QA/ QC aspects of HbA1c analysis
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Slide 1 :
Trefor Higgins FCACB, Director of Clinical Chemistry Dynacare Kasper Medical Laboratories Edmonton, Alberta QC and QA Aspects of HbA1c Testing
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Outline Definition Formation QC Expectations Reality QA tools NACB Guidelines Calculated A1c Peak shape ? checks Hand approach
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Definitions of HbA1c Original Definition: HbA1c: a peak on a chromatogram – reference range 4 – 6% IFCC Definition: HbA1c: Hb that is irreversibly glycated at one or both N-terminal valines of the ? chain - reference range 2 - 4%
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Uses of HbA1c Analysis Monitor glycemic control patient management clinical guidance and audit clinical trial design Independent risk factor for MI in non-diabetic patients Estimate of recovery from infection in ICU
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Formation of HbA1c
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HbA1c Formation Non-enzymatic 2 Stage reaction First reaction is 100 times faster than second reaction HbA1c formation is irreversible
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2002 National Academy of Clinical Biochemistry Guidelines Only methods certified by NGSP should be used Participation in CAP glycohemoglobin program Interassay CV < 5% (ideally < 3%) 2 control materials with different mean values Verify values below lower limit of reference interval by repeating testing Verify values above 15% by repeat testing Schiff base (labile HbA1c) should be removed before assay Reference range 4 - 6%
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QC Aspects Precision Requirements for HbA1c assay 2002 NACB Guidelines Skeie’s approach patient expectations physician response Kolatkar’s approach Phillipou’s approach Physician expectation
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Precision Requirements for HbA1c Methods 2002 NACB Guidelines Clin Chem 2002; 48: 436 – 472 Interassay CV 5% (ideally 3%)
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Precision Requirements for HbA1c Methods Kolatkar et al Clin Chem 1994; 40: 1608-9 Intensive diabetes management requires very precise testing of glycohemoglobin . CV should be in the range of 2 - 3%.
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Precision Requirements for HbA1c Methods Skeie et al. Clin Chem 2001; 47: 1212-7 Patients were asked to indicate change in HbA1c from 9.4% that they considered a true (real) change; from their responses we derived patient-derived quality specifications for HbA1c. Conclusion: HbA1c increasing CV 3.1% decreasing CV 3.2%
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Skeie’s Approach (2) Skeie et al. Clin Chem 2005; 51: 1145 - 53 Asked physicians to assess impact of rising/falling HbA1c value on their method of treatment . Physicians expect CV 3-5%.
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Physician Expectations: Diabetic endocrinologists were asked to state their expectations of reproducibility and significant differences between HbA1c results. The conclusion was that an analytical variation > 3% could give clinically significant HbA1c results. Precision Requirements for HbA1c Measurements
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Precision Requirements of HbA1c Methods Phillipou’s approach. Clin Chem 1993; 39: 2305 - 8 Based on biological variation of 1.7% Practical working CV of 3%
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Summary of Precision Requirements for HbA1c Measurements 1. NACB: ideally < 3% (<5% tolerated) 2. Kolatkar: CV should be < 2 - 3%. 3. Skeie: Patient-derived specifications 3.1% (HbA1c increasing) 3.2% (HbA1c decreasing) 4. Skeie: Physician response 3-5%. 5. Physician: preferably CV of < 3%. 6. Phillipou: practical CV of 3%.
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Critical Differences for HbA1c at Different Analytical CVs at a HbA1c of 7%
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Seasonal Variation in HbA1c Studies in the UK showed HbA1c is higher in January and February and lower in October. Possibly due to amount of food eaten
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Requirements vs. Reality for Precision on HbA1c Testing CAP/Healthmetrix data Cembrowski approach Tran’s approach
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Performance of HbA1c Methods on CAP Program
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Precision Requirements for HbA1c Methods Cembrowski’s Approach: Diabetic Technology and Therapeutics 2003; 5: 975-8. Took replicate measurements of HbA1c from patients using different methods.
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Average SD of HPLC (diamonds) and immunoassay (squares) vs. time between testing. The y-intercept represents the average analytic SD, while graphs represent both analytic and physiologic variation. Precision Requirements for HbA1c Measurements
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Precision Requirements for HbA1c Measurements Cembrowski’s Conclusion: Precision for different methods vary substantially but some are acceptable (defined as 3% or less).
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Precision Requirements for HbA1c MeasurementsDelta Checking HbA1c Values Tran et al - 33,000 differences from analyses of 150,048 HbA1c results 3 HPLC analyzers
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Tran’s Conclusion At a HbA1c of 7.16%, the CV is 3.7%. This variation is remarkably low given that HbA1c were measured over 2 years on any one of 3 analyzers.
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QA Aspects of HbA1c Analysis NACB guidelines for repeat testing – same v different analytical method monitoring of peak shapes calculated v measured HbA1c low hemoglobin Hb variants/HbF ? checks
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NACB recommendations for repeat testing: below reference interval above 15% Use of same method will give same results: review CBC data use method based on different principle QA – NACB Guidelines
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56 y f with fasting glucose 5.4 mmol/L, normal CBC HbA1c Results: Presence of ? chain variant interference on Bio-Rad VARIANT method
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Chromatograms of patient with interfering ? chain variant on: HbA1c program ? thalassemia program
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Five Steps in Identifying Presence of Variant - IRRCR Inspect chromatogram Repeat on same analyzer type Review cbc data Calculate HbA1c Repeat on analyzer based on alt. technology
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On HPLC, changes in retention time and peak shape may indicate the presence of a hemoglobin variant. Monitoring Peak Shape
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HbA1c in the presence of a hemoglobin variant has 2 problems: 1) the analytical problem 2) the physiological problem
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Interferences we can see: hemolysis - K+, glucose, LD lipemia - many chemistries icterus - acetaminophen Interferences we cannot see: ascorbic acid - glucose, cholesterol, triglyceride, uric acid methods heterophile antibody - all immunoassays In general, we do not have a clue if these are present in a sample. Hemoglobin Variant Interference - Analytical
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What ascorbic acid is to chemistry measurements and heterophile antibody is to immunoassay methods, hemoglobinopathies are to HbA1c analysis. Hemoglobin Variant Interference - Analytical
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The Analytical Problem - HPLC a) variant co-elutes with HbA - glycated species co-elute b) variant co-elutes with HbA – glycated species (if any) does not c) variant and HbA inadequately resolved
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The Analytical Problem - Immunoassay a) affinity of antibody for variant b) steric hindrance
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Error in HbA1c by Immunoassay Patient 28 y f with known C trait At HbA1c of 6%, Variant II bias is +0.82% (+0.85 at 7%). Little (Clin Chem 2004; 50S6; D-12) Bias from true HbA1c value of 6.15%.
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The Physiological Problem Normal red cell survival: 120 days ? thalassemia reported at 90 – 96 days Hemoglobin variants have decreased life cycle (HbS reported at 90 days) Preferential expression of normal HbA compared to variant
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NGSP requires bias from HbC or HbS trait of < 10% to be not clinically significant. Bio-Rad VARIANT II meets this criterion. Higgins et al. Clin Chem 2006; 52(s): C-88 Mongia et al. Presented at Academy of Clinical Laboratory Physicians and Scientists 2006 (Abstract # 2006-58)
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Effect of Presence of HbF % HbA1c vs. IFCC Reference Method Bias Plot - – samples with elevated HbF Effect of Elevated Fetal Hemoglobin on HbA1c Measurements: Five Common Assay Methods compared to the IFCC Reference Method. C. Rohlfing, S. Connolly, J. England, R. Little. Dept. of Pathology & Anatomical Sciences, U of Missouri School of Medicine, Columbia MO.
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Effect of Presence of HbF Effect of Elevated Fetal Hemoglobin on HbA1c Measurements: Five Common Assay Methods compared to the IFCC Reference Method. C. Rohlfing, S. Connolly, J. England, R. Little. Dept. of Pathology & Anatomical Sciences, U of Missouri School of Medicine, Columbia MO. Bias vs. IFCC RM
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Conclusions The G7 (when the HbF peak is properly recognized) and Variant II methods show minimal interference in HbA1c measurement up to HbF of ~30%. The DCA 2000, 2.2+ and CLC330/385 methods underestimate HbA1c in the presence of elevated HbF, especially at HbF levels > 20%. Physicians need to be aware of potential interferences when interpreting HbA1c results. Effect of Elevated Fetal Hemoglobin on HbA1c Measurements: Five Common Assay Methods compared to the IFCC Reference Method. C. Rohlfing, S. Connolly, J. England, R. Little. Dept. of Pathology & Anatomical Sciences, U of Missouri School of Medicine, Columbia MO.
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Interference from HbF – DKML initial results
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Limitations of HbA1c Analysis “Hemoglobinopathies may interfere with GHb analysis independent of their effects on erythrocyte survival.” “Moreover, hemoglobin variants cannot be identified by immunoassay.” Sacks DB. Clin Chem 2003; 49: 1245-7.
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The Effect of Low Hemoglobin A positive bias occurs in samples with low Hb using the Tina-quant II HbA1c immunoassay. Clin Biochem 2004: 37; 735 (51)
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Relationship of Glucose to HbA1c NGSP (glucose) = 1.98 (HbA1c) – 4.29 IFCC (glucose) = 1.84 (HbA1c) – 0.01 NGSP – DCCT method
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ADA Guidelines Treatment goal normal or near normal with HbA1c < 7%. Correlation between HbA1c and mean plasma glucose levels.
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Use of Calculated HbA1c ADA equation rearranged: HbA1c (calculated) = 0.5 (glucose) + 2.25 (SI units) HbA1c (calculated) = 0.0285 (glucose) + 2.15 (traditional units)
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Calculated HbA1c Fasting HbA1c (calculated) = 0.90 HbA1c (measured) – 0.14 (r = 0.733) 2 hr pc HbA1c (calculated) = 1.65 HbA1c (measured) – 4.52 (r = 0.83) Random HbA1c (calculated) = 1.39 HbA1c (measured) – 3.13 (r = 0.68)
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Use of Calculated HbA1c Best agreement found with HbA1c calculated from fasting glucose. Poorest agreement found with HbA1c calculated from random glucose.
Slide 52 :
ADA/CDA Recomendations for Measurement of HbA1c Frequency at least: 2 times/year in patients meeting treatment goals or with stable glycemic control 4 times/year in patients whose therapy has changed or who are not meeting glycemic controls
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HbA1c in Edmonton Region Compared absolute differences in HbA1c values 3 months (+/- 2 weeks apart).
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Conclusions Hemoglobin variants can challenge analytical methods. There are new QA tools that may be used. HbF interference +/-. Hb concentration may be a significant fact in immunoassay measurements. Imprecision of 3% desirable.
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