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Blood glucose
The main sugar in the blood and the body's key fuel. The fasting level is the standard screening number for prediabetes and diabetes.
Other names: Blood sugar, fasting blood glucoseSection: Blood chemistry
Updated: August 29, 2026·MedProtect editorial team Reference information — not a diagnosis or a prescription
In shortA normal fasting blood glucose is 70–99 mg/dL (3.9–5.5 mmol/L). Fasting values of 100–125 mg/dL (5.6–6.9 mmol/L) point to prediabetes, and 126 mg/dL (7.0 mmol/L) or higher usually means diabetes. A random result is normally 125 mg/dL (6.9 mmol/L) or lower. Fasting for at least 8 hours is required for the fasting version of the test.
Reference ranges
Group
Reference range
Fasting (8+ hours without food)
70–99 mg/dL (3.9–5.5 mmol/L)
Random (any time of day)
125 mg/dL (6.9 mmol/L) or lower
Values above are the common measurements quoted in the MedlinePlus Medical Encyclopedia. Laboratories may use slightly different intervals, and a venous blood test is considered more accurate than a fingerstick meter or a continuous glucose monitor.
Reference range vs optimal — not the same thing
The fasting reference is up to 6.1 mmol/L. Metabolically optimal is considered 3.9–4.7 mmol/L (70–85 mg/dL); 5.6–6.0 is a grey zone — ADA prediabetes criteria start at 5.6 mmol/L fasting.
Check your value
Enter the number from your lab report — see how it compares with the typical range. Mind the units shown in the chosen row.
A rough comparison with a typical range, not a diagnosis. Reference ranges vary between laboratories — check the range on your own report and let a clinician interpret the result.
What this marker is
Glucose is a simple sugar and the main source of energy for most cells, including brain cells. Carbohydrates from food — fruit, cereal, bread, pasta, rice — are quickly converted into glucose, which raises the blood level after meals. The hormone insulin keeps that level in check by moving glucose out of the blood and into cells.
The same test comes in three versions, and the reference numbers differ between them: a fasting test after at least 8 hours without food, a random test at any time of day, and a reading two hours after drinking a set amount of glucose (the oral glucose tolerance test, OGTT). Which version was used matters when the result is read.
Screening usually relies on the fasting test. MedlinePlus describes it as the most common diabetes screening test, generally starting at age 35 and repeated every 3 years when no other risk factors are present. Excess weight plus another risk factor — high blood pressure, unhealthy cholesterol, heart disease, a close relative with diabetes, gestational diabetes in the past, polycystic ovary syndrome, low physical activity, or membership in a high-risk ethnic group — is a reason to start earlier or test more often.
High blood sugar and early diabetes often cause no symptoms, which is exactly why screening exists. Symptoms that do bring people to the test include urinating more often than usual, blurred vision, recent weight gain, confusion, fainting spells, or a first-time seizure.
When it is high
Prediabetes (impaired fasting glucose)A fasting level of 100–125 mg/dL (5.6–6.9 mmol/L) means blood sugar is above normal but below the diabetes threshold. The risk of developing type 2 diabetes is increased, and lifestyle changes at this stage can delay or prevent it.
DiabetesA fasting level of 126 mg/dL (7.0 mmol/L) or higher usually means diabetes; a random level of 200 mg/dL (11.1 mmol/L) or higher points the same way. Providers typically confirm with a second test — a repeat fasting measurement, an A1C, or an OGTT.
Missed fasting windowIf food or a sugary drink was consumed within 8 hours of the draw, the 'fasting' result is not really fasting and can read high.
When it is low
Diabetes medicinesLow blood glucose (hypoglycemia) is most often caused by medicines for type 1 or type 2 diabetes, including insulin.
Kidney or liver conditionsIn people without diabetes, low glucose is much less common; when it happens, certain medicines and kidney or liver conditions are among the causes.
What can skew the result
These factors can shift the value without any disease — worth considering before a retest:
Food within the last 8 hoursTime of dayDiabetes medicinesHome meter vs. lab measurement
How to prepare
For the fasting version, nothing to eat or drink except water for at least 8 hours before the draw. No preparation is needed for a random test. The glucose tolerance test follows its own protocol with a measured glucose drink.
When a retest makes sense
A single high result is usually confirmed before a diagnosis: the provider orders a repeat fasting test, an A1C, or a glucose tolerance test. For screening with normal results and no risk factors, the described interval is every 3 years from age 35. Home meter readings that look off are a reason to confirm with a venous blood test, which is more accurate.
Who usually reviews this result
A result outside the reference range is usually reviewed by: your GP or primary care doctor; an endocrinologist if diabetes is confirmed. Bring your lab report and any previous results to the appointment.
Names used on lab reports
Your report may use any of these names: Blood glucoseBlood sugarFasting blood glucoseFasting blood sugarRandom blood glucose
Units and conversion
Measured in: mg/dL
mg/dL=mmol/L
mg/dL ÷ 18 = mmol/L. Example: 99 mg/dL = 5.5 mmol/L. Results are rounded to 2 decimal places.
Several markers on your report?
See how MedProtect explains results in the app: the whole report, a check of the recognized values, and a clear breakdown.
What is the difference between a glucose test and an A1C?
A glucose test shows the blood sugar level at the moment of the draw. An A1C shows the average over the past two to three months and does not require fasting. The two answer different questions and are often ordered together.
Can one high reading mean diabetes?
Usually not on its own. The MedlinePlus encyclopedia notes that providers typically confirm with another test — a repeat fasting glucose, an A1C, or an oral glucose tolerance test — before diagnosing diabetes.
Is a home glucose meter as accurate as the lab?
Not quite. A venous blood test processed by a laboratory is considered more accurate than fingerstick meter readings or continuous glucose monitors. Home devices are built for day-to-day management rather than diagnosis.
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