Blood Sugar Monitoring: When to Check and Why

Managing diabetes[1] is one part investigation and two parts action. Unlike some other diseases that rely primarily on professional medical treatment, diabetes treatment requires active participation by the person who has it. Monitoring your blood sugar level on a regular basis and analyzing the results is believed by many to be a crucial part of the treatment equation.

When someone is first diagnosed[2] with diabetes[3], he is usually given a blood sugar meter (or told to go buy one) and told how and when to use it, as well as what numbers to shoot for[4]. However, the advice a person receives on when to monitor and what the results should be generally depend on his type of diabetes, age and state of overall health. It can also depend on a health-care provider’s philosophy of care and which set of diabetes care guidelines he follows. At least three major health organizations have published slightly different recommendations regarding goals for blood sugar levels.

There is some common ground when it comes to blood sugar monitoring practices. For example, most people take a fasting reading before breakfast every morning. Some people also monitor before lunch, dinner and bedtime; some monitor after each meal; and some monitor both before and after all meals. However, when monitoring after meals, some people do it two hours after the first bite of the meal, while others prefer to check one hour after the start of a meal.

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To help sort out the whys and when of monitoring, three diabetes experts weigh in with their opinions. While they don’t agree on all the details, they do agree on one thing: Regular monitoring is critical in diabetes care.


Why monitor?

Self-monitoring is an integral part of diabetes management because it puts you in charge. Regardless of how you manage your diabetes — through diet[5] and exercise[6] alone or combined with oral medicines or insulin[7] — regular blood sugar monitoring provides immediate feedback on how your program is working.

“Checking your blood glucose gives you the freedom to make choices without worry, the confidence to learn from your actions, and the motivation to keep striving to do better,” says Linda Dale, RN, BA, CDE, Clinical Nurse Specialist in the Outpatient Diabetes Education Program at the University of Michigan Comprehensive Diabetes Center in Ann Arbor, Michigan. “Monitoring tells you that what you’re doing either is working or isn’t, and it serves as motivation to keep up actions that are working or to make changes.”

The important thing is to know how to interpret the numbers and take the necessary action. For example, if you take insulin and your blood sugar is high, you may need to bolus, or take more rapid-acting insulin, to bring your levels down into range. If you manage your type 2 diabetes[8] with diet and exercise, you might treat high blood sugar with a walk around the block.

People who use insulin and certain oral diabetes drugs are also at risk of developing low blood sugar, or hypoglycemia[9], which needs to be treated promptly when it occurs. Regular monitoring may enable you to catch and treat it early, and any symptoms of hypoglycemia should be checked with a meter reading.

Over time, blood sugar monitoring records can be analyzed for patterns of highs or lows that may suggest that a change is needed in the treatment regimen.

“Regular monitoring is especially helpful for showing the positive effects of exercise,” says Dale. “Say your readings have regularly been around 140 mg/dl, but you start taking a walk every day and you start getting more readings around 120 mg/dl. That will definitely boost your motivation.”

How often?

At least some studies have found that the more often people monitor their blood sugar with a conventional blood sugar meter, the better their glycosylated hemoglobin (HbA1c[10]) levels. (The HbA1c test is a measure of blood sugar control over the previous two to three months.) Other studies have reported similar benefits for continuous monitoring[11], in which a sensor worn under the skin transmits glucose measurements every minute (or every few minutes) to a receiver. The GuardControl Trial, for example, found that participants with type 1 diabetes[12] who used a continuous glucose monitor[13] for three months experienced a 1-percentage-point drop in their HbA1c levels.

“In a perfect world, people with type 1 diabetes should monitor six or seven times a day,” says Om Ganda, MD, Senior Physician at Joslin Diabetes Center[14] in Boston and Associate Clinical Professor of Medicine at Harvard Medical School[15]. “However, that’s often impractical because of time and resources.”

If a person is newly diagnosed, starting a new treatment, or having trouble with blood sugar control, most insurance companies will pay for more strips than usual for the person to monitor more frequently, he notes. A person whose Type 1 diabetes is in stable control should monitor a minimum of four times a day.

For people whose Type 2 diabetes in good control, Dr. Ganda recommends monitoring twice a day. But, he notes, the majority of his patients with Type 2 diabetes are not in good control and should check more often. Unfortunately, health insurance companies often cover only one or two test strips a day for people with Type 2 diabetes, which many experts feel isn’t really enough to offer useful information. In some cases, insurance companies will cover more strips if a doctor writes a prescription for more, and some people choose to buy more strips on their own, out of pocket.

Dale notes, “Sure, test strips are expensive, but the price has not gone up in the past 20 years. At about $1 a strip, they are no more expensive than a bottle of water or a cup of coffee. It’s a matter of making the choice of where you want to spend your money.”

For those who are limited to one or two strips a day, one way to get more information about blood sugar patterns is to check on different days at different times[16].

“If your post-breakfast reading is usually in range, skip that reading and alternate monitoring before and after lunch and before and after dinner,” Dr. Ganda advises. “It’s important to check at different times throughout the day to see the bigger picture of what’s going on.”

Although these experts endorse frequent monitoring regardless of type of diabetes or method of treatment, a recent study made headlines by finding little value in monitoring for people with Type 2 diabetes who are not taking insulin. In the study, participants were split into three groups: A control group had their HbA1c levels measured every three months but was not asked to use a blood sugar meter unless their doctor considered it essential to their management. A less-intensive group was asked to record three blood sugar values daily on two days during the week (one fasting reading and the other two readings before meals or two hours after meals). Participants were also given blood sugar goals for before and after meals and advised to consider contacting their doctor if their readings were consistently high or low. A more-intensive group was also told to monitor their blood sugar levels at home and was given training and support in timing, interpreting, and using the results of their blood sugar checks. At the end of the year, both home-monitoring groups saw a slight reduction in HbA1c levels. However, the researchers note the results were statistically insignificant.

“This study has some limitations,” says Dr. Ganda. “First, participants were asked to monitor less than once a day, which is not frequent enough to provide the information needed to make appropriate changes to behavior or medication. By the end of the study, roughly half of the participants were monitoring less than twice a week. For home monitoring to help you reach your goals, you need to check more frequently, analyze the data, and make changes as necessary.”

Fasting readings

A fasting blood sugar reading, taken first thing in the morning before you eat or drink anything, gives you a starting point for the day and helps to determine what is going on during the night. The American College of Endocrinology (ACE)[17] recommends aiming for fasting levels below 110 mg/dl, while the Joslin Diabetes Center[18] and the American Diabetes Association (ADA)[19] recommend a range of 80–130 mg/dl. If your readings are consistently higher than these goals, it may be because of the dawn phenomenon[20] or a result of the Somogyi effect[21].

In the dawn phenomenon, hormones released in the very early morning cause increased insulin resistance[22], resulting in higher blood sugar levels. This occurs in everyone, with diabetes or without. However, in people who don’t have diabetes, extra insulin is secreted, so the rise in blood sugar level is minimal. Common preventive treatments for high morning blood sugar caused by the dawn phenomenon include getting daily exercise, eating a carbohydrate-containing bedtime snack, or adding the drug metformin[23] (brand name Glucophage and others) to the diabetes control regimen.

The Somogyi effect, which is more likely to occur in people who use insulin, is a phenomenon in which low blood sugar during the night causes the body to release hormones that raise blood sugar levels, resulting in high morning levels. While a person’s first instinct for treating high morning readings may be to increase nighttime insulin, in fact, taking less insulin and going to bed with a higher blood sugar reading may be more effective at preventing the low that leads to the morning rise in glucose.

People who are experiencing high morning blood sugar levels are often encouraged to wake up at 3 AM on several occasions to check their blood sugar. High blood sugar at this time may point to the dawn phenomenon as the cause of the high morning readings, while low blood sugar at 3 AM may suggest the Somogyi effect.

Checking before meals

Similar to fasting readings, monitoring your blood sugar before meals gives you a baseline reading of your blood sugar before you eat. Some medical professionals call these preprandial readings.

“This is the best time to check your blood sugar, so you know what it is before you start the meal,” says Ananda Basu, MD, associate professor and consultant in the Division of Endocrinology at the Mayo Clinic[24] in Rochester, Minnesota. “Once you eat, your blood sugar is going to go up, but the baseline should be back to normal by the next meal.”

If your premeal readings are in the recommended range and your HbA1c test results are also in your target range, Dr. Basu says that monitoring after meals is not necessary. One reason is that blood sugar meters are most accurate when blood sugar levels are between 80 and 140 mg/dl, and after-meal spikes can exceed those levels. (For information about getting the most accurate readings from your blood sugar meter, check out “Getting Accurate Readings.”[25] And to learn about using alternate sites to monitor, see “When Your Fingers Have Had Enough.”[26])

“We don’t have any hard data from clinical studies showing the long-term effects of postmeal readings,” he adds. “We know that most people spike, but we don’t know if people who spike more have more complications[27] or those who spike less have fewer complications. Until more data prove that postmeal values are important, I find it hard to recommend postmeal monitoring.”

Checking after meals

Dr. Ganda agrees that premeal monitoring is a critical tool in diabetes management but says that sometimes postmeal readings are needed.

For example, postmeal readings are beneficial when a person’s fasting and premeal readings are in range, but his HbA1c level is high. Postmeal, or postprandial, readings are also important to assess a person’s response to short-acting pills that are taken just before meals or to the dose and timing of rapid-acting insulin given before meals. It can also give a sense of how much a person’s blood sugar level rises in response to certain foods or amounts of food.

“Diabetes is such an individualized disease that responses to foods vary from person to person,” Dr. Ganda says. “Unless people monitor after a meal, they will not understand the relationship between food and blood sugar. However, postmeal readings aren’t very useful unless you have premeal readings to compare them to.”

The most important time of life to check after meals is during pregnancy. “This is the only time we know for certain that postmeal readings have a direct and proven effect on outcomes,” Dr. Basu says. “For both mother and child, it is critical that pregnant women with diabetes monitor their blood sugar after meals.” Blood sugar goals for women with diabetes who are pregnant are “tighter” than those for adults who are not pregnant.

For people who choose to monitor after meals, the clock for when to check starts counting down at the start of the meal. Some people with diabetes monitor one hour after the start of meals in an effort to find their peak blood sugar level, then work to prevent spikes above certain levels. However, the experts say that’s not a good idea and that there isn’t any clinical, peer-reviewed data to support monitoring at one hour. The after-meal blood sugar goals for nonpregnant adults published by major diabetes organizations currently specify levels two hours after the start of meals.

“Looking at normal physiology, blood sugar peaks after you eat,” Dr. Ganda says. “For a person who doesn’t have diabetes, the peak occurs at 45 minutes to an hour after the meal. With diabetes, you peak a little later, and two hours is the standardized amount of time for blood sugar to come back down. There’s no need to check at one hour, because it is hard to interpret the numbers. It may be the peak at one hour, or the peak may not have occurred yet. Every person is different.”

Exactly what your blood sugar level should be two hours after meals is up for debate. As of 2017[28], ACE guidelines call for the tightest control, with two-hour readings below 140 mg/dl. The Joslin Diabetes Center[18] and the American Diabetes Association[29] suggest postmeal readings below 180 mg/dl.

“Postprandial goals are somewhat controversial,” Dr. Ganda says. “We haven’t really looked at this as a predictor of complications. Most of the research we have is based on HbA1c numbers, not postmeal readings. But, in my experience, to get an HbA1c result below 6.5 percent, your fasting blood sugar levels need to routinely be below 110 mg/dl, and your two-hour readings need to be less than 140 mg/dl. Unless you do that, you won’t be able to reach that goal.”

A change in attitude

For many people with diabetes, striving for tight control is a full-time job, and numbers outside the parameters of your goals can make you crazy. Dale, the diabetes educator from the University of Michigan, suggests a shift in perception that can help avoid knee-jerk reactions to high or low numbers: Instead of “testing” your blood sugar, “monitor” it.

“When you ‘test,'” she says, “the results can be interpreted to mean that you’ve ‘passed’ or ‘failed.’ It’s emotionally charged. When you ‘monitor’ instead, you gather information and make adjustments as necessary. You just need to ask, ‘What can I learn from this? Was my serving of pasta too large? Do I need to lower my insulin dose before exercise? What can I do better to prevent this from happening in the future?’ That’s how it should be for everyone.”

Want to learn more about managing blood sugar? Read “What Is a Normal Blood Sugar Level?”[30] “Managing Your Blood Glucose Ups and Downs”[31], and “Making Your Blood Glucose Monitor Work for You,”[32] then see our blood sugar chart[33].

Originally published May 13, 2009.

  1. diabetes:
  2. diagnosed:
  3. with diabetes:
  4. what numbers to shoot for:
  5. diet:
  6. exercise:
  7. insulin:
  8. type 2 diabetes:
  9. hypoglycemia:
  10. HbA1c:
  11. continuous monitoring:
  12. type 1 diabetes:
  13. continuous glucose monitor:
  14. Joslin Diabetes Center:
  15. Harvard Medical School:
  16. different days at different times:
  17. American College of Endocrinology (ACE):
  18. Joslin Diabetes Center:
  19. American Diabetes Association (ADA):
  20. dawn phenomenon:
  21. Somogyi effect:
  22. insulin resistance:
  23. metformin:
  24. Mayo Clinic:
  25. “Getting Accurate Readings.”:
  26. “When Your Fingers Have Had Enough.”:
  27. complications:
  28. As of 2017:
  29. American Diabetes Association:
  30. “What Is a Normal Blood Sugar Level?”:
  31. “Managing Your Blood Glucose Ups and Downs”:
  32. “Making Your Blood Glucose Monitor Work for You,”:
  33. blood sugar chart:

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