HbA1c may underestimate glucose exposure in sickle cell trait
medwireNews: Black people with sickle cell trait (SCT) have lower glycated hemoglobin (HbA1c) levels than those without at the same blood glucose level, shows a study in JAMA.
“These results could have clinically significant implications,” say Mary Lacy (Brown University School of Public Health, Providence, Rhode Island, USA) and co-researchers.
“As a screening tool, an HbA1c value that systematically underestimates long-term glucose levels may result in a missed opportunity for intervention.”
The study included 4620 Black participants of the CARDIA and Jackson Heart Study cohorts. The 7.9% who had SCT had an average HbA1c of 5.7%, compared with 6.0% in participants without SCT, despite there being no significant difference in concurrently measured 2-hour glucose levels, at 118.5 and 113.0 mg/dL, respectively.
Indeed, the HbA1c level for any given blood glucose level was significantly lower in participants with than without SCT for both fasting glucose and 2-hour glucose tests, and after accounting for confounders. And the team notes that the accuracy of the HbA1c assay used in both cohorts is reportedly unaffected by the presence of SCT.
As a consequence, HbA1c was 70% accurate for detecting fasting glucose-defined prediabetes or diabetes in participants with SCT, compared with 77% in those without, with corresponding accuracies of 60% and 74% for prediabetes or diabetes defined from 2-hour glucose levels.
Writing in an accompanying editorial, Anthony Bleyer and Joseph Aloi, both from Wake Forest School of Medicine in Winston-Salem, North Carolina, USA, say: “It would appear beneficial for black patients with diabetes mellitus to be tested for SCT and have the results included in the medical record.”
They add: “Several studies have shown that many black patients are uncertain of their SCT status.”
However, they also highlight mounting evidence of an altered relationship between HbA1c and blood glucose in Black people without SCT relative to White people – an issue that potentially affects a much larger number of patients than the problems caused by SCT.
Because HbA1c appears to be higher in Black than White people for any given glucose level, Black people may be overdiagnosed based on HbA1c measurements, and may also run a higher risk for hypoglycemia if intensive glycemic control is attempted, the editorialists note.
“At higher HbA1c levels, patients uniformly meet diagnostic thresholds and require treatment,” they conclude. “In contrast, as the HbA1c level approaches the diagnostic threshold of 6.5%, physicians should consider that black race and presence of SCT may affect the HbA1c measurement and consider performing oral glucose tolerance testing.”
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