Glycated Hemoglobin - Interpretation of Results

Interpretation of Results

Laboratory results may differ depending on the analytical technique, the age of the subject, and biological variation among individuals. Two individuals with the same average blood sugar can have A1C values that differ by as much as 3 percentage points. Results can be unreliable in many circumstances, such as after blood loss, for example, after surgery, blood transfusions, anemia, or high erythrocyte turnover; in the presence of chronic renal or liver disease; after administration of high-dose vitamin C; or erythropoetin treatment. In general, the reference range (that found in healthy persons), is about 20–40 mmol/mol (4%–5.9%).

Higher levels of HbA_1c are found in people with persistently elevated blood sugar, as in diabetes mellitus. While diabetic patient treatment goals vary, many include a target range of HbA_1c values. A diabetic person with good glucose control has a HbA_1c level that is close to or within the reference range. The International Diabetes Federation and American College of Endocrinology recommend HbA_1c values below 48 mmol/mol (6.5%), while American Diabetes Association recommends that the HbA_1c be below 53 mmol/mol (7.0%) for most patients. Recent results from large trials suggest that a target below 53 mmol/mol (7%) may be excessive: Below 53 mmol/mol (7%) the health benefits of reduced A1C become smaller, and the intensive glycemic control required to reach this level leads to an increased rate of dangerous hypoglycemic episodes.

A retrospective study of 47,970 diabetes patients found that patients with an A1C less than 48 mmol/mol (6.5%) had an increased mortality rate, but a later international study contradicted these findings.

Practitioners must consider an individual patient's health, his/her risk of hypoglycemia, and his/her specific health risks when setting a target A1C level. For example, patients at high risk of microvascular complications may gain further benefits from reducing A1C below 53 mmol/mol (7%). Because patients are responsible for averting or responding to their own hypoglycemic episodes, the patient's input and the doctor's assessment of the patient's self-care skills are also important.

Persistent elevations in blood sugar (and, therefore, HbA_1c) increase the risk for the long-term vascular complications of diabetes such as coronary disease, heart attack, stroke, heart failure, kidney failure, blindness, erectile dysfunction, neuropathy (loss of sensation, especially in the feet), gangrene, and gastroparesis (slowed emptying of the stomach). Poor blood glucose control also increases the risk of short-term complications of surgery such as poor wound healing.

Lower-than-expected levels of HbA_1c can be seen in people with shortened red blood cell lifespan, such as with glucose-6-phosphate dehydrogenase deficiency, sickle-cell disease, or any other condition causing premature red blood cell death. On the converse, higher-than-expected levels can be seen in people with a longer red blood cell lifespan, such as with Vitamin B₁₂ or folate deficiency.

The approximate mapping between HbA_1c values given in DCCT percentage (%) and eAG (estimated average glucose) measurements is given by the following equation:

eAG(mg/dl) = 28.7 × A1C − 46.7
eAG(mmol/l) = 1.59 × A1C − 2.59
Data in parentheses are 95% confidence intervals