A fasting glucose test or fasting blood glucose (FBG) is a test that is used for screening, diagnosing, and monitoring diabetes mellitus. It is also used to identify people with prediabetes. It is a blood test that measures the levels of glucose after no caloric intake for a minimum of 8 hours, which is also known as a fast.1 It can be performed at the doctor’s office or at home by pricking your finger and using a glucometer. The laboratory test is used for screening and diagnosis, while the at-home glucometer is used to monitor blood glucose levels for people with diabetes. In a laboratory the glucose hexokinase method or the glucose oxidase peroxidase method are used for glucose testing.2 When used at the doctor’s office, readings from a glucometer help provide information for counseling and can guide treatment.1
Some advantages of this test are that it is easy to complete, low-cost, readily available, accurate, and fast.1 However, the downsides are that it reflects the glycemia only in the moment of sampling and can be influenced by other factors. Biological factors that can influence the results of a fasting glucose test include acute illness, duration of fasting, pharmaceutical drugs, ethnicity, sex, and age. Some other factors that can influence the results are sample instability, errors during the application and extreme environmental conditions.2,3 Because FBG results might be influenced by other factors and it measures one moment of glycemia, a HbA1c test might be preferred for confirmation of diagnosis of diabetes since it measures long-term glycemia.2,3
The expected values are:1
- Normoglycemic when <100 mg/dL,
- Prediabetic when 100-125 mg/dL
- Diabetic when 126 mg/dL
References:
- Committee ADAPP. 2. Diagnosis and Classification of Diabetes: Standards of Care in Diabetes—2024. Diabetes Care. 2023;47(Supplement_1):S20-S42. doi:10.2337/dc24-S002
- Sacks DB, Arnold M, Bakris GL, et al. Guidelines and Recommendations for Laboratory Analysis in the Diagnosis and Management of Diabetes Mellitus. Diabetes Care. 2023;46(10):e151-e199. doi:10.2337/dci23-0036
- Rodriguez-Segade S, Rodriguez J, Camiña F, et al. Prediabetes defined by HbA(1c) and by fasting glucose: differences in risk factors and prevalence. Acta Diabetol. Sep 2019;56(9):1023-1030. doi:10.1007/s00592-019-01342-5
