Glucose
Glucose is the sugar in your blood and your body's main fuel. This test shows how much is there at the moment the sample is drawn.
AI-verified against cited sources. Awaiting expert reviewAlso called GLU, FPG, FBGLOINC 2345-7
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What it is
Glucose comes from the carbohydrates you eat, and your liver also makes it. The hormone insulin moves it from your blood into your cells to use as energy. A glucose test measures how much is in your blood at the moment of the draw. It is often done after an overnight fast, and it is also part of routine panels like the BMP and CMP, which may be drawn without fasting.
Why it matters
Blood glucose that stays high is the main sign of prediabetes and diabetes, which over time can harm the eyes, kidneys, nerves and blood vessels. Glucose that drops too low can cause shakiness, sweating and confusion, and in severe cases fainting or seizures. Glucose moves up and down with meals, illness and stress, so one result is a snapshot. Clinicians usually read it together with an A1c (a 2 to 3 month average) and may repeat it before drawing any conclusion.
If it’s low
In people with diabetes, the ADA calls glucose below 70 mg/dL (3.9 mmol/L) low, and below 54 mg/dL (3.0 mmol/L) the more serious level. In people without diabetes, truly low glucose is uncommon. A low number on a lab report can also come from a sample that sat too long before it was processed.
Common causes
- A sample that sat for hours before the blood cells were separated, because the cells use up glucose in the tube
- A long gap since your last meal, or intense exercise
- Heavy alcohol use, especially without food
- In people with diabetes, some diabetes medicines such as insulin or sulfonylureas, which are a frequent cause of low glucose
- Liver or kidney disease
- Underactive adrenal glands (Addison disease), certain pituitary conditions or an underactive thyroid
- Poor nutrition
- Rarely, a pancreas tumor that makes too much insulin
What can help
- If you have diabetes, ask your care team how to prevent and treat low glucose, and what to do when it happens
- If low glucose happens when you don't have diabetes, ask your clinician whether more testing is needed to find the cause
- Ask whether the timing of your meals or exercise could be involved
If it’s high
A fasting result of 100 to 125 mg/dL (5.6 to 6.9 mmol/L) falls in the range the American Diabetes Association (ADA) calls prediabetes. A fasting result of 126 mg/dL (7.0 mmol/L) or higher falls in the diabetes range, but the ADA says it takes two results in the diabetes range to make that call, either from two different tests (such as glucose and A1c, which can come from the same blood draw) or from the same test repeated at a different time. A single high number can also have everyday explanations.
Common causes
- Not fasting: a meal, juice or sweetened coffee in the past 8 hours
- Stress on the body from illness, infection, surgery or injury
- Some medicines, such as corticosteroids
- Prediabetes or diabetes
- Hormone conditions, such as an overactive thyroid or Cushing's syndrome
- Problems with the pancreas, such as pancreatitis
What can help
- Ask your clinician whether to repeat the test after an overnight fast or to add an A1c
- Regular physical activity, a balanced eating pattern and a healthy weight are the main lifestyle levers clinicians discuss for glucose that runs high
- Ask whether any of your medicines could be raising your glucose
Usual ranges
What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.
For adults: 70–99 mg/dL. Source: Labcorp, American Diabetes Association (Diabetes Care), Diabetes Care (NACB and ADA), New England Journal of Medicine. Labs treat below 55 mg/dL or above 450 mg/dL as urgent.
- Adults (this one applies)70–99 mg/dL
Sources: Labcorp, American Diabetes Association (Diabetes Care), Diabetes Care (NACB and ADA), New England Journal of Medicine
- 1+, non fasting70–140 mg/dL
Source: Mayo Clinic Laboratories (read through the testcatalog.org mirror)
See Glucose by age and sex, with the U.S. survey numbers where they exist.
ADA fasting plasma glucose categories (2026)
- Normalbelow 100 mg/dL
- Prediabetes (impaired fasting glucose)100 to under 126 mg/dL
- Diabetes range (needs a confirming result)126 and above mg/dL
ADA hypoglycemia levels (people with diabetes)
- Level 2: below 54 mg/dLbelow 54 mg/dL
- Level 1: 54 to 69 mg/dL54 to under 70 mg/dL
- 70 mg/dL or higher70 and above mg/dL
If this applies to you
Glucose means a little more for some people. Open the one that fits you.
Prediabetes or diabetes
Fasting glucose is one of the standard tests for prediabetes and diabetes. In conditions that change how A1c relates to blood glucose, the ADA suggests glucose-based criteria instead of A1c.
Keto or low-carb eating
Low-carb eating can lower glucose. The ADA notes that insulin and other diabetes medicines may need adjusting to prevent low blood sugar on very-low-carb eating plans, and it advises regular medical follow-up for people with diabetes who eat this way.
On a GLP-1 medicine
These medicines lower blood glucose. The labels say that taken with insulin or a sulfonylurea they can raise the chance of low blood sugar, and the Wegovy and Zepbound labels call for monitoring glucose before and during treatment in people with diabetes.
Taking isotretinoin
The label notes that some people on isotretinoin have had trouble with blood sugar control, and new cases of diabetes have been found during use.
How common is your level?
NHANES describes the US civilian, non-institutionalized population, not a 'healthy' reference group. People with disease, on medication, fasting or not (except fasting-subsample markers), smokers and so on are all included. A percentile says where a value sits among Americans of that sex and age, not whether it is normal.
Use the left and right arrow keys to read each age. Shift moves five years.
Reference data derived from CDC/NCHS National Health and Nutrition Examination Survey (NHANES), cycles 2015-2016, 2017-March 2020 (pre-pandemic), August 2021-August 2023. Analysis by bloodtesttracker.com; not endorsed by CDC, NCHS or HHS.
Explore Glucose by age and sex: every percentile as a table, with a download.
What can affect your result
- Fasting: the ADA defines fasting as no calories for at least 8 hours (water is fine)
- Sample handling: glucose in an unspun tube falls about 5 to 7% per hour
- Sample type: plasma and serum run about 11% higher than whole blood, and home meters and sensors measure different samples (many meters convert to a plasma-equivalent value)
- Illness, surgery, injury and other physical stress
- Medicines, including corticosteroids and diabetes medicines
- Hormone conditions involving the thyroid, adrenal glands or pituitary
- Pregnancy, which has its own testing and targets
- A sample that sits unseparated can read falsely low: blood cells use up glucose at about 5 to 7% per hour. Fast processing or special collection tubes prevent this.
- Not fasting: a recent meal or sweet drink raises glucose for a few hours, so a non-fasting result is judged against a higher range.
- Illness, surgery, injury and other physical stress raise glucose, as can some medicines (for example corticosteroids).
- Fingerstick meters and continuous glucose monitors use a different sample and can differ from a lab result. Plasma glucose runs about 11% higher than whole-blood glucose, but many meters are calibrated to report a plasma-equivalent value, so don't adjust a meter reading yourself.
Morning, after an overnight fast, is the standard for the fasting range. Routine panels may be drawn at any time. Reflects a single moment and changes within minutes to hours. For a 2 to 3 month average, look at A1c.
When to talk to a clinician
Contact the clinician who ordered the test today if your lab flagged the result as critical. Many labs use limits of roughly below 40 to 55 mg/dL (2.2 to 3.1 mmol/L) or above 400 to 500 mg/dL (22 to 28 mmol/L). Call 911 or your local emergency number for confusion, fainting or seizures, or for very high glucose with vomiting, belly pain, fast breathing or extreme drowsiness. If a fasting result is 126 mg/dL (7.0 mmol/L) or higher and no one has discussed it with you, book a visit to confirm it.
Questions you could ask
- Was this a fasting sample, and which range is my lab comparing it with?
- Should I track my glucose and A1c together over time?
- Should we repeat this after an overnight fast, or add an A1c to confirm?
- Could a recent illness, stress or one of my medicines explain this result?
- How often should I be rechecked?
- Could the timing of my last meal or how the sample was handled explain this?
Questions people ask
Does a high glucose result mean I have diabetes?
Not by itself. A single high result can come from a recent meal, illness or stress. The ADA needs two results in the diabetes range to diagnose diabetes, either from two different tests (for example glucose and A1c, which can be drawn at the same time) or from repeating the same test. Your clinician decides what to do next.
Do I need to fast before a glucose test?
For the fasting range of 70 to 99 mg/dL, yes: no calories for at least 8 hours, and water is fine. Glucose is also on routine panels that may be drawn without fasting, and then labs compare it with a higher range, such as 70 to 140 mg/dL at Mayo Clinic Laboratories. Check whether your report says fasting or random.
Why doesn't my lab result match my home meter or CGM?
They measure different samples. A meter reads a drop of whole blood and a sensor reads fluid under the skin, while the lab measures plasma or serum. Plasma glucose runs about 11% higher than whole-blood glucose, but many meters are calibrated to report a plasma-equivalent number, so the gap is usually smaller than that. Meters and sensors also have their own margin of error. Home devices are useful for day-to-day trends, and the lab result is the one used for diagnosis.
How do I convert mg/dL to mmol/L?
Divide mg/dL by 18.016, so 100 mg/dL is about 5.6 mmol/L. To go the other way, multiply mmol/L by 18.016. Some countries report in g/L, where 1 g/L equals 100 mg/dL.
Can my glucose be normal while my A1c is high, or the other way around?
Yes. Glucose is a snapshot, while A1c is a 2 to 3 month average that can also be shifted by things like iron deficiency or inherited hemoglobin variants. If the two keep disagreeing, the ADA suggests clinicians check for a problem with one of the tests.
Show the science and sources
Units
- 1 mg/dL = 1 mg/dL
- 1 mg/dL = 0.05551 mmol/L
- 1 mg/dL = 0.01 g/L
Open the Glucose unit converter
Is a change real?
Glucose naturally varies about 5% from test to test in the same healthy person. Between two results from the same lab, a change of roughly 15% or more is unlikely to be day-to-day noise or lab variation (95% reference change value). Source: Critical appraisal and meta-analysis of biological variation studies on glycosylated albumin, glucose and HbA1c (Ricos C et al.).
For clinicians and researchers
Glucose is measured in venous plasma or serum, by an enzymatic method (hexokinase or glucose oxidase in virtually all US analyses, per Sacks 2011). The NACB/ADA laboratory guideline (Sacks 2011) advises that diagnosis use venous plasma glucose measured in an accredited laboratory, not a point-of-care meter. In unseparated blood, glycolysis lowers glucose by about 5 to 7% per hour (roughly 0.6 mmol/L or 10 mg/dL per hour). The guideline advises an ice-water slurry with plasma separated within 30 minutes, or a tube with a fast glycolysis inhibitor such as citrate buffer, because fluoride alone leaves glycolysis running for up to 4 hours. Plasma glucose runs about 11% higher than whole-blood glucose at a normal hematocrit; the guideline recommends that meters report plasma-equivalent glucose, and many are calibrated to do so. ADA 2026 criteria for diabetes are a fasting plasma glucose of 126 mg/dL (7.0 mmol/L) or higher, a 2-hour value of 200 mg/dL (11.1 mmol/L) or higher on a 75 g OGTT, an A1C of 6.5% or higher, or a random glucose of 200 mg/dL or higher with classic symptoms. Without unequivocal hyperglycemia, two abnormal results are needed, from two different tests (which may be drawn at the same time, e.g. A1C and FPG) or from the same test at two time points. Impaired fasting glucose is 100 to 125 mg/dL (5.6 to 6.9 mmol/L). The WHO and several other groups set the lower limit at 110 mg/dL (6.1 mmol/L), which the ADA lowered to 100 in 2003. When glucose and A1C consistently and substantially disagree, the ADA advises checking for a problem or interference with one of the tests. Risk is not zero inside the reference interval: among 13,163 young Israeli men with fasting glucose under 100 mg/dL, type 2 diabetes risk rose progressively from about 87 mg/dL upward compared with under 81 mg/dL (Tirosh 2005, observational).
LOINC 2345-7 (primary), 1558-6, 14749-6, 14771-0, 74774-1, 77135-2, 2339-0, 15074-8. Specimen: serum-or-plasma.
Sources
- Glucose (test 001032), Labcorp
- Glucose, Random, Serum (test GLURA), Mayo Clinic Laboratories test catalog, Mayo Clinic Laboratories (read through the testcatalog.org mirror)
- 2. Diagnosis and Classification of Diabetes: Standards of Care in Diabetes-2026, American Diabetes Association (Diabetes Care) (2026)
- 6. Glycemic Goals, Hypoglycemia, and Hyperglycemic Crises: Standards of Care in Diabetes-2026, American Diabetes Association (Diabetes Care) (2026)
- Guidelines and recommendations for laboratory analysis in the diagnosis and management of diabetes mellitus, Diabetes Care (NACB and ADA) (2011)
- Normal fasting plasma glucose levels and type 2 diabetes in young men, New England Journal of Medicine (2005)
- Glucose (PubChem compound 5793), PubChem, National Library of Medicine
- Critical Values List (CORP-APPEND-0104A, Rev. 46), ARUP Laboratories (2026)
- Critical (Panic) Values, with adult reference intervals (document DX_TL_811850, 01/19/2024), Labcorp (2024)
- Quest Diagnostics Priority Result Reporting Policy (Client Synopsis), Quest Diagnostics (2025)
- Critical Value Policy, Michigan Medicine Laboratories (MLabs) (2025)
- Diagnostic Laboratories: Critical Values, Houston Methodist
- Appendix 1: Critical Value List, University of Chicago Medicine
AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.