Comparing Tests for Diabetes and Prediabetes: A Quick Reference Guide
On this page:
- Confirming Diagnosis of Type 2 Diabetes and Prediabetes
- Interpreting Laboratory Results
- Comparing Diabetes Blood Tests
- Comparing Diagnoses
This fact sheet compares the following tests:
- A1C test
- fasting plasma glucose (FPG) test
- oral glucose tolerance test (OGTT)
- random plasma glucose (RPG) test
In addition, the National Diabetes Education Program (NDEP) offers a pocket guide, Diabetes Numbers At-a-Glance, which can be ordered at www.ndep.nih.gov. Both resources utilize current American Diabetes Association (ADA) clinical recommendations for diagnosing and managing diabetes and prediabetes.1
Confirming Diagnosis of Type 2 Diabetes and Prediabetes
Diagnosis must be confirmed unless symptoms are present. Repeat the test using one of the following methods:
- Repeat the same test on a different day—preferred.
- If two different tests are used—e.g., FPG and A1C—and both indicate diabetes, consider the diagnosis confirmed.
- If the two different tests are discordant, repeat the test that is above the diagnostic cut point.
If diagnosis cannot be confirmed using the results of two tests, but at least one test indicates high risk, health care providers may wish to follow the patient closely and retest in 3 to 6 months.1
Interpreting Laboratory Results
When interpreting laboratory results health care providers should
- consider that all laboratory test results represent a range, rather than an exact number2
- be informed about the A1C assay methods used by their laboratory2
- send blood samples for diagnosis to a laboratory that uses an NGSP-certified method for A1C analysis to ensure the results are standardized3
- consider the possibility of interference in the A1C test when a result is above 15% or is at odds with other diabetes test results1
- consider each patient’s profile, including risk factors and history, and individualize diagnosis and treatment decisions in discussion with the patient1
With a coefficient of variation as large as that allowed by the National Glycohemoglobin Standardization Program, a reported A1C result of 7.0% could indicate a true A1C of anywhere from ~6.5 to 7.5%.
*See www.ngsp.org for information on A1C interference and recommended testing methods.
**See the NIDDK publication The A1C Test and Diabetes at www.diabetes.niddk.nih.gov.
With a coefficient of variation 5.7% (typical biological variation within the same person), an FPG test result of 126 mg/dL could indicate a true FPG of anywhere from ~110 to 142 mg/dL.
‡Adapted from Sacks DB. A1C versus glucose testing: a comparison. Diabetes Care. 2011;34(4):518–523.
†American Diabetes Association. Standards of medical care in diabetes—2011. Diabetes Care. 2011;34(Supp 1):S11–S61.
*Testing for GDM is not covered in this publication.
In some people, a blood glucose test may indicate a diagnosis of diabetes even though an A1C test does not.
The reverse can also occur—an A1C test may indicate a diagnosis of diabetes even though a blood glucose test does not.
Because of these variations in test results, health care providers should repeat tests before making a diagnosis. People with differing test results may be in an early stage of the disease, where blood glucose levels have not risen high enough to show on every test.
- American Diabetes Association. Standards of medical care in diabetes—2011. Diabetes Care. 2011;34(Supp 1):S11–S61.
- Sacks DB. A1C versus glucose testing: a comparison. Diabetes Care. 2011;34(4):518–523.
- See www.ngsp.org for information on A1C test interference and recommended testing methods.
Publications produced by the Clearinghouse are carefully reviewed by both NIDDK scientists and outside experts. This publication was originally reviewed by Randie Little, Ph.D., University of Missouri-Columbia and David B. Sacks, M.B., Ch.B., F.R.C.Path., NIH Clinical Center. The updated version of this publication was reviewed by David Aron, M.D., Louis Stokes Department of Veterans Affairs Medical Center.
This publication is published by the NIDDK in collaboration with the NDEP. This publication is available at www.diabetes.niddk.nih.gov.
Another quick-reference tool, Diabetes Numbers At-a-Glance 2012, is offered by the NDEP and is available at www.ndep.nih.gov.
National Diabetes Education Program
1 Diabetes Way
Bethesda, MD 20814–9692
Phone: 1–888–693–NDEP (1–888–693–6337)
The National Diabetes Education Program is a federally funded program sponsored by the U.S. Department of Health and Human Services’ National Institutes of Health and the Centers for Disease Control and Prevention and includes over 200 partners at the federal, state, and local levels, working together to reduce the morbidity and mortality associated with diabetes.
National Diabetes Information Clearinghouse
1 Information Way
Bethesda, MD 20892–3560
The National Diabetes Information Clearinghouse (NDIC) is a service of the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). The NIDDK is part of the National Institutes of Health of the U.S. Department of Health and Human Services. Established in 1978, the Clearinghouse provides information about diabetes to people with diabetes and to their families, health care professionals, and the public. The NDIC answers inquiries, develops and distributes publications, and works closely with professional and patient organizations and Government agencies to coordinate resources about diabetes.
This publication is not copyrighted. The Clearinghouse encourages people to duplicate and distribute as many copies as desired.
This publication may contain information about medications, and, when taken as prescribed, the conditions they treat. When prepared, this publication included the most current information available. For updates or for questions about any medications, contact the U.S. Food and Drug Administration toll-free at 1–888–INFO–FDA (1–888–463–6332) or visit www.fda.gov. Consult your health care provider for more information.
NIH Publication No. 14–7850
Page last updated June 18, 2014