Spotlight on limitations of the HbA1c test
A recent position statement called for use of one-hour glucose tolerance testing for diabetes diagnosis, a review described limitations of point-of-care HbA1c testing, and a study identified differences in the relationship between fasting glucose and HbA1c level by race, age, and sex.
Several recent publications delved into the limitations of HbA1c testing in the diagnosis and management of diabetes.
A position statement from the International Diabetes Federation called for more use of one-hour oral glucose tolerance testing. The statement, published by Diabetes Research and Clinical Practice on March 6, offered evidence that a plasma glucose result of 155 mg/dL (8.6 mmol/L) or above one hour after a 75-gram glucose load is highly predictive for detecting progression to diabetes, risk of micro- and macrovascular complications, liver disease, and mortality in patients with risk factors. A level of 209 mg/dL (11.6 mmol/L) or above is diagnostic of diabetes, it noted. Using one-hour glucose testing instead of fasting plasma glucose or an HbA1c level alone "provides an opportunity to avoid misclassification of glycaemic status" and "allows early detection of high-risk people for intervention to prevent progression" to diabetes, the statement says. "The substantive data presented in the Position Statement provides strong evidence for redefining current diagnostic criteria" for prediabetes and diabetes, according to the international group of authors.
A review article, published by Diabetes Care on March 29, offered caveats and considerations on point-of-care (POC) HbA1c testing. The review described how standardization of A1C testing was achieved and summarized advantages and disadvantages of POC testing. It recommended that practices using this technology implement in-depth personnel training, with written policies and procedures and documented quality control testing at recommended intervals. Clinicians who order the POC test should be aware of its limitations, including potential interference by hemoglobin variants, lower accuracy than laboratory HbA1c testing, and known bias in the measurements. "Making cut point–based diagnoses or treatment decisions based solely on POC A1C results should be avoided, and a more holistic view of glycemia should be obtained with additional data," the review said. It concluded that POC testing is convenient and, on average, can lead to similar glycemic control as traditional testing methods, as long as clinicians are "considering the limitations of these devices and following correct testing procedures to ensure that accurate A1C results are obtained for optimal care of patients."
The final study, funded by Abbott Diabetes Care and published by BMJ Open Diabetes Research & Care on March 4, looked at factors affecting the relationship between fasting plasma glucose (FPG) and HbA1c level. It used both of these lab values from 12,531 participants in the National Health and Nutrition Examination Survey to calculate the apparent glycation ratio of red blood cells. The study found that at a FPG of 7 mmol/L (126 mg/dL), Black patients had a significantly higher HbA1c than White patients (50.2 vs. 47.4 mmol/mol). Patients under age 21 years had lower HbA1c than older patients (47.9 vs. 48.3 mmol/mol) and men had lower levels than women (47.0 vs. 49.2 mmol/mol). Researchers calculated that, at the same FPG, there would be a 0.46% difference in HbA1c level between Black women over age 50 years and White men under age 21 years. "Therefore, a single HbA1c alone is insufficient to give an accurate reflection of average glycemia across diverse populations resulting in some cohorts being underdiagnosed while others are being incorrectly labeled as having diabetes and consequently started on unnecessary therapies," the study authors wrote. "Careful attention should be given to older adults of black origin who are at higher risk of incorrect diagnosis of diabetes with the sole use of HbA1c."