HbA1c test results on mobile device (not fasting glucose)

HbA1c vs. fasting glucose: understanding the similarities and differences

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.

Table of contents

Key takeaways

  • The HbA1c and fasting plasma glucose (FPG) are blood tests healthcare providers use to diagnose diabetes.
  • The HbA1c test measures your blood sugar over the previous 2 to 3 months and is less sensitive to acute illness and stress than the FPG test.
  • The FPG test measures your blood sugar after an extended fasting period and is more sensitive to acute illness and stress than the HbA1c test.

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.

What is an HbA1c test?

The HbA1c test measures blood glucose (i.e., sugar) levels over the last two to three months [3]. The HbA1c test is a standardized test unaffected by acute changes, such as recent illness or stress [3]. 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]:

  • Overweight or obesity (BMI ≥25 kg/m2 or ≥23 kg/m2 in Asian Americans)
  • First-degree relative with diabetes
  • High-risk race/ethnicity (e.g., African American, Latino, Native American, Asian American, Pacific Islander)
  • History of heart disease
  • High blood pressure (≥140/90 mmHg or on therapy for hypertension)
  • HDL cholesterol level <35 mg/dL (0.90 mmol/L) and/or a triglyceride level >250 mg/dL (2.82 mmol/L)
  • Women with polycystic ovary syndrome
  • Physical inactivity
  • Patients with prediabetes (A1C ≥5.7% [39 mmol/mol], impaired glucose tolerance, or impaired fasting glucose]) should be tested yearly.
  • Women diagnosed with gestational diabetes (i.e., diabetes during pregnancy) must undergo lifelong testing every three years.
  • HIV

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]:

  • Healthy: less than 5.7%
  • Prediabetes: 5.7-6.4%
  • Diabetes: greater than 6.5%

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 [6]. A1C tests are routinely used, but their results often have the following problems:

  • Lower sensitivity (ability to identify positive individuals)
  • Affected by conditions such as sickle cell disease, pregnancy, hemodialysis, blood loss or transfusion, and treatment with erythropoietin (increases red blood cell production)
  • Not validated in non-white populations

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.

What is a fasting plasma glucose test?

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:

  • Healthy: 100 mg/dL and below
  • Prediabetes: 100 to 125 mg/dL
  • Diabetes: 126 mg/dL and greater

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.

HbA1c vs. FPG: what are the similarities?

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.

HbA1c vs. FPG: what are the differences?

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.

What is the effect of exercise on blood sugar?

What are the GLP-1 drugs?

Prediabetic range for HbA1c

What is normal HbA1c by age?

References

  1. American Diabetes Association. Standards of medical care in diabetes-2022 abridged for primary care providers. Clin Diabetes. 2022;40(1):10-38. doi:10.2337/cd22-as01. URL
  2. Davidson KW, Barry MJ, Mangione CM, et al.. Screening for Prediabetes and Type 2 Diabetes. JAMA. 2021;326(8):736. doi:10.1001/jama.2021.12531. URL
  3. Goyal R, Jialal I. Diabetes Mellitus Type 2. In: StatPearls [Internet]. StatPearls Publishing; 2022. URL
  4. Kaiafa G, Veneti S, Polychronopoulos G, et al. Is HbA1c an ideal biomarker of well-controlled diabetes? Postgrad Med J. 2021;97(1148):380-383. doi:10.1136/postgradmedj-2020-138756. URL
  5. Kaur G, Lakshmi PVM, Rastogi A, et al.. Diagnostic accuracy of tests for type 2 diabetes and prediabetes: A systematic review and meta-analysis. PLOS ONE. 2020;15(11):e0242415. doi:10.1371/journal.pone.0242415. URL
  6. Mongraw-Chaffin M, Bertoni AG, Golden SH, et al. Association of Low Fasting Glucose and HbA1c With Cardiovascular Disease and Mortality: The MESA Study. J Endocr Soc. 2019;3(5):892-901. Published 2019 Mar 1. doi:10.1210/js.2019-00033. URL
  7. Owora AH. Commentary: Diagnostic Validity and Clinical Utility of HbA1c Tests for Type 2 Diabetes Mellitus. Curr Diabetes Rev. 2018;14(2):196-199. doi:10.2174/1573399812666161129154559. URL
  8. Pajunen P, Peltonen M, Eriksson JG, et al. HbA(1c) in diagnosing and predicting Type 2 diabetes in impaired glucose tolerance: the Finnish Diabetes Prevention Study: HbA1cand diabetes diagnosis in impaired glucose tolerance. Diabet Med. 2011;28(1):36-42. doi:10.1111/j.1464-5491.2010.03183.x. URL
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