Blood Glucose Levels: Normal Range, High & Low Causes Explained
Blood glucose is the concentration of sugar in your blood. A glucose test measures how much glucose is present and is used to screen for diabetes, prediabetes, and other metabolic conditions.
Disclaimer
Content is informational and not a substitute for professional medical advice. Consult your healthcare provider for significant abnormalities.
Get a personalized interpretation of your lab results - free, fast, and private.
Informational only - not a medical diagnosis.
What does a blood glucose test measure?
Glucose is a simple sugar that your cells use as their primary fuel source. Insulin, a hormone produced by the pancreas, moves glucose from your bloodstream into cells. When this system works correctly, blood glucose stays within a narrow range throughout the day.
Blood glucose can be tested in several ways depending on the clinical question.
Fasting glucose is the standard test. You avoid food for 8-12 hours before the blood draw. It reflects your baseline blood sugar level, free from the influence of a recent meal.
Random glucose can be drawn at any time regardless of meals. It is mainly used when acute diabetes symptoms are present.
2-hour post-load glucose is part of the oral glucose tolerance test (OGTT). It shows how efficiently your body clears glucose after a carbohydrate challenge.
This content is informational and not a substitute for professional medical advice.
Blood glucose normal range: reference values by category
The values below are based on widely used clinical standards for fasting venous plasma glucose. A single elevated result is not a diagnosis - confirmation on a second test is required.
| Test | Normal | Prediabetes | Diabetes (provisional) |
|---|---|---|---|
| Fasting glucose (mg/dL) | 70-99 | 100-125 | >= 126 |
| 2-hr OGTT glucose (mg/dL) | < 140 | 140-199 | >= 200 |
| Random glucose (mg/dL) | - | - | >= 200 with symptoms |
Source: LabCorp Reference Ranges. Diagnostic thresholds align with American Diabetes Association Standards of Care. Diagnosis requires confirmation on a second separate test.
How to convert blood glucose units: mmol/L and mg/dL
Lab reports in the US use mg/dL; most of the world uses mmol/L. If you are comparing results from different sources or using a glucometer set to different units, use these formulas.
- mmol/L -> mg/dL: multiply by 18.016
- mg/dL -> mmol/L: divide by 18.016
Conversion formulas:
Key threshold equivalents:
| mmol/L | mg/dL | Clinical meaning |
|---|---|---|
| 3.9 | 70 | Lower limit of normal |
| 5.5 | 99 | Upper limit of normal (fasting) |
| 5.6 | 101 | Start of prediabetes range |
| 6.9 | 124 | Upper limit of prediabetes |
| 7.0 | 126 | Provisional diabetes threshold (fasting) |
| 11.1 | 200 | Diabetes threshold (2-hr OGTT or random with symptoms) |
Conversion factor: 1 mmol/L = 18.016 mg/dL. LOINC code for this biomarker: 2339-0.
What does high blood glucose mean?
A single elevated glucose result does not automatically mean you have diabetes. Several temporary factors can push glucose above the normal range.
- eating within 8 hours of the blood draw
- acute stress, illness, or intense physical activity before the test
- certain medications, especially corticosteroids and some antipsychotics
Functional causes (unrelated to diabetes):
- fasting glucose consistently in the prediabetes range (100-125 mg/dL)
- prediabetes combined with other risk factors such as excess weight, low physical activity, or a family history of type 2 diabetes
Results worth discussing with your doctor:
When fasting glucose is borderline, doctors typically order an HbA1c and sometimes an OGTT to get a complete picture of glucose metabolism.
What does low blood glucose mean?
Low blood glucose (hypoglycemia) is less common than high glucose but can be serious.
- prolonged fasting (more than 12-14 hours without food)
- intense exercise without adequate carbohydrate intake beforehand
- alcohol consumption, especially without eating
- in people with diabetes - too much insulin or certain diabetes medications
Common causes of low glucose:
Symptoms of significant hypoglycemia include shakiness, sweating, confusion, and rapid heartbeat. Glucose below 70 mg/dL with symptoms warrants immediate attention.
How Ivorus helps you track blood glucose
Ivorus is a lab result interpretation platform.
Example: if your fasting glucose reads 108 mg/dL (above the normal range of 70-99), Ivorus flags it as borderline elevated, surfaces related markers (HbA1c, fasting insulin, triglycerides), and shows whether the level has been rising across multiple tests or is an isolated result.
Upload your lab results and the platform compares your markers against reference ranges, tracks trends over time, and highlights patterns worth discussing with your doctor.
This content is informational only and does not constitute a medical diagnosis or recommendation. Consult your healthcare provider for significant abnormalities.
Frequently Asked Questions
Do I need to fast before a blood glucose test?
Yes, for the standard fasting glucose test you should avoid food and caloric drinks for 8-12 hours before the draw. Water is fine. If you did not fast, the result may be temporarily elevated and will not reflect your true baseline blood sugar level.
Is a fasting glucose of 102 mg/dL something to worry about?
102 mg/dL falls in the prediabetes range (100-125 mg/dL). It is not a diabetes diagnosis, but it is worth discussing with your doctor. Lifestyle changes - regular physical activity and a balanced diet - can often bring fasting glucose back into the normal range.
What is the difference between fasting glucose and HbA1c?
Fasting glucose is a snapshot of your blood sugar at a specific moment in time. HbA1c reflects your average glucose level over the past 2-3 months. Doctors often use both tests together because they provide complementary information about blood sugar control.
What else can affect my glucose result besides food?
Stress, acute illness, vigorous exercise, poor sleep, and certain medications (particularly corticosteroids) can temporarily raise blood glucose. If your result was unexpected, ask your doctor when it makes sense to retest under more controlled conditions.
How often should I get a glucose test?
For healthy adults with no risk factors, once a year as part of a routine metabolic panel is typical. If you have risk factors (excess weight, family history of diabetes, prediabetes), your doctor may recommend testing every 6-12 months. People with diagnosed diabetes test more frequently, as directed by their care team.
Can glucose results differ between labs?
Fasting glucose reference ranges are fairly standardized (70-99 mg/dL is widely used), but slight variations exist between laboratories. Always compare your result against the reference range printed on your specific lab report, not a generic online chart.
What is an oral glucose tolerance test (OGTT)?
An OGTT measures your glucose fasting and then again 2 hours after drinking a standard glucose solution. It is more sensitive than fasting glucose alone and is commonly used to diagnose prediabetes, gestational diabetes, and type 2 diabetes when fasting results are borderline.
Related biomarkers
Get a personalized interpretation of your lab results - free, fast, and private.
Informational only - not a medical diagnosis.