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When faced with a challenging diagnosis, does one test fit all?
You’re sitting in one of your exam rooms, in front of the computer screen (we all stare at those things for far too long, every day!). Your patient’s hemoglobin A1c (HbA1c) is 5.0%, yet her fasting plasma glucose is 108 mg/dL. Or the reverse – the HbA1c is 6.0%, but the fasting plasma glucose (FPG) is 95. So, does this patient have diabetes? What would you advise her?
For many years, the sole test for diagnosing diabetes mellitus (DM) that was practical and recommended was an FPG. There’s the oral glucose tolerance test (OGTT), but this is expensive, cumbersome, and thus, used only for those testing for gestational diabetes, or for research purposes.
The main drawback, was, and is, that the patient had to have obtained this after an overnight fast. HbA1c had been recommended only to gauge the level of glycemic control once a diagnosis of diabetes had been made, but was not yet ready for prime time, so to speak, for diagnosis. That changed in 2009, when the A1c was standardized globally. From that point forward, an A1c of 6.5 or higher could make the diagnosis of DM.
The question is: should clinicians use one test or the other only? You could certainly make a very persuasive argument for the A1c, as the patient does not have to fast. There is less biologic variability with this test than with the FPG.
Countering this is the observation that the A1c identifies fewer individuals with diabetes than either the FPG or the 2-hour OGTT.1-4 Interestingly, a study of adults in the US without diabetes, those with an FPG > 126 mg/dL but with an A1c diagnostic of DM was 0.5%. Those with an FPG indicating DM, but with a non-DM A1c was 1.8%; those with both tests diagnostic of DM was only 1.8%.1
So, what’s a clinician to do? Several, including Silvio Inzucchi, MD, have suggested checking both A1c and FPG “either simultaneously or in sequence, a strategy that might be considered for patients at highest risk.”5
You may be further asking, “What if my patient gets both tests, and they are discordant?” Here, the waters are no less murky. The American Diabetes Association does recommend repeating the abnormal test only, and if the abnormal value is confirmed, to let that determine the diagnosis. This approach could be reasonable if the other lab is close to, but not abnormal.
If there’s a wider gap between the two tests, (for instance, if the A1c is 6.3% and the FPG is 90), expert opinion suggests repeating both.
Please share any thoughts and experiences with diagnostic testing - dilemmas, questions, puzzles, or your own approach to what tests you order, and when.
1. Carson AP, et al. Comparison of A1C and fasting glucose criteria to diagnose diabetes among U.S. adults. Diabetes Care. 2010;33:95-97.
2. Lipska KJ, et al. Identifying dysglycemic states in older adults: implications of the emerging use of hemoglobin A1c. J Clin Endocrinol Metab. 2010;95:5289-5295.
3. Jorgensen ME, et al. New diagnostic criteria for diabetes: is the change from glucose to HbA1c possible in all populations? J Clin Endocrinol Metab. 2010;95:E333-E336.
4. Olson DE, et al. Screening for diabetes and pre-diabetes with proposed A1C-based diagnostic criteria. Diabetes Care. 2010;33:2184-2189.
5. Inzucchi SE. Diagnosis of diabetes. N Engl J Med. 2012; 367:542-550.