EDUCATION :: DIABETES


Three things laboratories need to know While HbA1c testing has significantly advanced diabetes diagnosis, monitoring, and manage- ment, there are often under-recognized factors that can undermine testing accuracy. These include hemoglobin variants, red blood cell lifespan, and lipe- mia. Understanding these issues permits greater confi- dence in results for laboratorians and clinicians at the forefront of patient diabetes diagnosis and monitoring.


Hemoglobin variants The hemoglobin molecule is comprised of oxygen- carrying heme and four globin subunits. In normal adult hemoglobin, HbA, the globin subunits are two Į and two ȕ chains. Fetal hemoglobin (HbF), in contrast, is comprised of two ɲ and two Ȗ chains. Variant hemo- globins are those in which a germline DNA mutation results in a change in the amino acid sequence of a globin chain. These mutations may affect the glycation of hemoglobin, potentially impacting HbA1c results. The most common hemoglobin variants are HbS, HbE, HbC, and HbD, all of which involve the substitution of a single ȕ-chain amino acid.9


be present in patients with variant hemoglobins.10


HbF elevation may also In


some patients, the presence of a variant hemoglobin is clinically evident, so laboratorians and clinicians will likely recognize the limitations of HbA1c testing in those instances. Sickle cell anemia, for instance, affects one in 12 African Americans and one in 100 Hispanic Americans or Latinos,9


and pres-


ents with severe anemia and other symptoms. Hemoglobin E, on the other hand, is common in people from Southeast Asia,11


and may not be


clinically evident. A variant should be suspected if HbA1c results are inconsistent with results of blood glucose monitoring, as assays vary in their performance in patients with variant hemoglobins.10


Red blood cell lifespan In addition to variability in hemoglobin glycation, patients with variant hemoglobins may also experi- ence shortened red blood lifespan and increased red blood cell turnover. As noted above, in some patients the presence of a condition that results in increased red blood cell turnover is clinically evident, so labora- torians and clinicians may recognize the limitations of HbA1c testing in those instances. In other instances, increased red blood cell turnover may not be clini- cally evident or suspected, or the clinician may not be aware of the potential for discrepancy between blood glucose and HbA1c levels. Note that hemolytic ane- mias may be either congenital (such as thalassemias) or acquired (thrombotic thrombocytopenia purpura or autoimmune hemolytic anemia). Thalassemias are a type of hemoglobinopathy that may occur together with a variant hemoglobin, further complicating the clinical picture.11


In a world that is becoming increasingly migratory, it is helpful for laboratorians to stay mindful of the makeup of patient populations in their regions. Those patients with conditions or variants that affect red blood cell turnover or hemoglobin may require alter- native tests. Information is available from the National Glycohemoglobin Standardization Program.12


54 JULY 2019 MLO-ONLINE.COM


Finally, any red blood cell transfusion for any reason may affect HbA1c results.13


Again, discrep-


ancies between HbA1c and blood glucose monitor- ing should be investigated, and a blood transfusion history established.


Lipemia


Lipemia refers to an overabundance of emulsi- fied fat, or lipids, in the blood. Some individu- als have high baseline lipid levels—typically hypertriglyceridemia—but postprandial lipemia is also seen in otherwise healthy patients following the consumption of a high-fat meal. Any assay that relies on a turbidimetric readout, including some HbA1c assays, may be affected by high levels of lipemia, as these assays rely on changes in light transmission through the sample. Importantly, although clinicians are generally aware of the need to run certain tests like fasting glucose and lipid profiles on fasting or, at least, non-postprandial samples, they are not nec- essarily aware of the potential for interference with turbidometric assays and are unlikely to know which assays are turbidometric. Recent research suggests that enzymatic and immu- noassay HbA1c assays can be vulnerable to lipemia interference.14


The choice of assay is driven by the


workflow needs of the laboratory as well as the patient population. Immunoassays are popular due to their relatively low cost and ease of incorporation into the remainder of laboratory testing, but as assays vary in their susceptibility to interference from vari- ant hemoglobins, laboratories may also consider the expected frequency of the various common variant hemoglobins in their community.12


Investigations of discrepancies between HbA1c and glucose monitoring should include an assessment of a particular assay’s susceptibility to lipemia interfer- ence, and, potentially, measurement of patient tri- glyceride levels. Postprandial lipemia may be avoided by advising patients to avoid a high-fat meal prior to phlebotomy.


Summary


HbA1c testing is an important tool in the fight against diabetes. As with all in vitro diagnostic tests, preanalytic and analytic issues can have an impact on accuracy. For HbA1c assays, issues to consider include hemoglobin variants, red blood cell turnover, and lipe- mia. Clinicians and laboratorians can work together to help ensure the accuracy of results, strengthening confidence in patient care decisions.


Please visit mlo-online.com for references.


Jeannine T. Holden, MD, serves as chief medical officer and vice president of medical and scientific affairs for Beckman Coulter Diagnostics.


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62  |  Page 63  |  Page 64  |  Page 65  |  Page 66  |  Page 67  |  Page 68  |  Page 69  |  Page 70  |  Page 71  |  Page 72