the Clinical Laboratory Improvement Amendments of 1988 (CLIA’88). The NGSP has significantly reduced the variability in HbA1c testing.5 In 1995, the International Federation of Clinical Chemistry and Laboratory Medicine (IFCC) established a working group to standardize HbA1c, anchored on a metrologically sound international reference measurement system.6 The IFCC working group developed a reference method based on enzymatic cleavage of intact hemoglobin to obtain hexapeptides of HbA1c and nonglycated hemoglobin.6 The hexapeptides are separated by high-performance liquid chromatography and quantified by electrospray ionization–mass spectrometry or capillary electrophoresis. A network of 14 reference laboratories using the reference procedure assign HbA1c target values to reference materials, reference panels of blood samples, and control materials necessary for the reference measurement system.7 Results obtained with the new standardization system can be related to the previous harmonization initiatives and therefore, to the DCCT. The NGSP and other networks produce HbA1c values different from the IFCC method. This is attributable to the lack of specificity of the original high-performance liquid chromatography methods. Nevertheless, there is a linear correlation with the IFCC method, allowing reliable linear regression equations to be devised. These published equations7 can be used to calculate DCCT values from IFCC results and vice versa. Ongoing network comparisons demonstrate a stable relationship between NGSP and IFCC networks.7 Two important factors must be considered. The IFCC reference methods are not designed for routine testing of patient samples, but rather for setting standards for use by manufacturers producing routine methods. Moreover, the IFCC recommends that laboratories report results in SI units, namely mmol/mol rather than percent. Using the derived equation, an HbA 1c value of 7% would be 53 mmol/mol. Several countries in Europe and Asia have adopted SI units for HbA1c. Measurement accuracy reflects the closeness of a result to the true value and repeatability (or precision) is closeness of agreement of repeated measurements coefficient of variation. How precise and accurate should HbA1c measurements be? Recent guidelines recommend intralaboratory and interlaboratory coefficients of variation of less than 2% of the measured value and less than 3.5%, respectively.8 This is similar to the coefficient of variation of less than 2.9%, recommended for glucose measurement.8 At an HbA1c
jama.com
JAMA June 11, 2014 Volume 311, Number 22
Copyright 2014 American Medical Association. All rights reserved.
Downloaded From: http://jama.jamanetwork.com/ by a Western Oregon University User on 05/24/2015
2271
Opinion Viewpoint
concentration of 7.0% (which is the therapeutic target for the majority of patients with diabetes), 95% of the more than 3200 laboratories that participated in the 2014 College of American Pathologists survey use methods with between-laboratory coefficients of variation less than 3.5%.4 In general, blood tests should have sufficient accuracy to detect clinically relevant differences. The National Institute for Clinical Excellence recommends that treatment regimens for patients with diabetes should be evaluated based on a measured change in HbA1c of at least 0.5%. By 2010, 80% of laboratories used an HbA 1c method that could accurately distinguish a change of less than 0.5% HbA1c.5 Evaluation of HbA1c every 3 to 6 months (as recommended)2 will reduce measurement uncertainty. Methods to measure HbA1c continue to improve and variability will likely be reduced further. Aside from technical issues influencing HbA1c measurement, some clinical conditions may affect the ability of HbA1c to reflect temporally averaged blood glucose control. Formation of HbA1c is dependent on the hemoglobin circulating being predominantly HbA. There is a widely held misconception that HbA1c cannot be measured accurately in individuals with mutated Hb (also known as variant Hb). The prevalence of hemoglobinopathies (non-HbA) varies among countries, with HbS or HbC trait present in 10% of the 26 million US African Americans.9 More than 95% of instruments can accurately measure HbA1c in patients with these Hb variants; however, it is always advisable to discuss this with the laboratory. Similarly, by selecting the correct instrument, accurate HbA1c results can be obtained in patients with HbAE or HbAD.4,9 Some hemoglobinopathies alter erythrocyte survival, which will influence all HbA1c measurements. ARTICLE INFORMATION Conflict of Interest Disclosures: Both authors have completed and submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest and none were reported. Funding/Support: This work is supported in part by the Intramural Program of the National Institutes of Health. Role of the Sponsor: The Intramural Program of the National Institutes of Health had no role in the preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication. REFERENCES 1. The Diabetes Control and Complications Trial Research Group. The effect of intensive treatment of diabetes on the development and progression of
2272
Hemolytic anemia, from whatever cause, lowers HbA1c values because of reduced red blood cell survival. However, irondeficiency anemia can lead to an inappropriate increase in HbA1c of 1% to 1.5%, which declines after successful iron replacement. Thalassemia is an inherited disorder with decreased synthesis of one of the globin chains. Disease severity determines whether HbA1c reflects average glycemia. Mild disease may not alter erythrocyte lifespan, but for patients with more severe disease (especially if requiring transfusion), HbA1c measurements will not be reliable. Other patient factors that may influence HbA1c concentrations are age and race. The molecular mechanisms are poorly understood and the clinical relevance, if any, remains to be established. In addition to hemoglobin, other proteins in the blood are glycated. Albumin is the most abundant protein in plasma and tests to quantify glycated albumin (ie, fructosamine and glycated albumin) have been developed. However, these tests have not been evaluated in large clinical trials and no evidence validates their ability to predict complications. Importantly, no therapeutic targets for these tests have been widely adopted. Factors other than glucose (eg, change in blood albumin levels) also can alter glycated albumin concentrations. Another marker of long-term glycemia, 1,5anhydroglucitol, has similar problems, with limited evaluation in clinical studies, exclusion from use in certain patients (notably in renal impairment), and limited availability in clinical laboratories. In conclusion, HbA1c remains the only test that can predict the microvascular complications of diabetes and for which there are generally accepted therapeutic targets. HbA1c can be measured accurately in the majority of patients and provides valuable information to help guide treatment decisions.
long-term complications in insulin-dependent diabetes mellitus. N Engl J Med. 1993;329(14):977986. 2. American Diabetes Association. Standards of medical care in diabetes—2014. Diabetes Care. 2014;37(suppl 1):S14-S80.
6. Hoelzel W, Weykamp C, Jeppsson JO, et al; IFCC Working Group on HbA1c Standardization. IFCC reference system for measurement of hemoglobin A1c in human blood and the national standardization schemes in the United States, Japan, and Sweden: a method-comparison study. Clin Chem. 2004;50(1):166-174.
3. Berg AH, Sacks DB. Haemoglobin A1c analysis in the management of patients with diabetes: from chaos to harmony. J Clin Pathol. 2008;61(9):983987.
7. Weykamp C, John WG, Mosca A, et al. The IFCC Reference Measurement System for HbA1c: a 6-year progress report. Clin Chem. 2008;54(2):240-248.
4. National Glycohemoglobin Standardization Program. Harmonizing hemoglobin A1c testing. http://www.ngsp.org. Accessed May 20, 2014.
8. Sacks DB, Arnold M, Bakris GL, et al. Guidelines and recommendations for laboratory analysis in the diagnosis and management of diabetes mellitus. Clin Chem. 2011;57(6):e1-e47.
5. Little RR, Rohlfing CL, Sacks DB; National Glycohemoglobin Standardization Program (NGSP) Steering Committee. Status of hemoglobin A1c measurement and goals for improvement: from chaos to order for improving diabetes care. Clin Chem. 2011;57(2):205-214.
9. Bry L, Chen PC, Sacks DB. Effects of hemoglobin variants and chemically modified derivatives on assays for glycohemoglobin. Clin Chem. 2001;47(2): 153-163.
JAMA June 11, 2014 Volume 311, Number 22
Copyright 2014 American Medical Association. All rights reserved.
Downloaded From: http://jama.jamanetwork.com/ by a Western Oregon University User on 05/24/2015
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