Fasting Blood Glucose Testing in Hypertensive Patients

Fasting Duration Requirements

The American Diabetes Association, European Society of Cardiology, and European Association for the Study of Diabetes recommend a minimum 8‑hour overnight fast with no caloric intake before fasting plasma glucose measurement in adults, including hypertensive patients at cardiovascular risk. 1
Evidence from major guidelines confirms that shorter fasting periods (e.g., 3–6 hours) are not endorsed, reinforcing the 8‑hour minimum for accurate results. 2

Blood should be drawn in the early morning after the overnight fast to minimize diurnal variation and standardize glucose values. 6

Level A evidence supports routine diabetes screening in patients with hypertension, making precise fasting glucose measurement essential in this population. 1
In hypertensive men with normal fasting glucose, a 2‑hour oral glucose tolerance test (OGTT) independently predicts cardiovascular mortality; the OGTT also requires an 8‑hour overnight fast before the 75‑g glucose load. 1

Patients should consume an unrestricted diet containing at least 150 g of carbohydrate daily for three days before testing to avoid falsely low glucose values. 5
Normal physical activity should be maintained in the days preceding the test; abstaining may alter glucose metabolism. 5
Acute illness, acute stress, or recent vigorous exercise should be avoided because they can falsely affect glucose results. 5
During the testing period, patients should remain seated and refrain from smoking to prevent transient glucose alterations. 5

Place the sample tube immediately in an ice‑water slurry and separate plasma within 30 minutes to prevent glycolysis‑related glucose loss. 6
If immediate processing is not feasible, use tubes containing citrate buffer (glycolysis inhibitor). 6
At room temperature, untreated samples lose glucose by approximately 5‑7 % per hour due to glycolysis. 5
Do not delay plasma separation beyond 30 minutes unless glycolysis inhibitors are employed, to avoid artifactually low glucose concentrations. 6

Hemoglobin A1c testing does not require fasting and can serve as an alternative diagnostic test, whereas fasting plasma glucose specifically mandates an 8‑hour fast. 3
In the absence of unequivocal hyperglycemia, a diabetes diagnosis requires two abnormal test results (e.g., fasting glucose, OGTT, or HbA1c) obtained on separate occasions or from different test modalities. 3