Either fasting plasma glucose (FPG) or hemoglobin A1c (HbA1c or A1c) can identify people at elevated risk for developing diabetes within the next five years, but together the two tests better predict excess risk than does either one alone, Japanese researchers report.
While fasting glucose has been the long-standing diabetes screening and diagnostic test, the American Diabetes Association (ADA) in 2010 endorsed A1c for the same purposes. Previously, A1c had been used to monitor control of diabetes. Now an A1c of more than 6.5%, like a fasting plasma glucose level of more than 126 mg/dl, is considered diagnostic for diabetes. An A1c between 5.7% and 6.4%, or FPG between 100 and 125 mg/dL, indicates increased risk of development of diabetes, sometimes called "prediabetes."
Writing in the July 9 issue of The Lancet, the researchers note some controversy surrounding use of A1c as a diagnostic tool. Part of this is because many diagnosed with prediabetes by FPG do not meet the ADA's A1c criteria for the condition, leading many to fear that A1c testing alone may miss many cases.
The researchers sought to determine if using both FPG and A1c together could identify the people most likely to progress to diabetes and allow early intervention. The researchers conducted a study that followed participants over several years and included nondiabetic Japanese men and women ages 24 to 82 who, between 1997 and 2003, had initial routine health exams with both FPG and A1c tests. With the initial (baseline) tests, 4103 participants had both levels within the normal range, 1167 had elevated fasting glucose only, 382 had elevated A1c only, and 256 had both elevated glucose and A1c. The researchers assessed these patients' rate of progression to diabetes over several years.
When evaluated on average 4.7 years after the first tests were done, 338 patients of 5903 (5.7%) had progressed to diabetes. Of these, 292 (86%) had been identified as prediabetic at baseline. About 8-9% of the patients with only one of the tests elevated at baseline were diagnosed with diabetes at 4.7 years. In contrast, 60% (154 of 256) of those with both tests elevated at baseline were diagnosed with diabetes after 4.7 years. Patients who were identified as prediabetic by both tests at baseline were 32 times more likely to progress to diabetes than those who had normal results and 6 times more likely to progress to diabetes than those who had only one test elevated.
"Our results suggest that introduction of the A1c criterion in addition to assessment of fasting plasma glucose could effectively target patients who are most likely to progress to diabetes and allow for early intervention," the researchers write.
This study adds fuel to the debate about the use of A1c to diagnose diabetes and prediabetes, write Anoop Misra of Fortis Hospital, New Delhi, and Satish Garg of the University of Colorado Denver, Aurora in a commentary in the same issue of The Lancet. The fact that using both tests identified more high-risk individuals suggests that each plays a different role in predicting diabetes. The tests measure different facets of glucose metabolism. Impaired fasting glucose shows insulin resistance, A1c reflects chronic high blood sugar, and oral glucose tolerance tests—which weren't included in the study—determine muscle insulin resistance, the authors point out.
Results of the Japanese study are similar to those from a U.S. study that found FPG and A1c measure different aspects of abnormal regulation of blood sugar and that together the two tests provide more sensitive and specific prediction of excess risk for diabetes than does either one alone. The identification of high risk populations using both tests may lead to more effective interventions to prevent the progression to diabetes.
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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.
Y. Heianza et al. HbA1c 5.7—6.4% and impaired fasting plasma glucose for diagnosis of prediabetes and risk of progression to diabetes in Japan (TOPICS 3): a longitudinal cohort study. The Lancet. Available online at http://www.thelancet.com/journals/lancet/article/PIIS0140-6736(11)60472-8/abstract through http://www.thelancet.com. Published July 9, 2010. Accessed August 16, 2011.
Misra A, Garag S. HbA1c and Fasting Plasma Glucose for the Diagnosis of Diabetes, Commentary. The Lancet 2011, 378:104-106.
B. Soloway. Does the HbA1c Criterion for Prediabetes Predict Incident Diabetes? JournalWatch General Medicine. Journal Watch Specialties. Available online at http://general-medicine.jwatch.org/cgi/content/full/2011/707/1?q=etoc_jwgenmed through http://general-medicine.jwatch.org. Published July 7, 2011. Accessed August16, 2011.
S. Boschert. HbA1c Plus Fasting Glucose More Accurately Identifies Prediabetes. Elsevier Global Medical News. Available at http://infoviewer.biz/infodisplay/story/imn062520111146126056.html?APP=7&CU=imn5804 through http://infoviewer.biz. Accessed August 16, 2011.
D. Mann et al. Impact of A1C Screening Criterion on the Diagnosis of Pre-Diabetes Among U.S. Adults. Diabetes Care. Available online at http://care.diabetesjournals.org/content/33/10/2190.abstract through http://care.diabetesjournals.org. Published October 2010. Accessed August 16, 2011.