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Why would someone have a log HgbA1C (4.3%) yet have an elevated fasting glucose (105 mg/dl?

First and foremost, it’s important to gather more information, including confirming that the client was fasting (except for water) for at least 8 hours prior to testing in order to obtain a truly fasting glucose level.

Check to see if they have a history of elevated fasting glucose, it is always best to assess changes over time. Tests should be repeated to confirm.

Check whether they are on medications that can elevate fasting glucose in the short term including steroids and antidepressants. Assess whether they may be under increased stress or have an active infection, both of which can increase glucose.

Consider conducting an oral glucose tolerance test (OGTT). It is important to measure plasma glucose 1 and 2 hours after meals to reveal fluctuations that can be detrimental but not picked up by hemoglobin A1C or fasting glucose.

Fasting insulin and C-peptide would be important to obtain to assess whether insulin resistance or possibly decreased insulin production is occurring.

Testing fructosamine and glycated albumin can provide further information about glucose control over time.

“The Atherosclerosis Risk in Communities (ARIC) study, which evaluated fructosamine and glycated albumin in blood samples from 11.348 adults without diabetes and 958 adults
diagnosed with diabetes mellitus (both type 1 and 2), observed an independent association with risk of incident diabetes, prevalent retinopathy and incident chronic kidney disease. The authors also showed that these markers have potential for identifying individuals at risk for incident diabetes and may be useful complements to HbA1c in clinical practice, mainly when HbA1c testing is inaccessible or when the result might not be reliable.”

Ribeiro, Rogério Tavares et al. “HbA1c, Fructosamine, and Glycated Albumin in the Detection of Dysglycaemic Conditions.” Current diabetes reviews vol. 12,1 (2016): 14-9. doi:10.2174/1573399811666150701143112 [R]

Interestingly, the blood sample source and how the blood is handled can influence results. The length of time the blood spends in the test tube can affect glucose levels which can decrease by 5-7% per hour. Therefore, actual blood glucose levels may be even higher than reported. Also, whole blood measurement of glucose can be 11% lower than that of plasma, the preferred testing medium.

Sacks, David B. “A1C versus glucose testing: a comparison.” Diabetes care vol. 34,2 (2011): 518-23. doi:10.2337/dc10-1546 [R]