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Glucose — illustration

Glucose (Blood sugar, Blood glucose, Fasting blood glucose (FBG), Fasting plasma glucose (FPG), Fasting blood sugar (FBS), GLU)

Glucose — blood sugar — is the fuel almost every cell in your body prefers, and your brain depends on it almost exclusively. After you eat, carbohydrates break down into glucose and enter the bloodstream; between meals, your liver releases stored glucose to keep the supply steady. The pancreas manages this constantly, releasing insulin to move sugar into cells when levels rise and glucagon to raise them when they fall. A healthy body keeps fasting glucose in a remarkably tight band, roughly 70–99 mg/dL.

When this system drifts, it usually drifts high. Insulin resistance — cells responding sluggishly to insulin — develops silently over years, showing up first as fasting readings creeping above 100 mg/dL (prediabetes) and later crossing 126 mg/dL (diabetes). About one in three US adults has prediabetes, and most don't know it, which is exactly why this simple test appears in nearly every routine blood panel. Low glucose is less common outside of diabetes treatment but matters just as much, because the brain feels a fuel shortage within minutes.

The number on your lab report is a single snapshot, not a verdict. It moves with when you last ate, how you slept, what medications you take, and even how the sample was handled. That's why every major guideline requires abnormal results to be confirmed before a diagnosis is made — and why the trend across tests tells you more than any one value.

Reference ranges

Adults

Most US labs list a fasting glucose reference range of about 70–99 mg/dL, following American Diabetes Association criteria; glucose ranges are not split by sex. Low readings are tiered using the ADA's hypoglycemia levels — 54–69 mg/dL is level 1 (an alert value) and below 54 mg/dL is level 2 (clinically serious) — and most US labs additionally treat values below about 40 mg/dL as a critical (panic) value, though the exact callback threshold varies slightly by lab. There is no recognized 'borderline low' category for glucose; 70 mg/dL is the accepted lower cutoff.

Range (mg/dL)Classification
20–40 Critically low (panic value)
40–54 Severely low (serious hypoglycemia)
54–70 Low (mild hypoglycemia)
70–100 Normal
100–126 Borderline high (prediabetes range)
126–200 High (diabetes range)
200–300 Very high
300–400 Severely high

How to read your result

First, check whether your test was fasting — the bands on this page assume it was. Reading the number: below 70 mg/dL is low, with real urgency beginning under 54 mg/dL and a medical emergency below about 40 mg/dL, where labs phone results as panic values; 70–99 mg/dL is the healthy fasting zone; 100–125 mg/dL signals prediabetes; 126 mg/dL and above is diabetes territory once a second test confirms it; and readings past 200–300 mg/dL call for prompt medical attention rather than routine follow-up.

High readings usually mean insulin isn't keeping up — from insulin resistance, insufficient insulin production, medications, or acute stress. Low readings usually trace back to diabetes medications, prolonged fasting, alcohol, or (rarely) hormone problems. In both directions, symptoms sharpen the picture: an abnormal number with matching symptoms carries far more weight than an isolated surprise on otherwise routine bloodwork.

What can shift this value

  • How long you fasted — even coffee with cream or a shortened fast can raise the result
  • Diabetes medications: insulin and sulfonylureas lower glucose, sometimes too far
  • Other medications: corticosteroids, thiazide diuretics, some antipsychotics, and niacin raise it; beta-blockers can mask low-sugar symptoms
  • Acute stress, pain, or illness — stress hormones push glucose up
  • Time of day: early-morning hormone surges (dawn phenomenon) nudge fasting values higher
  • Exercise: a hard workout can lower glucose for up to 24 hours afterward
  • Alcohol, which can drop glucose hours later, especially on an empty stomach
  • Sample handling: blood left sitting before processing reads falsely low, a few mg/dL per hour
  • Pregnancy, which changes glucose handling and uses entirely different cutoffs
  • Lab versus home meter: fingerstick meters are allowed a margin of error and often differ from lab draws by several points

When it's tested

Glucose is included in the basic metabolic panel (BMP) and comprehensive metabolic panel (CMP), so it shows up in nearly all routine bloodwork. It's also ordered on its own as a fasting glucose, as part of an oral glucose tolerance test (OGTT), and alongside an A1C when screening for or monitoring diabetes. People with diabetes also track it daily with home meters or continuous glucose monitors, which use the same units but slightly different accuracy standards.

US labs report glucose in mg/dL, while the UK and most other countries use mmol/L — multiply mg/dL by 0.0555 to convert (so {value} {unit} is about {si_value} {si_unit}).

Frequently asked questions

Is your value mg/dL a normal glucose level?

For a fasting test, most US labs consider 70–99 mg/dL normal. Where your value mg/dL falls depends on whether you were truly fasting — after a meal, readings up to 140 mg/dL are common and expected.

Does a fasting glucose of your value mean I have diabetes?

Diabetes is diagnosed when fasting glucose is 126 mg/dL or higher on two separate tests, or 200 mg/dL or higher with classic symptoms. One number alone is rarely a diagnosis — confirmation with a repeat test or A1C is standard.

What blood sugar level is dangerous?

On the low side, glucose below 54 mg/dL is clinically serious and below about 40 mg/dL is a lab panic value — severe lows can cause seizures or loss of consciousness. On the high side, sustained readings above 300 mg/dL risk diabetic ketoacidosis or a hyperosmolar state, and many labs flag values around 400 mg/dL and above as critical. Symptoms matter as much as the number: confusion, vomiting, or labored breathing alongside an extreme reading means emergency care.

Do I need to fast before a glucose test, and does coffee affect it?

A fasting glucose test requires 8–12 hours with nothing but water — food, juice, soda, and even coffee with cream or sugar can raise the result. Black coffee has little direct sugar but can still nudge glucose up slightly in some people, so most labs ask you to skip it. If you didn't fast properly, tell your clinician; the fix is usually just repeating the test, not worrying about the number.

How can I lower my blood sugar?

The proven levers are losing 5–7% of body weight if you're overweight, getting about 150 minutes of moderate activity a week, cutting sugary drinks, and prioritizing sleep. If lifestyle changes aren't enough or your level is high, medications like metformin are safe, well-studied next steps — your clinician can match the plan to your numbers.

What's the difference between a glucose test and an A1C test?

A glucose test is a snapshot — your blood sugar at the moment of the draw, which shifts with meals, stress, and sleep. A1C reflects your average glucose over the past 2–3 months by measuring sugar attached to red blood cells. Clinicians often use both: glucose to catch the day-to-day picture and confirm fasting status, A1C (normal below 5.7%, diabetes at 6.5% or higher) for the long-term trend.

Browse Glucose values

All 381 values from 20 to 400 mg/dL

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Educational information, not medical advice. Full disclaimer