Glucose Tests: FBS, PPBS, GCT, OGTT, DIPSI, HbA1c & More
Blood glucose testing is not just one test. Depending on the clinical question, the laboratory may perform fasting glucose, random glucose, post-meal glucose, glucose tolerance tests, pregnancy glucose testing, HbA1c, capillary glucose or continuous glucose monitoring. This complete LABDECODED guide explains what each test measures, when it is used, how the sample is collected, important cut-offs, and how to interpret the result.
- What is blood glucose?
- Fasting Blood Sugar / Fasting Plasma Glucose
- Random Blood Sugar / Random Plasma Glucose
- PPBS / Post-Prandial Blood Sugar
- OGTT / GTT
- 50-g GCT in pregnancy
- DIPSI test
- 75-g pregnancy OGTT
- 100-g 3-hour OGTT
- HbA1c
- Glucometer / Capillary glucose
- Continuous Glucose Monitoring
- Diabetes diagnostic criteria
- Prediabetes / intermediate hyperglycaemia
- Important laboratory factors
- Interactive glucose calculator
- Pregnancy glucose calculator
- Frequently Asked Questions
🩸 What Is Blood Glucose?
Glucose is one of the body's major energy substrates. Blood glucose is regulated mainly by insulin, glucagon, the liver, skeletal muscle and adipose tissue.
A glucose result can change substantially depending on whether the patient is fasting, recently ate a meal, received a glucose load, is pregnant, exercised, received medication or has an acute illness.
1️⃣ Fasting Blood Sugar — FBS / FPG
Fasting Plasma Glucose (FPG) is measured after the patient has had no caloric intake for at least 8 hours.
Water is generally permitted, but tea, coffee with sugar or milk, juice, biscuits and other caloric drinks can invalidate the fasting condition.
| FPG | mg/dL | mmol/L | General interpretation* |
|---|---|---|---|
| Normal range | <100 | <5.6 | Normal fasting glucose |
| Impaired fasting glucose | 100–125 | 5.6–6.9 | Prediabetes range |
| Diabetes range | ≥126 | ≥7.0 | Requires diagnostic confirmation when appropriate |
*Non-pregnant adult diagnostic categories based on ADA 2026 criteria. Diagnosis should not be made from an isolated abnormal result unless the clinical situation provides unequivocal hyperglycaemia.
2️⃣ Random Blood Sugar — RBS / RPG
Random Plasma Glucose (RPG) can be collected at any time of day regardless of when the patient last ate.
A random plasma glucose of ≥200 mg/dL (≥11.1 mmol/L) can meet the diagnostic criterion for diabetes when accompanied by classic symptoms of hyperglycaemia or a hyperglycaemic crisis.
3️⃣ PPBS — Post-Prandial Blood Sugar
PPBS is generally measured approximately 2 hours after the start of a meal, depending on the laboratory or clinical protocol.
It is commonly used in people with diabetes to assess glucose after food and to monitor treatment.
4️⃣ OGTT / GTT — Oral Glucose Tolerance Test
An Oral Glucose Tolerance Test evaluates how the body handles a defined glucose load.
For non-pregnant adults, the standard diagnostic OGTT uses a 75-g anhydrous glucose load, with fasting and 2-hour plasma glucose measurements.
| 75-g OGTT | Result | Interpretation in non-pregnant adults |
|---|---|---|
| Fasting | <100 mg/dL | Normal fasting glucose |
| Fasting | 100–125 mg/dL | Impaired fasting glucose |
| 2-hour | 140–199 mg/dL | Impaired glucose tolerance |
| 2-hour | ≥200 mg/dL | Diabetes range |
5️⃣ 50-g GCT — Glucose Challenge Test
The 50-g Glucose Challenge Test (GCT) is commonly used as the first step in a two-step approach to gestational diabetes screening.
Unlike a fasting OGTT, the GCT is generally performed without requiring fasting.
- Give 50 g oral glucose.
- Measure plasma glucose approximately 1 hour later.
- If the screening threshold is reached, proceed to a diagnostic OGTT according to the clinical protocol.
| 1-hour GCT threshold | Use |
|---|---|
| 130 mg/dL | More sensitive screening threshold used in some protocols |
| 135 mg/dL | Used in some clinical protocols |
| 140 mg/dL | Common screening threshold |
6️⃣ DIPSI Test — Important in India
DIPSI stands for Diabetes in Pregnancy Study Group of India.
The DIPSI approach uses a 75-g oral glucose load and measures venous plasma glucose after 2 hours, irrespective of whether the woman is fasting or non-fasting.
Glucose Load
Oral glucose load
Sample
After glucose administration
Cut-off
mg/dL threshold used by DIPSI
| DIPSI parameter | Protocol |
|---|---|
| Glucose | 75 g oral glucose |
| Fasting required? | No — DIPSI is designed for non-fasting testing |
| Blood sample | Venous plasma glucose |
| Timing | 2 hours after glucose load |
| GDM threshold | ≥140 mg/dL |
7️⃣ 75-g OGTT in Pregnancy — One-Step Approach
A commonly used one-step pregnancy approach uses a fasting 75-g OGTT with measurements at fasting, 1 hour and 2 hours.
| Time | Threshold | mmol/L |
|---|---|---|
| Fasting | ≥92 mg/dL | ≥5.1 |
| 1 hour | ≥180 mg/dL | ≥10.0 |
| 2 hours | ≥153 mg/dL | ≥8.5 |
8️⃣ 100-g 3-Hour OGTT — Carpenter–Coustan
In the traditional two-step approach, women who screen positive on the 50-g GCT undergo a fasting 100-g, 3-hour OGTT.
| Time | Carpenter–Coustan threshold | mmol/L |
|---|---|---|
| Fasting | ≥95 mg/dL | ≥5.3 |
| 1 hour | ≥180 mg/dL | ≥10.0 |
| 2 hours | ≥155 mg/dL | ≥8.6 |
| 3 hours | ≥140 mg/dL | ≥7.8 |
9️⃣ HbA1c — The Long-Term Glucose Marker
HbA1c reflects average glycaemic exposure over approximately the preceding 2–3 months, with greater influence from more recent weeks.
| HbA1c | General category |
|---|---|
| <5.7% | Normal |
| 5.7–6.4% | Prediabetes range |
| ≥6.5% | Diabetes range |
🔟 Capillary Glucose / Glucometer
Point-of-care glucose meters measure glucose in capillary blood and are extremely useful for immediate monitoring.
Home Monitoring
Useful for fasting and post-meal monitoring.
Hospital
Useful when rapid bedside glucose information is required.
Laboratory Diagnosis
Diagnostic decisions should follow validated laboratory protocols.
1️⃣1️⃣ Continuous Glucose Monitoring — CGM
Continuous Glucose Monitoring systems estimate glucose in interstitial fluid at frequent intervals and can provide a much richer picture of glucose patterns than isolated blood glucose measurements.
CGM reports may include:
- Time in range
- Time above range
- Time below range
- Glucose management indicator
- Glucose variability
- Daily glucose profile
1️⃣2️⃣ Diabetes Diagnostic Criteria — Non-Pregnant Adults
According to the ADA Standards of Care 2026, diabetes can be diagnosed using HbA1c or plasma glucose criteria.
| Test | Diabetes threshold |
|---|---|
| HbA1c | ≥6.5% |
| Fasting plasma glucose | ≥126 mg/dL (≥7.0 mmol/L) |
| 2-hour 75-g OGTT | ≥200 mg/dL (≥11.1 mmol/L) |
| Random plasma glucose | ≥200 mg/dL with classic symptoms or hyperglycaemic crisis |
1️⃣3️⃣ Prediabetes / Intermediate Hyperglycaemia
| Test | Prediabetes range |
|---|---|
| HbA1c | 5.7–6.4% |
| Fasting plasma glucose | 100–125 mg/dL |
| 2-hour 75-g OGTT | 140–199 mg/dL |
1️⃣4️⃣ What About Low Glucose?
Glucose interpretation is not only about high glucose. Low glucose may also be clinically important, especially in people receiving insulin or glucose-lowering medication.
1️⃣5️⃣ Laboratory Factors That Can Change Glucose Results
⏱️ Delay in separating plasma from cells
Glycolysis continues after blood collection. If the sample is not handled appropriately, glucose can fall during transport or storage.
🧪 Sample type matters
Plasma and serum glucose values should not be treated as identical under every circumstance. Modern diagnostic recommendations generally specify plasma glucose.
🥤 Fasting status
A fasting sample should be genuinely fasting. Even small caloric intake before collection can affect the result.
🏃 Exercise
Recent strenuous exercise can influence glucose metabolism and may alter the measured result.
💊 Medication
Insulin, sulfonylureas, steroids and several other medications can affect glucose concentrations.
🤒 Acute illness
Infection, surgery, trauma and physiological stress can cause significant changes in glucose.
1️⃣6️⃣ Glucose Sample Collection — Laboratory Checklist
| Step | What to check |
|---|---|
| Patient identification | Correct patient and request form |
| Test requested | FBS, RBS, PPBS, OGTT, GCT, DIPSI or other protocol |
| Timing | Record exact time of glucose load and blood collection |
| Fasting | Confirm fasting duration where required |
| Glucose load | Verify correct dose and preparation |
| Sample | Use the validated specimen type and collection system |
| Transport | Minimize delay and follow laboratory stability requirements |
1️⃣7️⃣ Glucose Tests at a Glance
| Test | Glucose load | Fasting? | Sample timing | Main use |
|---|---|---|---|---|
| FPG/FBS | None | Yes | Fasting | Screening/diagnosis |
| RPG/RBS | None | No | Any time | Clinical assessment |
| PPBS | Normal meal | Usually no | Post-meal | Glucose monitoring |
| 75-g OGTT | 75 g | Yes | Fasting + 2 h | Diabetes diagnosis |
| 50-g GCT | 50 g | No | 1 h | GDM screening |
| DIPSI | 75 g | No | 2 h | GDM screening/diagnosis protocol |
| 75-g pregnancy OGTT | 75 g | Yes | 0, 1 and 2 h | GDM diagnosis |
| 100-g OGTT | 100 g | Yes | 0, 1, 2 and 3 h | Two-step GDM diagnosis |
| HbA1c | None | No | Single blood sample | Long-term glycaemic assessment |
🧮 Interactive Glucose Conversion & Interpretation Calculator
Enter a glucose value and select the test. The calculator converts mg/dL ↔ mmol/L and provides a protocol-based educational interpretation.
Glucose Test Calculator
LABDECODED TOOL🤰 Pregnancy Glucose Test Calculator
Pregnancy glucose testing can follow different protocols. Select the protocol and enter the available glucose values.
Pregnancy Glucose Test Interpreter
GCT • DIPSI • 75-g OGTT • 100-g OGTT1️⃣8️⃣ Key Takeaways
FBS
Requires fasting and is one of the principal tests for diagnosing diabetes.
RBS
Can be performed at any time, but diagnostic interpretation depends strongly on symptoms and clinical context.
OGTT
Uses a standardized glucose load and specific timed samples.
DIPSI
Uses 75 g glucose and a 2-hour value without requiring fasting.
GCT
A 50-g, 1-hour pregnancy screening test in two-step protocols.
HbA1c
Provides an estimate of longer-term glycaemic exposure.
❓ Frequently Asked Questions
Is FBS the same as FPG?
FBS usually refers to fasting blood sugar, while FPG means fasting plasma glucose. In clinical laboratory practice, FPG is the more specific terminology when plasma glucose is being measured.
Can I drink water before an FBS test?
Plain water generally does not provide calories and is usually permitted during fasting. Follow the specific instructions given by your laboratory or clinician.
Is 200 mg/dL always diabetes?
Not necessarily from every test. A random plasma glucose ≥200 mg/dL has diagnostic significance when accompanied by classic symptoms or hyperglycaemic crisis. Other glucose tests have their own criteria.
Is DIPSI the same as a 75-g OGTT?
They both use 75 g glucose, but they are not necessarily performed in the same way. DIPSI uses a 2-hour measurement irrespective of fasting status, whereas the one-step pregnancy OGTT uses fasting, 1-hour and 2-hour measurements.
Is GCT the same as OGTT?
No. The 50-g GCT is generally a screening test. A diagnostic OGTT is performed according to the protocol when screening indicates further testing.
Why are glucose results reported in mg/dL and mmol/L?
Both units are used internationally. To convert glucose: mmol/L = mg/dL ÷ 18 and mg/dL = mmol/L × 18.
Can a glucometer diagnose diabetes?
Point-of-care glucose testing is valuable for monitoring and clinical decisions, but diagnostic testing should follow validated laboratory and clinical protocols.
Why can two glucose tests give different results?
Different tests have different glucose loads, timing, fasting requirements, specimen types and analytical methods. Patient factors and sample handling can also contribute.
🧠 Final Thoughts
Glucose testing becomes much easier to understand once the test name, timing and glucose load are separated clearly.
FBS, RBS, PPBS, GCT, DIPSI, 75-g OGTT, 100-g OGTT and HbA1c answer different clinical questions.
For laboratory professionals, the most important principle is to document the exact protocol: fasting status, glucose dose, collection time, sample type and applicable interpretation criteria.
FBS, PPBS, GCT, DIPSI, OGTT or HbA1c?
Share your experience in the comments below.
🔬 Explore More Laboratory Guides
LABDECODED brings together practical laboratory medicine, interpretation guides, calculators and diagnostic explanations.
Explore more laboratory resources at LABDECODED
This article is intended for educational and laboratory information purposes only. It does not replace professional medical advice, diagnosis or treatment.
Glucose and gestational diabetes criteria can differ between professional organizations, countries and clinical protocols. Always use the protocol specified by the treating clinician, laboratory or applicable guideline.
Do not diagnose diabetes or gestational diabetes solely from this calculator or from an isolated laboratory result.
📚 References & Further Reading
-
American Diabetes Association.
Standards of Care in Diabetes—2026: Diagnosis and Classification of Diabetes.
Diabetes Care.
ADA — Diagnosis and Classification of Diabetes -
World Health Organization.
Pregnancy assessment and gestational diabetes diagnostic criteria.
WHO — Assessment Methods During Pregnancy -
Diabetes in Pregnancy Study Group of India (DIPSI).
Published evidence describing the 75-g non-fasting 2-hour approach.
DIPSI Evaluation Study — Indian Journal of Medical Research -
Carpenter–Coustan pregnancy OGTT criteria.
ADA Standards — Pregnancy Glucose Criteria
0 Comments