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Blood Sugar (Fasting)

Diabetes Biomarker

Sample Needed

Collection Type: Blood

Body System

Related System: Diabetes

Overview

Fasting blood sugar (fasting plasma glucose) measures the concentration of glucose in the blood after an overnight (usually 8–12 hour) fast. It reflects how well the body maintains baseline blood glucose through insulin secretion and glucose uptake by tissues. The test is used to screen for and monitor diabetes mellitus and prediabetes, and to evaluate unexplained low-glucose episodes (hypoglycemia). Symptoms prompting testing include increased thirst, frequent urination, unexplained weight loss, fatigue, blurred vision, faintness, or sweating. Results can vary with age (older adults often have higher fasting levels), pregnancy (different thresholds), acute illness, medications (steroids, diuretics), and ethnicity; men and women have similar reference ranges but differ in risk profiles.

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Test Preparation

Fasting for 8-12 hours before test

Why Do I Need This Test

Profile: Included in the Diabetes screening/monitoring profile.
Symptoms: Polyuria, polydipsia, unexplained weight loss, fatigue, dizziness, sweating.
Diagnoses/monitoring: Detects/monitors diabetes, prediabetes (impaired fasting glucose), and evaluates hypoglycemia.
Causes of abnormal levels: insulin resistance, pancreatic beta-cell dysfunction, acute stress/illness, medications.
Biological meaning: High fasting glucose = inadequate glucose regulation; low = excess insulin or deficient glucose production.
Lifestyle/family: Poor diet, obesity, inactivity, alcohol use, and family history of type 2 diabetes increase need for testing.

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Symptom Checker

Understanding Test Results

<70 mg/dL: Consistent with hypoglycemia; may cause sweating, tremor, confusion, syncope; causes include excess insulin or oral hypoglycemics, prolonged fasting, alcohol, liver disease, or endocrine disorders.
Immediate evaluation/treatment required if symptomatic.
70–99 mg/dL: Normal fasting glucose indicating adequate basal glycemic control.
100–125 mg/dL (5.6–6.9 mmol/L): Impaired fasting glucose (prediabetes) elevated risk for progression to type 2 diabetes and cardiovascular disease; lifestyle intervention recommended.
≥126 mg/dL (≥7.0 mmol/L) on two separate fasting tests: Diagnostic of diabetes mellitus.
A single elevated value usually requires repeat confirmation or corroboration with HbA1c or a 2-hour OGTT.
In pregnancy, different thresholds apply for gestational diabetes screening and diagnosis.

Normal Range

70-99 mg/dL OR 3.9-5.5 mmol/L

FAQs

Q: What is a normal fasting blood sugar range?

A: A normal fasting blood sugar (glucose) level is generally between 70 and 99 mg/dL (3.9–5.5 mmol/L). Levels of 100–125 mg/dL indicate prediabetes, and a fasting level of 126 mg/dL or higher on two separate tests suggests diabetes. Individual targets may vary with age, pregnancy, or health conditions—discuss personal targets with your healthcare provider.

Q: How many hours of fasting for a blood sugar test?

A: For a fasting blood sugar (fasting plasma glucose) test, fast 8–12 hours beforehand—most labs require at least 8 hours, commonly done after an overnight fast. Only plain water is allowed; avoid food, alcohol, and sugary drinks. Continue usual medications only if your clinician advises it. Schedule the test in the morning and avoid heavy exercise or smoking before the draw.

Q: Is fasting blood sugar 8 or 12 hours?

A: Fasting blood sugar is measured after an overnight fast of at least 8 hours; many labs accept a fasting period of 8–12 hours. For diagnostic fasting plasma glucose, a minimum 8-hour fast is standard, though some recommendations allow up to 12 hours. Only water is permitted; avoid food and caloric drinks, and follow your lab or clinician’s instructions.

Q: How to lower fasting blood sugar in pregnancy?

A: To lower fasting blood sugar in pregnancy: monitor levels and share results with your care team; eat balanced, consistent evening meals avoiding late high‑GI carbs; have a small bedtime snack combining protein and low‑GI carbohydrate; increase post‑meal light activity (e.g., walking); maintain healthy sleep and stress habits; and follow prescribed insulin or medication adjustments only under clinician guidance. Frequent follow‑up ensures maternal and fetal safety.

Q: What is the danger level of fasting blood sugar?

A: Fasting blood sugar (FBS) normal: <100 mg/dL; 100–125 mg/dL indicates prediabetes; ≥126 mg/dL (on two tests) indicates diabetes. Hypoglycaemia is <70 mg/dL (levels <54 mg/dL are severe). Very high fasting levels (e.g., >250–300 mg/dL) increase risk of diabetic ketoacidosis or hyperosmolar crisis seek urgent care for extreme thirst, vomiting, breathlessness, confusion, or very high readings.

Q: What are 5 signs your blood sugar is too high?

A: Five common signs your blood sugar is too high are increased thirst, frequent urination, increased hunger, persistent fatigue, and blurred vision. You may also notice slow wound healing or recurrent infections; if symptoms occur, check blood glucose and consult a healthcare professional. Immediate medical advice is important if levels are very high.