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Articles |
1
Departments of Child Health and Pathology, University of Missouri School of Medicine, 1 Hospital Dr., Columbia, MO 65212.
2
Centers for Disease Control and Prevention, Division of Laboratory Sciences, National Center for Environmental Health (F25), 4770 Buford Hwy. NE, Chamblee, GA 30341-3724.
3
Brigham & Women’s Hospital, Department of Pathology, Harvard Medical School, Thorn 430, 75 Francis St., Boston, MA 02115.
aAddress correspondence to this author at: Department of Child Health, Room M767, University of Missouri School of Medicine, 1 Hospital Dr., Columbia, MO 65212. Fax 573-884-8823; e-mail LittleR{at}health.missouri.edu.
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Abstract |
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 Top Abstract IntroductionThe NGSPChanges in Certification...Progress in StandardizationUse of DCCT-Traceable ResultsInternational StandardizationSummaryReferences |
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Approach: We review the NGSP process and summarize progress in standardization through analysis of CAP data.
Content: Since 1996, the number of methods and laboratories certified by the NGSP as traceable to the DCCT has steadily increased. CAP GH2-B survey results reported in December 2000 show marked improvement over 1993 data in the comparability of GHB results. In 2000, 90% of surveyed laboratories reported GHB results as hemoglobin A1c (HbA1c) or equivalent, compared with 50% in 1993. Of laboratories reporting HbA1c in 2000, 78% used a NGSP-certified method. For most certified methods in 2000, between-laboratory CVs were <5%. For all certified methods in 2000, the mean percent HbA1c was within 0.8% HbA1c of the NGSP target at all HbA1c concentrations.
Summary: The majority of laboratories in the US are now reporting results that are traceable to DCCT/UKPDS outcomes.
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Introduction |
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TopAbstractIntroductionThe NGSPChanges in Certification...Progress in StandardizationUse of DCCT-Traceable ResultsInternational StandardizationSummaryReferences |
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The correlation between GHB and outcome risks demonstrated in the DCCT and later in the UKPDS underscore the need to measure GHB with high precision and in such a manner that results can be directly related to these studies and therefore to outcome risks. However, in 1993, at the end of the DCCT, GHB assay methods were not standardized among methods or laboratories, making it difficult for physicians and patients to relate test results to DCCT-based treatment goals.
Because of the positive impact that standardization of GHB determinations would have on the care of diabetic patients, and in anticipation of the report of DCCT results, the AACC Standards Committee established a GHB standardization subcommittee in April 1993. The goal of the subcommittee was to develop a plan for GHB standardization that would ultimately allow individual clinical laboratories to relate their GHB assay results to those from the DCCT. This would allow individual clinical laboratories to provide diabetic patients and their healthcare providers with test results that could be related directly to risks for development and/or progression of diabetes-related chronic complications.
Although the DCCT was completed in 1993, the GHB assay systems from the study remained in place as part of another National Institutes of Health-sponsored long-term diabetes study called the Epidemiology of Diabetes Interventions and Complications (EDIC) (6). To initiate a standardization program in a timely fashion, the AACC subcommittee on GHB standardization recommended that the DCCT reference method be used as the designated comparison method for GHB standardization while studies were being performed to evaluate candidate definitive and reference methods and to develop purified GHB standards.
Early efforts to standardize GHB values among clinical laboratories by use of a "universal calibrator" were feasible with some assay methods (7)(8). Later studies (9)(10) showed, however, that such an approach did not work for several existing methods. It was found that materials prepared for use as calibrators, quality-control materials, and proficiency-testing samples were often subjected to preparative processes that could cause them to yield results that differed appreciably from those of patient specimens, i.e., matrix effects (10). However, if appropriate calibrators or conversion equations were used for each method, calibration was feasible with all methods (11). Therefore, because an important goal was to allow standardization of all assay methods regardless of the methodology used, the GHB standardization subcommittee proposed that standardization to the DCCT reference could be performed best at the manufacturing level, where the most appropriate materials and standardization format for each method could be determined. Verification of method traceability was to be based on fresh sample comparisons with the Designated Comparison Method.
Once the development of a standardization protocol was completed, the subcommittee was dissolved and the National Glycohemoglobin Standardization Program (NGSP) was organized to implement the protocol developed by the AACC subcommittee. The NGSP approach to standardization of GHB was modeled after the Cholesterol Reference Method Laboratory Network program (12), which provides a means for manufacturers to establish traceability to the National Reference System for Cholesterol via split sample comparisons. Here we describe the NGSP, changes that were made in the protocol based on certification and proficiency testing (PT) data during first 3 years of implementation, and the resulting improvements seen in GHB measurements as indicated by PT data.
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The NGSP |
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TopAbstractIntroductionThe NGSPChanges in Certification...Progress in StandardizationUse of DCCT-Traceable ResultsInternational StandardizationSummaryReferences |
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outlines the basic components of the NGSP and the way in which they interact with each other as well as with manufacturers and clinical laboratories.
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steering committee
The Steering Committee works with the Laboratory Network to implement the GHB Standardization Program according to the protocol. The committee is responsible for reviewing policy/protocol changes and certification data submitted by the Laboratory Network.
laboratory network
The Laboratory Network consists of an Administrative Core (NETCORE), a Central Primary Reference Laboratory (CPRL), back-up Primary Reference Laboratories (PRLs) in the US and in Europe, and Secondary Reference Laboratories (SRLs) in the US and in Europe. By mid-1996, a Laboratory Network consisting of a CPRL, PRLs, and SRLs was established; monitoring of each network laboratory began in May 1996. SRL methods now include ion-exchange HPLC, affinity HPLC, capillary electrophoresis, and immunoassay (see list of network laboratory participants in the Appendix).
NETCORE.
The Administrative Core coordinates the certification process and communicates directly with the Steering Committee. The Core analyzes all certification and network monitoring data, sends certification and monitoring data to the Steering Committee, and issues certificates to laboratories and manufacturers after review of the data by the Steering Committee.
CPRL.
The CPRL analyzes GHB (HbA1c) by HPLC using Bio-Rex 70 resin and according to the existing CPRL method protocol used throughout the DCCT (13) and sets the initial calibration for the standardization program based on the "set-point" used in the DCCT. Although the shortcomings of this particular method are well known with regard to its specificity (e.g., interference from HbF, variant hemoglobins, and nonglucose adducts) and low throughput, its long-term stability was the primary factor in its selection as the anchor or point of reference for the NGSP (Fig. 2
). The CPRL method has shown long-term CVs of <3% throughout and after the DCCT. Backup PRLs and SRLs calibrate their assays so that results from fresh blood specimens agree with the CPRL. The CPRL provides comparison data for certification of network laboratories and administers a monitoring program for all certified PRLs and SRLs.
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PRLs.
PRLs serve as back-up laboratories for the CPRL to ensure that the CPRL function would continue without interruption in the event that the CPRL could not meet the program needs. All PRLs analyze GHB (HbA1c) by the Designated Comparison Method according to the CPRL method protocol. PRLs must be certified as traceable to the CPRL method (see discussion of criteria below).
SRLs.
SRLs work directly with manufacturers to assist them in standardizing their methods and in providing comparison data for certification of traceability. Each SRL analyzes GHB by a precise method (documented total imprecision 
3%) that is calibrated to the Designated Comparison Method. Although most SRLs use methods that are commercially available, SRL methods are specifically calibrated for the NGSP to match the CPRL. SRLs must be certified as traceable to the CPRL method according to specific, strict criteria (see "Certification Criteria" below). In addition, the precision and bias of network laboratories are evaluated monthly to ensure that they are within specific limits of the CPRL (see "Performance Monitoring of Network Laboratories" below).
certification of network laboratories
A proposed network laboratory (PRL or SRL) must perform precision testing according to NCCLS EP5-A guidelines (14) and participate in a fresh-blood sample comparison (n = 100) for estimation of bias from the CPRL according to modified NCCLS EP9-A guidelines (15).
performance monitoring of network laboratories
Monthly surveys are administered by the CPRL to monitor performance of the network PRLs and SRLs. Each month, the CPRL ships 10 fresh whole-blood specimens over the desired clinical range (4–14% HbA1c) to each network laboratory within 2 days of collection. The CPRL, PRLs, and SRLs analyze the specimens in two separate analytical runs on 2 separate days. Data analysis includes a measure of precision (SD of the differences in replicates) and bias (mean of the differences between the network laboratory and the CPRL). An example of one SRL monthly monitoring report showing precision and bias over a 12-month period is shown in Fig. 3
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ngsp protocol
The NGSP Laboratory Network interacts with laboratories and manufacturers through three different processes: calibration, certification, and PT (Fig. 1
).
Calibration.
The process by which a method is standardized so that results are comparable to those of the NGSP Network is an informal one and is determined by the manufacturer. The process depends on the specific method, e.g., calibration may be accomplished by assignment of calibrator values or by a conversion equation.
Certification.
Certification is a formal process consisting of assessments of both precision and bias from the NGSP (DCCT equivalent results). Both manufacturers and clinical laboratories can participate in this process. Precision testing follows NCCLS EP5-A guidelines (14). Method comparison for bias estimation is done with fresh (or fresh frozen) blood samples (n = 40), with duplicate analysis by the manufacturer or laboratory being certified and a SRL. Methods and laboratories that pass NGSP certification criteria are awarded a certificate of traceability to the DCCT.
PT.
The ADA recommends that all laboratories participate in a PT program administered by the College of American Pathologists (CAP) that uses fresh blood samples (16). Unlike other GHB PT surveys provided in the US, the CAP GH2 survey provides pooled fresh blood specimens at three GHB concentrations with target values assigned by the NGSP Network (mean of results from NGSP SRLs). Matrix effects are avoided by use of fresh blood, and sample deterioration is minimized by shipment on cold packs within 1 week of collection. The NGSP value assignment allows laboratories to directly compare their results to those of the NGSP/DCCT as well as to other laboratories that use the same method. PT data are used to assess the effectiveness of the standardization program by (a) estimation of bias from the target (e.g., by laboratory, by method, by method type) and by (b) interlaboratory comparability within and between method peer groups.
certification criteria
Manufacturers and laboratories are awarded Certificates of Traceability to the DCCT for successfully completing precision testing and fresh sample comparisons for the specific reagent lots, calibrator lots, and instrumentation used. Final certification of traceability is issued by the NETCORE after review of the data by the Steering Committee. It is recommended that manufacturers recertify their methods annually. Current certification criteria are described below.
Network laboratory certification.
PRLs and SRLs must fulfill the following criteria for network laboratory certification: (a) total imprecision (CV) must not be significantly (
2 analysis) >3%; (b) no more than one within-method and one between-method outlier is acceptable; and (c) the 95% confidence interval (CI) for predicted bias should overlap the ± 3% range of the CPRL at two GHB concentrations (6% and 9% HbA1c). All data analyses are performed by the NETCORE, according to NCCLS EP5-A (precision) and modified EP9-A (bias) guidelines.
Manufacturer and level II laboratory certification.
For a commercial method or a laboratory to be considered traceable to the DCCT (a) total imprecision (CV) must not be statistically significantly >5% (14) and (b) the 95% CI of the differences between methods (test method vs SRL) must fall within the clinically significant limits of ± 1% GHB (17). Outliers (more than the mean + 3 SD of differences between pairs) are identified but used for investigational purposes only, and not to pass or fail a method.
Level I laboratory certification.
This type of certification, which includes both yearly certification and quarterly monitoring, is recommended for large laboratories that are involved in research studies or clinical trials where long-term precision of GHB measurement is critical. The certification process is the same as for manufacturers, but the precision and bias criteria are more stringent: (a) total imprecision (CV) must not be statistically significantly >3% (14), and (b) the 95% CI of the differences between methods (test method vs SRL) must fall within ± 0.75% GHB (17). The quarterly monitoring process and criteria are identical to those used for monthly network laboratory monitoring.
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Changes in Certification Criteria in December 1999 |
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TopAbstractIntroductionThe NGSPChanges in Certification...Progress in StandardizationUse of DCCT-Traceable ResultsInternational StandardizationSummaryReferences |
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shows certification data from two methods that passed NGSP certification using modified NCCLS EP-9 guidelines. In both cases, the methods show little bias from the NGSP network laboratory, and both would pass the old bias criteria. However, one method shows considerable scatter around the regression line. Because NCCLS EP9-A requires only that the 95% CI overlap the decision limit, which is ± 5% of the network laboratory result at 6% and 9% HbA1c, the amount of scatter in individual results is not taken into account. In fact, it is easier for methods that show more scatter around the regression line to pass the bias criteria because this widens the 95% CI of the predicted bias, making it easier to overlap the 5% limit.
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The NGSP addressed this problem by changing the bias criteria for certification of manufacturer and level I and II laboratories from estimation of bias according to NCCLS EP9-A (15) to assessment of agreement according to Bland and Altman, indicated in Table 1
(17). The method shown in Fig. 4B
would fail the new assessment of agreement criteria. In addition, the outlier criteria were changed slightly and were no longer used to pass or fail a method (Table 1
).
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Progress in Standardization |
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TopAbstractIntroductionThe NGSPChanges in Certification...Progress in StandardizationUse of DCCT-Traceable ResultsInternational StandardizationSummaryReferences |
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diabetes/ngsp.html).
cap results
In 2000, based on CAP GH2 2000B summary report data (18), 90% of laboratories reported results as HbA1c or equivalent, compared with only 50% in 1993 when the DCCT ended (19) and 66% in 1996 when the NGSP first began. Of those laboratories reporting results as HbA1c, 78% (70% of the total) used a NGSP-certified method. The mean, high, and low values for each method, compared with the DCCT/NGSP target values in 1993 and 2000, are shown in Fig. 5
. Compared with results in 1993, many more methods and laboratories reported results that were close to the DCCT target. For most certified methods in 2000, between-laboratory CVs were <5%. For all certified methods in 2000, the mean %HbA1c was within 0.6%, 0.5%, and 0.8% HbA1c of the NGSP target at the low, middle, and high HbA1c, respectively (CAP 2000 GH2-B). Overall, NGSP-certified methods are more precise and consistent among laboratories.
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Use of DCCT-Traceable Results |
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TopAbstractIntroductionThe NGSPChanges in Certification...Progress in StandardizationUse of DCCT-Traceable ResultsInternational StandardizationSummaryReferences |
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Despite the use of many different GHB assay methodologies, 70% of laboratories are currently using NGSP-certified methods and are thus reporting DCCT-traceable results (18). DCCT-traceable results are now being used on a national level in the US as a means of measuring the quality of diabetes care and educating people with diabetes and their healthcare providers. For example, HbA1c is 1 of 11 variables used by the ADA to assess diabetes care for its Provider Recognition Program. For this program, the frequency of HbA1c measurement and the proportion of patients with HbA1c <8% and <10% (by a NGSP-certified method) is assessed. The Diabetes Quality Improvement Project "accountability measures" include the percentage of patients receiving one or more GHB tests per year and the percentage of patients with HbA1c >9.5% (those at highest risk for complications); these measures were subsequently adopted by the National Committee for Quality Assurance. More recently, a consensus statement from the American Medical Association, the Joint Commission on Accreditation of Healthcare Organizations, and the National Committee for Quality Assurance recommended the use of Diabetes Quality Improvement Project measures and use of certified methods. The National Diabetes Education Program will promote the ADA goals for diabetes treatment through the development and implementation of diabetes awareness and education activities. Standardization of GHB to an outcomes-based reference system is critical to these processes.
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International Standardization |
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TopAbstractIntroductionThe NGSPChanges in Certification...Progress in StandardizationUse of DCCT-Traceable ResultsInternational StandardizationSummaryReferences |
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Summary |
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TopAbstractIntroductionThe NGSPChanges in Certification...Progress in StandardizationUse of DCCT-Traceable ResultsInternational StandardizationSummaryReferences |
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Appendix |
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Acknowledgments |
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Footnotes |
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References |
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TopAbstractIntroductionThe NGSPChanges in Certification...Progress in StandardizationUse of DCCT-Traceable ResultsInternational StandardizationSummaryReferences |
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R. Hanas Psychological Impact of Changing the Scale of Reported HbA1c Results Affects Metabolic Control Diabetes Care, November 1, 2002; 25(11): 2110 - 2111. [Full Text] [PDF]
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P. Luraschi, S. Brambilla, R. Mozzi, G. Cattozzo, and C. Franzini Monitoring Analytical Quality in Routine Glycohemoglobin Measurements Clin. Chem., September 1, 2002; 48(9): 1594 - 1597. [Full Text] [PDF]
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L. D. Youngman, S. Clark, S. Manley, R. Peto, and R. Collins Reliable Measurement of Glycated Hemoglobin in Frozen Blood Samples: Implications for Epidemiologic Studies Clin. Chem., September 1, 2002; 48(9): 1627 - 1629. [Full Text] [PDF]
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T Lahousen, R E Roller, R W Lipp, and W J Schnedl Silent haemoglobin variants and determination of HbA1c with the HPLC Bio-Rad Variant II J. Clin. Pathol., September 1, 2002; 55(9): 699 - 703. [Abstract] [Full Text] [PDF]
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D. B. Sacks, D. E. Bruns, D. E. Goldstein, N. K. Maclaren, J. M. McDonald, and M. Parrott Guidelines and Recommendations for Laboratory Analysis in the Diagnosis and Management of Diabetes Mellitus Clin. Chem., March 1, 2002; 48(3): 436 - 472. [Abstract] [Full Text] [PDF]
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), HbA1 (
), and total GHB (•).