The biochemical hallmark of diabetes is elevated blood glucose. Therefore, diagnosis of diabetes is made by estimation of glucose concentrations in the blood.
When the fasting plasma glucose is ≥ 126 mg/dl or random blood glucose ≥ 200 mg/dl on more than one occasion.1
Fasting Plasma Glucose:
Elevated fasting plasma glucose is always regarded to have a high degree of specificity for the diagnosis of diabetes. It is more consistent and reproducible than postprandial plasma glucose because there are more variable in the latter, such as timing and carbohydrate load. FPG may be easier to control with medications than PPG. An overnight fasting for 8 – 12 hours is considered desirable.
The ADA and WHO have recommended FPG value of ≥ 126 mg/dl as the diagnostic value for diabetes and the value of 110 – 125 mg/dl have been termed as impaired fasting glucose which is a prediabetic stage.1
Postprandial Blood Glucose:
The word postprandial means after a meal and hence it refers to plasma glucose concentration after food intake. The optimal time to measure postprandial glucose concentrations is 2 hr after the start of a meal. An elevated PPG concentration is one of the earliest abnormalities of type 2 diabetes, and represents an independent risk for cardiovascular disease. Postprandial changes precipitate atherosclerosis before FPG concentrations are affected. The recommended PPG goal of treatment is a value of <160mg/dl.
Oral Glucose Tolerance Test
It is recommended for diagnosis/exclusion for diabetes.
The Oral Glucose Tolerance Test (OGTT) is done in the morning after 8-10hrs of overnight fast (water may be taken). A fasting blood sample should be taken before giving glucose load. The person then drinks 75gm of glucose in 250-300 ml of water (the glucose load should be consumed over a period of five minutes). A further blood sample must be collected 2 hours after the load.
Diabetes is diagnosed if the fasting value is ≥ 126 mg/dl or 2 hour plasma glucose is ≥ 200mg/dl.1
Impaired Glucose Tolerance is present when the 2 hour value is in the range of ≥140 mg/dl - < 200 mg/dl.
Impaired Fasting Glucose is present when the fasting level is ≥ 110mg/dl and ≤ 125mg/dl and the 2 hour value is < 140mg/dl.
Glucose tolerance is normal when the fasting and the 2 hour values are < 110mg/dl and < 140mg/dl respectively.
Hemoglobin A1c (HBA1c)
The red blood cells that circulate in the body live for about three months before they die off. When sugar sticks to these hemoglobin proteins in these cells, it is known as glycosylated hemoglobin or hemoglobin A1c (HbA1c). Measurement of HbA1c gives us an idea of how much sugar is present in the bloodstream for the preceding three months. In most labs, the normal range is 4%-5.6 %. In poorly controlled diabetes, its 8.0% or above, and in well controlled patients it's less than 7.0% (optimal is <6.5%). The benefits of measuring A1c is that gives a more reasonable and stable view of what's happening over the course of time (three months), and the value does not vary as much as finger stick blood sugar measurements.
While there are no guidelines to use HbA1c as a screening tool, it gives a physician a good idea that someone is diabetic if the value is elevated. Right now, it is used as a standard tool to determine blood sugar control in patients known to have diabetes. The recommended cut-off points are 2
Normal range: 4.5 – 5.6%
Prediabetes range: 5.7 – 6.4%
Diabetic range: ≥ 6.5%
Gestational Diabetes Diagnosis:3
75-g OGTT with Plasma Glucose measurement fasting and at 1 h and 2 h, at 24-28 wks in women not previously diagnosed with overt diabetes
Perform OGTT in the morning after overnight fast ( ≥8 h)
GDM diagnosis made if PG values meet or exceed:
Fasting : 92 mg/dL (5.1 mmol/L)
1 hour : 180 mg/dL (10.0 mmol/L)
2 hour : 153 mg/dL (8.5 mmol/L)
50-g GLT (nonfasting) with Plasma Gluocse measurement at 1 h (Step 1), at 24-28 wks in women not previously diagnosed with overt diabetes
If Plasma Glucose at 1 h after load is ≥140 mg/dL (7.8 mmol/L), proceed to 100-g OGTT
(Step 2), performed while patient is fasting
GDM diagnosis made when two or more Plasma Glucose levels meet or exceed:
Fasting : 95 mg/dL or 105 mg/dL (5.3/5.8 mmol/L)
1 hour : 180 mg/dL or 190 mg/dL (10.0/10.6 mmol/L)
2 hour : 155 mg/dL or 165 mg/dL (8.6/9.2 mmol/L)
3 hour : 140 mg/dL or 145 mg/dL (7.8/8.0 mmol/L)
- Definition and Diagnosis of Diabetes and Intermediate Hyperglycemia – Report of a WHO/IDF consultation. Geneva: Whorl Health Organization; 2006.
- World Health Organization. Use of Glycated Haemoglobin in the Diagnosis of Diabetes Mellitus. Abbreviated Report of a WHO Consultation. Geneva: WHO; 2011.
- Standards of Medical Care in Diabetes—2015, Diabetes Care Volume 38, Supplement 1, January 2015