The A1C test is a blood test that provides information about your average levels of blood glucose, also called blood sugar, over the past 3 months. The A1C test can be used to diagnose type 2 diabetes and prediabetes.The A1C test is also the primary test used for diabetes management.
Diabetes & Prediabetes Tests
This fact sheet compares the following tests:
A1C test
fasting plasma glucose (FPG) test
oral glucose tolerance test (OGTT)
random plasma glucose (RPG) test
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
Comparing Diabetes Blood Tests‡
View the content below as a table in the PDF Version (460 KB) .
A1C Test
Uses:
Screening and diagnosis of prediabetes
5.7–6.4%†
Screening and diagnosis of type 2 diabetes
≥ 6.5%†
repeat for confirmation of diagnosis
Monitoring of diabetes
Technical Features:
Diagnosis requires a laboratory test certified by the NGSP, not meter—point-of-care A1C tests are only suitable for monitoring
Sample any time of day, no fasting
Sample: anticoagulated whole blood
Sample stability: superior
Sensitivity: less than the FPG test and the OGTT
Coefficient of variation: assay variability, see www.ngsp.org
Courtesy of David Aron, M.D., Louis Stokes Department of Veterans Affairs Medical Center
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%.
Pros:
Reflects long-term blood glucose concentration
Unaffected by acute changes in glucose levels due to stress or illness
Highly correlated with risks for complications such as retinopathy and cardiovascular disease (CVD)
Convenient for patient and health care providers
Most stable sample after collection
Low within-patient variability
Established international standardization of lab tests
Accuracy of test is monitored
Cons:
Lower sensitivity: identifies fewer cases of diabetes than the glucose tests
Interference resulting in falsely increased or lowered results due to*
genetic variants including HbS, HbC, HbD, and HbE traits and HbF:** affects people of African, Mediterranean, and Southeast Asian heritage
kidney disease
liver disease
iron deficiency anemia
heavy bleeding
Not recommended for rapidly progressing diabetes, e.g., type 1 diabetes in children
May not be available in some laboratories/areas of the world
Expensive
FPG Test
Uses:
Screening and diagnosis of prediabetes or impaired fasting glucose (IFG)
100–125 mg/dL†
Screening and diagnosis of diabetes
≥126 mg/dL†
repeat for confirmation of diagnosis
Technical Features:
Diagnosis requires a laboratory test, not meter
Sample in morning, after 8-hour fast
Sample: sodium fluoride plasma preferred
Sample stability: low—requires processing within 30 minutes
Sensitivity: greater than the A1C test, less than the OGTT
Coefficient of variation: assay variability:
Courtesy of David Aron, M.D., Louis Stokes Department of Veterans Affairs Medical Center
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.
Pros:
Low cost
Assay is widely available
Assay is automated
Cons:
Indicates single-point blood glucose level
Affected by short-term lifestyle changes: stress or illness
Less tightly linked to diabetes complications than A1C
Not convenient for patient or health care provider: requires fasting and scheduling a morning appointment or return visit
Diurnal variation
Sample not stable after collection
High within-patient variability
Many laboratories measure serum, which is not recommended
Inadequate standardization of assays
OGTT
Uses:
Screening and diagnosis of prediabetes or impaired glucose tolerance (IGT)
140–199 mg/dL at 2 hr.†
Screening and diagnosis of diabetes
≥200 mg/dL at 2 hr.†
repeat for confirmation of diagnosis
Screening and diagnosis of gestational diabetes mellitus (GDM)*
Technical Features:
Sample in morning, after 8-hr. fast and 2 hrs. after glucose load
Sample stability: low—requires processing within 30 minutes
Patients should ingest at least 150 g/day of carbohydrates for 3 days prior
Sensitivity: greater than the A1C or the FPG tests
Range of variability: 16.7%
Pros:
Sensitive indicator of risk of developing diabetes
Early marker of impaired glucose balance
Cons:
Affected by short-term lifestyle changes: stress, illness, and medications
Not convenient for patient or health care provider: requires fasting and scheduling a morning appointment or return visit
Extensive patient preparation
Sample not stable after collection
High within-patient variability
Low reproducibility
Expensive
RPG Test
Uses:
Diagnosis of diabetes—used only with classic symptoms of hyperglycemia or hyperglycemic crisis:
polyuria, polydypsia, and unexplained weight loss
200 mg/dL†
Technical Features:
Sample any time, no fasting
Sample stability: low—requires processing in fewer than 2 hours
Pros:
Convenient
Part of basic metabolic panel screen
Cons:
Indicates single-point blood glucose level
Used only in symptomatic patients, not recommended for screening
Insensitive measurement
Greater within-patient variability
Affected by short-term lifestyle changes and prandial state
Comparing Diagnoses
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.
References
Additional Links
This content is provided as a service of the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK), part of the National Institutes of Health. The NIDDK translates and disseminates research findings through its clearinghouses and education programs to increase knowledge and understanding about health and disease among patients, health professionals, and the public. Content produced by the NIDDK is carefully reviewed by NIDDK scientists and other experts.
The NIDDK would like to thank:
Randie Little, Ph.D., University of Missouri-Columbia; David B. Sacks, M.B., Ch.B., F.R.C.Path., NIH Clinical Center; David Aron, M.D., Louis Stokes Department of Veterans Affairs Medical Center
Source: NIDDK, NIH