Fasting Blood Sugar Testing: Common Questions Answered

Q: Fasting Blood Sugar Testing: Common Questions Answered

Question 1： The diagnostic criteria for diabetes published by the American Diabetes Association in 2010 are as follows： a. <br>Fasting plasma glucose level ≥ 126 mg/dl after at least 8 hours of fasting. <br>b. <br>Plasma glucose level ≥ 200 mg/dl during a 2-hour oral glucose tole

Please ask again about the timing for blood glucose testing and any other related questions?

Hello, Dr.
Hsiao! I have read your previous response regarding my question about blood sugar testing times and other related issues, but I still have some confusion.
I would like to ask again about the timing of blood sugar tests and other questions.
Thank you for your help.

1.
In your previous response, you mentioned that clinically, it is emphasized to fast for 8 to 12 hours; there is no strict time point.
Does this mean that fasting for anywhere between 8 to 12 hours is acceptable for testing fasting blood sugar and other tests?
2.
If the fasting period exceeds 12 hours, will this affect the accuracy of the fasting blood sugar and other test results?
3.
The hemoglobin A1c test does not require fasting, meaning it can be drawn at any time, correct? If it is done while fasting, does it also mean fasting for 8 to 12 hours is acceptable? If the fasting period exceeds 12 hours, will this affect the hemoglobin A1c results? Is there a difference in values between fasting and non-fasting hemoglobin A1c tests?
4.
I have been fasting for 8 to 12 hours, and sometimes even exceeding 12 hours, for my hemoglobin A1c tests.
Can these results be trusted? Does the hemoglobin A1c reflect values from the past 2 months or 3 months?
5.
You previously mentioned that the postprandial blood sugar test should be done 2 hours after starting to eat, with the eating time being approximately 30 minutes.
Does this mean the timer starts when I take the first bite of food, and I should draw blood exactly 2 hours after finishing my meal? For example: if I start eating at 9:05 AM, finish at 9:25 AM, should I draw blood at 11:05 AM? Is it necessary for the eating time to be around 30 minutes if I finish in 20 minutes?
6.
You also mentioned not to focus too much on single blood sugar and lipid values, but rather to pay attention to the overall patient condition and changes in test values.
What does this refer to specifically?
Finally, I apologize for the inconvenience, but my family doctor can only see me in September, so I kindly ask for your clarification on each question to help me feel more at ease.
Thank you very much for your assistance!

ntyu3_5, 30~39 year old female. Ask Date: 2015/04/26

Dr. Xiao Yongxun reply Internal Medicine

Question 1: The diagnostic criteria for diabetes published by the American Diabetes Association in 2010 are as follows: a.
Fasting plasma glucose level ≥ 126 mg/dl after at least 8 hours of fasting.
b.
Plasma glucose level ≥ 200 mg/dl during a 2-hour oral glucose tolerance test.
c.
HbA1c ≥ 6.5% (must be confirmed with a method that is certified by the National Glycohemoglobin Standardization Program (NGSP) and meets the standards of the Diabetes Control and Complications Trial (DCCT)).
d.
Clinical symptoms of diabetes (such as polydipsia, polyuria, polyphagia, and unexplained weight loss) with a random plasma glucose level ≥ 200 mg/dl.
Criteria a, b, and c must be confirmed on two separate occasions; criterion d only requires one occurrence, thus fasting for at least 8 hours is necessary.
Question 2: Prolonged periods of time can lead to hypoglycemia in normal individuals, and diabetic patients may also have blood glucose levels close to normal, which can result in misinterpretation.
Question 3: Glycated hemoglobin (HbA1c) refers to the irreversible binding of glucose molecules in the blood to hemoglobin within red blood cells.
As blood glucose levels rise, the percentage of glycated hemoglobin increases.
Since this is an irreversible reaction and the lifespan of red blood cells is approximately 90 to 120 days, HbA1c reflects the average blood glucose levels over the past six weeks.
For HbA1c levels to show a decrease, the already glycated red blood cells must die, and new hemoglobin must bind to fewer glucose molecules.
Therefore, the HbA1c test is largely independent of fasting status.
Question 4: As mentioned above, it reflects the average over the past six weeks.
Question 5: The starting point is two hours after the initiation of eating, referred to as the postprandial glucose level at 2 hours; the duration of the meal (whether 20 minutes or 30 minutes) should not be considered.
Question 6: The patients we treat are living individuals, not just laboratory values.
Patients have various diseases, lifestyles, dietary habits, exercise routines, stressors, acute events, mental states, and personality traits that must all be taken into account, as these factors can significantly influence any laboratory values; this influence often exceeds concerns about fasting duration or the timing of blood draws post-meal.

Reply Date: 2015/04/26

More Info

Understanding fasting blood sugar testing and related queries is crucial for managing your health, especially if you are concerned about diabetes or other metabolic conditions. Here’s a comprehensive breakdown of your questions:
1. Fasting Duration for Blood Sugar Testing: When it is stated that fasting should be for 8 to 12 hours, it means that you can have your blood drawn anytime within that window. For example, if you fast for 8 hours, you can have your blood drawn at that point, or you can wait until 12 hours of fasting. The key is to maintain a consistent fasting period to ensure accurate results.

2. Impact of Extended Fasting: If you exceed the 12-hour fasting period, it may not significantly affect the fasting blood sugar results, but it can lead to changes in your metabolic state. Prolonged fasting can sometimes cause your blood sugar levels to drop lower than normal, which may not reflect your usual levels. Therefore, while it is generally acceptable to fast longer than 12 hours, it’s best to stick within the recommended timeframe for the most reliable results.

3. Testing for Glycated Hemoglobin (HbA1c): HbA1c testing does not require fasting, meaning you can have your blood drawn at any time of the day. However, if you choose to fast before the test, the same 8 to 12-hour fasting guideline applies. Exceeding 12 hours of fasting should not significantly impact the HbA1c results, as this test reflects your average blood sugar levels over the past 2 to 3 months. The values obtained from fasting versus non-fasting tests for HbA1c should not differ significantly.

4. Validity of HbA1c Results After Extended Fasting: If you have fasted for 8 to 12 hours or even slightly longer, the results of your HbA1c test can still be trusted. This test is designed to provide an average of your blood glucose levels over the previous two to three months, so it is not as sensitive to short-term fluctuations in blood sugar levels as fasting glucose tests.

5. Postprandial Blood Sugar Testing: For postprandial (after eating) blood sugar testing, the timing is crucial. You should start the timer from the moment you take your first bite of food. If you finish eating at 9:25 AM, you should have your blood drawn at 11:05 AM, exactly two hours after starting your meal. While it is ideal to take about 30 minutes to eat, it is not strictly necessary; the key is to ensure that you wait the full two hours after starting to eat.

6. Overall Health Monitoring: The advice to not focus solely on single blood sugar or lipid values emphasizes the importance of looking at trends over time. This means you should consider how your values change with lifestyle modifications, dietary changes, or medication adjustments. A single test result can be influenced by many factors, including stress, illness, or even the time of day. Therefore, monitoring your overall health and understanding the context of your test results is essential for effective management.

In conclusion, understanding the nuances of blood sugar testing can empower you to take charge of your health. If you have further concerns or specific symptoms, it is always best to consult with a healthcare professional who can provide personalized advice based on your medical history and current health status. Regular monitoring and open communication with your healthcare provider are key to managing your health effectively.

Similar Q&A

Understanding Fasting Blood Sugar Levels: Key Questions Answered

Hello, doctor. Currently, I measured my fasting blood glucose at a nearby pharmacy, and it was 136 (using a blood glucose meter). Question 1: I fasted for 8 hours (with water), and I finished eating at 1:00 AM. I was fasting from 1:00 AM until around 9:00 AM the next morning (w...

Dr. Xiao Yongxun reply Internal Medicine
Question 1: No problem. Question 2: According to the standards published by the American Diabetes Association (ADA, 2011), a fasting plasma glucose level of ≥ 126 mg/dL after fasting for more than eight hours should raise the suspicion of diabetes. Question 3: Although I haven&#x...

Understanding Blood Sugar Issues: Insights on PCOS and Metabolic Health

Question 1: Recently, due to polycystic ovary syndrome, I was prescribed oral metformin and SGLT2 inhibitors. Although these are not for diabetes treatment, I underwent blood tests due to gynecological examinations revealing metabolic issues. My fasting blood glucose was 106 mg/d...

Dr. Lin Jianliang reply Internal Medicine
1. On the day of fasting blood glucose testing, do not eat or take any medications. 2. Fasting blood glucose is 106 mg/dL, and hemoglobin A1c is 6.2%, indicating prediabetes. 3. Avoid all types of sugars, including brown sugar, black sugar, white sugar, cube sugar, fructose, and ...

Essential Tests for Diabetes Diagnosis and Dietary Guidance

When visiting a medical laboratory for tests, what should be checked? Should I test for fasting blood glucose levels, hemoglobin, or other items? Additionally, if someone has diabetes, can they consume sugar-free beverages or sugar-free soy milk? Lastly, is it true that teas that...

Dr. Xiao Yongxun reply Family Medicine
Dear Mr. Luka: According to the diagnostic criteria for diabetes set by the American Diabetes Association, a. Fasting plasma glucose level ≥126 mg/dL after fasting for more than 8 hours, b. Plasma glucose level ≥200 mg/dL during a 2-hour oral glucose tolerance test, c. HbA1c ≥6.5...

Understanding Fasting Blood Sugar Variations: What You Need to Know

Hello doctor, I have a habit of regularly checking my blood sugar. This morning, I went to the clinic and had my blood sugar tested, and the result was 124. About three months ago, at the same clinic, it was in the 90s. Then, about 10 minutes later, I went to another clinic and h...

Dr. Xiao Yongxun reply Internal Medicine
Dear Mr. Cheng, 1. According to the diagnostic criteria for diabetes set by the American Diabetes Association: a. A fasting plasma glucose level of ≥126 mg/dL after fasting for at least 8 hours, b. A 2-hour plasma glucose level of ≥200 mg/dL during an oral glucose tolerance test...