Also seen on reports as: FBS, FPG, Fasting Glucose
What is Fasting Blood Sugar?
Fasting Blood Sugar (FBS) measures the level of glucose in the blood after the body has gone several hours without food — usually 8 to 12 hours of overnight fasting, with only plain water allowed during that period.
Because food has not been digested for several hours, FBS shows what the body's baseline blood sugar is, without the influence of a recent meal. It is one of the oldest and most widely used screening tests for diabetes.
The test is run on a small blood sample drawn from a vein. Most Indian labs report the result in milligrams per decilitre (mg/dL). Some labs also report a parallel value in millimoles per litre (mmol/L); to convert mg/dL to mmol/L, divide by 18.
Why doctors order it
FBS is most commonly ordered as a screening test for diabetes and prediabetes in adults — particularly during routine health checks, when symptoms suggesting high or low sugar are present, or when there is a family history of diabetes, obesity, polycystic ovary syndrome (PCOS), or known cardiovascular risk.
ICMR guidance recommends screening for diabetes in Indian adults from age 30 onwards, or earlier if risk factors are present. In someone already managing diabetes, FBS is used alongside HbA1c and post-meal sugar values to understand the day-to-day sugar pattern.
FBS is also part of the standard package for pregnancy bookings, pre-operative assessments, and routine corporate or pre-employment medicals across India.
What the result means
Indian and international diabetes guidelines (ICMR-RSSDI, ADA) use the same fasting-glucose categories. They are screening categories — a diabetes diagnosis is usually confirmed with a second test (either a repeat FBS, an HbA1c, or an OGTT) before treatment decisions are made.
The 'lean diabetic' phenotype is more common in Indians than in many Western populations, meaning fasting sugars can be in the diabetes range even when body weight looks normal. A normal weight is not, by itself, a reason to skip screening.
An isolated raised FBS does not always mean diabetes. Stress, acute illness, certain medicines, and pre-test fasting that ran longer than expected (true overnight fasting >12 hours) can shift the value. Doctors typically retest and combine with HbA1c or OGTT for clarity.
Reference range
Values in mg/dL (after at least 8 hours of fasting)
- Normal
- Below 100 mg/dL
- Prediabetes (Impaired Fasting Glucose)
- 100 – 125 mg/dL
- Diabetes (screening)
- 126 mg/dL or higher, confirmed on a repeat test
- Hypoglycaemia (low) — needs urgent attention
- Below 70 mg/dL, especially with symptoms
These cut-offs follow ICMR-RSSDI / ADA guidance and are used by most NABL-accredited Indian labs. Pregnancy, certain medications, and acute illness shift interpretation — your doctor reads the value in context.
In India
What this test means for an Indian reader
ICMR diabetes screening criteria recommend fasting blood sugar testing for adults from age 30, or earlier when risk factors are present. The 'lean diabetic' phenotype — type-2 diabetes at normal BMI — is more common in Indians than in most Western populations, so a normal weight is not a reason to skip screening.
ICMR-RSSDI follows the ADA cutoffs: normal <100 mg/dL, prediabetes 100–125 mg/dL, diabetes ≥126 mg/dL on two occasions. Borderline FBS is often followed up with HbA1c or an OGTT.
Indian guidelines & references
- ICMR National Diabetes Guidelines
- RSSDI (Research Society for the Study of Diabetes in India)
- Association of Physicians of India (API)
Common factors that affect the result
Several common factors can push FBS up or down on the day of the test. Inadequate or excessive fasting (less than 8 hours or more than 12) can both cause distorted readings. Acute infection, fever, recent surgery, poor sleep, and high stress on the morning of the test can raise the result. Some medicines — particularly steroids, certain diuretics, and some antipsychotics — can also push FBS higher.
On the other side, prolonged fasting beyond 14 hours, recent intense exercise, certain antidiabetic medicines taken inadvertently the morning of the test, and significant alcohol intake the night before can lower the reading. People with anaemia or recent illness should mention this to the doctor.
Pregnancy lowers fasting sugar levels on average; thresholds for diagnosing gestational diabetes are different from those used in non-pregnant adults.
When to talk to your doctor
- Your FBS is in the prediabetes range (100–125 mg/dL) for the first time, or has crept up from previous values.
- Your FBS is 126 mg/dL or higher on screening — a repeat test or HbA1c will usually be advised.
- Your FBS is below 70 mg/dL, particularly if you have symptoms like sweating, shakiness, dizziness, or confusion. Levels under 54 mg/dL need urgent attention.
- You are already on diabetes treatment and your FBS is consistently above or below your doctor's set target.
- You are planning a pregnancy, are pregnant, or have PCOS — sugar control matters earlier and at stricter targets in these situations.
Related tests
Sources
- ICMR Guidelines for Management of Type 2 Diabetes — Indian Council of Medical Research
- RSSDI Clinical Practice Recommendations — Research Society for the Study of Diabetes in India
- ICMR-INDIAB Study — Indian Council of Medical Research
- Standards of Care in Diabetes — American Diabetes Association
- Classification and Diagnosis of Diabetes Mellitus — World Health Organization
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