Glucose tolerance test explained in one article

Author: Hou Jiayi, Deputy Chief Technician, Shanxi Provincial Hospital of Traditional Chinese Medicine

Reviewer: Li Ning, deputy chief physician, Shanxi Provincial People's Hospital

When a physical examination finds that fasting blood sugar is elevated, doctors often ask patients to do a glucose tolerance test. Why do we need to do a glucose tolerance test? How does a glucose tolerance test help diagnose diabetes? The following will answer your questions one by one.

1. What is a glucose tolerance test?

The glucose tolerance test is a glucose load test, which is divided into two forms: oral and intravenous. The most commonly used in clinical practice is the "oral glucose tolerance test", which detects the body's ability to regulate blood sugar by taking glucose once, and is used for screening prediabetes and diagnosing diabetes. The sugar regulation mechanism of normal people is normal. Blood sugar will rise temporarily after eating, reaching the highest peak after 0.5 to 1 hour, and will return to normal levels in 2 to 3 hours. When there is insulin resistance or abnormal insulin secretion in the body, the body's ability to absorb and utilize sugar decreases. After taking a certain amount of glucose, blood sugar concentration will increase significantly and cannot return to normal levels in a short time. This situation indicates that the body's ability to regulate blood sugar is reduced. This phenomenon is called impaired glucose tolerance.

2. How to do a glucose tolerance test

(1) Three days before the test, the sugar content in the subject's daily food should not be less than 150 g, and the subject should be able to maintain normal activities and stop taking drugs that may affect the test (such as insulin).

(2) After fasting for 10 to 16 hours, draw venous blood in a sitting position to measure fasting blood glucose.

(3) Dissolve 75g of anhydrous glucose (or 82.5g of glucose powder containing 1 molecule of water) in 250-300ml of water and drink it within 5 minutes. The dosage for pregnant women is 100g, and the oral glucose dosage for children is calculated at 1.75g/kg body weight, and the total amount should not exceed 75g.

(4) Draw venous blood to measure blood glucose concentration 1 hour, 2 hours, and 3 hours after taking sugar. Draw venous blood 4 times in total during the whole test. During the blood draw, the "seven no's" should be followed, i.e. no insulin injection, no drinking, no eating, no medication, no infusion, no smoking, and no strenuous exercise. Among them, the blood glucose concentration 2 hours after taking sugar is the key to clinical diagnosis.

Figure 1 Glucose powder containing 1 molecule of water

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3. How to interpret the results of a glucose tolerance test

(1) Normal glucose tolerance: fasting blood glucose 3.90-6.11 mmol/L, blood glucose peaks at 6.70-11.10 mmol/L 1 hour after taking sugar, blood glucose <7.80 mmol/L 2 hours after taking sugar, and blood glucose ranges from 3.89 to 6.11 mmol/L 3 hours after taking sugar.

(2) Impaired fasting blood glucose: fasting blood glucose 6.10-7.00 mmol/L, blood glucose <7.80 mmol/L 2 hours after taking sugar.

(3) Impaired glucose tolerance: mild glucose tolerance that has not yet reached the standard of diabetes. Fasting blood sugar is 6.11-7.00 mmol/L, and blood sugar ranges from 7.80 to 11.10 mmol/L 2 hours after taking sugar.

(4) Diabetes: fasting blood glucose ≥7.00 mmol/L and/or blood glucose ≥11.10 mmol/L 2 hours after taking sugar.

Figure 2 A normal glucose tolerance test result and curve

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4. In what situations is a glucose tolerance test necessary?

This test can be used to determine whether the patient has diabetes when blood sugar is abnormally elevated but has not yet reached the diagnostic criteria for diabetes.

(1) Urine sugar is positive but fasting blood sugar is normal or slightly higher than normal.

(2) Fasting blood sugar is normal but the postprandial blood sugar level is at a critical level.

(3) Age ≥40 years, fasting blood glucose ≥5.60 mmol/L or random blood glucose ≥6.50 mmol/L.

If the following conditions occur, screening should be conducted every year.

(1) Overweight (BMI ≥ 24 kg/m2) or obese (BMI ≥ 28 kg/m2).

(2) Patients with atherosclerotic cardiovascular and cerebrovascular diseases.

(3) Those with a history of delivering a macrosomia (fetal weight ≥ 4.0 kg) or a history of gestational diabetes.

(4) Those with a history of hypertension (blood pressure ≥140/90 mmHg) or who are currently receiving antihypertensive treatment.

(5) Those with high-density lipoprotein cholesterol ≤ 0.9 mmol/L, triglycerides ≥ 2.22 mmol/L, or those who are receiving lipid-lowering treatment.

(6) History of impaired glucose regulation in previous screening.

(7) People who lead a sedentary lifestyle.

(8) Those with a history of transient steroid diabetes.

(9) Patients with polycystic ovary syndrome.

(10) Patients who have been receiving long-term treatment with antipsychotics and/or antidepressants.

(11) Those with a first-degree relative with a history of diabetes.