Written on November 28, 2022 by Theresa Vuskovich, DMD. To give you technically accurate, evidence-based information, content published on the Everlywell blog is reviewed by credentialed professionals with expertise in medical and bioscience fields.
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Diabetes, whether type 1, type 2, or prediabetes, occurs when the body's ability to produce and respond to insulin is impaired. As a result, sugar levels are elevated in the blood. To measure the sugar level in your blood, HbA1c and fasting plasma glucose tests are performed to diagnose and monitor diabetes. Here we'll break down these two tests, including their similarities and differences.
The HbA1c test measures blood glucose (i.e., sugar) levels over the last two to three months . The HbA1c test is a standardized test unaffected by acute changes, such as recent illness or stress . The test measures the amount of glycated hemoglobin (HbA1c), red blood cells with sugar attached to them.
The HbA1c test is often called an A1C test or hemoglobin A1C and is used to diagnose diabetes and other tests and symptoms. If you are older than 35 years old and/or have any of the following signs, your healthcare provider may recommend an A1C test [1-3]:
You can take an A1C test with your healthcare provider or an at-home AC1 test. The following results indicate whether or not you have diabetes [1,3]:
A positive diabetes diagnosis requires two positive tests from the same sample or a repeat test [1,3]. If your test results are normal, your healthcare provider will repeat the test every three years in the absence of symptoms.
Your healthcare provider will help you translate your A1C results and determine if you have diabetes. A1C test results can also indicate your risk for long-term complications of diabetes, such as heart disease . A1C tests are routinely used, but their results often have the following problems:
Your A1C results are only a part of the diabetes diagnosis process. Researchers have found that A1C tests are not always reliable indicators of diabetes [4,5,7,8]. In addition to the A1C test, healthcare providers use the fasting plasma glucose test to confirm a diagnosis of diabetes.
Fasting plasma glucose (FPG) or fasting blood sugar (FBS) tests are usually performed in the morning before breakfast and measure blood sugar levels when fasting, which means no food or drink for eight hours before the test. The FPG detects if blood sugar levels remain high even after an extended period without sugar, indicating a problem with sugar metabolism. The following FPG test results indicate whether you are healthy or may have diabetes: [1,3]
The FPG test results indicate the following:
Your healthcare provider will use FPG and A1C test results to determine whether you have diabetes, prediabetes, or are healthy. Now that you know the basics of both tests let's examine their similarities and differences.
Both tests help your healthcare provider understand how you metabolize sugar and help them determine if you have diabetes. Both tests are also venous blood tests.
FPG tests and HbA1c tests differ in their preparation steps and what they measure. A fasting period of at least 8 hours is required for the FPG test, whereas no preparation is needed for the HbA1c test.
HbA1c measures your blood sugar levels over the past 2 to 3 months, while FPG measures your immediate blood glucose levels. As a result, FPG is more sensitive to illnesses or acute stress than an HbA1c test.
Your healthcare provider will help you determine which tests are appropriate for you and translate the results. Everlywell provides an at-home HbA1c test to help track your blood sugar levels over time. Learn more about Everlywell's at-home HbAc test.