When to Test Your Blood Sugar

Monitoring your blood sugar helps your doctor determine if your treatment plan is working and if your medication needs adjusting. Blood sugar that’s too high, referred to as hyperglycemia, or too low, known as hypoglycemia can cause health problems such as dizziness, confusion, blurred vision and even coma. The good news: “A patient with well-controlled type 2 diabetes who’s treated with metformin (an oral medication) may only check their glucose a few times a week or month,” says Dr. Kohlenberg. (If you have type 1 diabetes and administer multiple insulin injections each day, you’ll need more frequent monitoring.)

“Blood sugar vacillates throughout the day due to many factors,” says Megan Wolf, a registered dietitian and diabetes care and education specialist at Cleveland Clinic in Ohio. The preferred way to keep track of it? Whatever works best for you. “This may include journaling numbers into a logbook or typing blood sugar readings into an app,” says Wolf. “A lot of my patients find continuous glucose monitors helpful.” Glucometers and continuous glucose monitoring systems store glucose data that can be uploaded to a computer. Then the patient can review their blood sugar data on their device or keep a detailed written log.

Testing your blood sugar both right before and after meals will give you an idea of how food affects your sugar levels. Ideally, you’re looking for numbers of 80-130mg/dL before and less than 180 mg/dL one to two hours after eating. According to Wolf, “Someone should check blood sugar before meals when they are on a medication, such as insulin, that requires them to adjust their dose based on their blood sugar reading.” This so-called correction dose is given if your blood sugar is above your target. It is helpful to check blood sugar one to two hours after eating as well because it tells you how your body responded to your meal.

It’s important to check blood sugar more frequently if your physician changes your diabetes medications or if you’re prescribed other medications, including steroids and certain antibiotics, which can affect blood sugar, notes Wolf. Depending on the drug, it can do wonky things to your blood sugar soon after taking it, sometimes within a few hours. “For example, glucocorticoids [steroids] such as prednisone can cause hyperglycemia (elevated glucose) via multiple mechanisms,” adds Dr. Kohlenberg. His advice: Anytime you start a new medication, review it with your physician to see if this will impact your glucose and diabetes treatment plan.

Testing your blood sugar before you go to sleep and after you awaken will reveal if it is high or low—and what might be causing that. Normally, the body releases insulin in the early morning to keep glucose levels, which naturally rise in the a.m., in check. If you have diabetes and your blood sugar is high when you awaken, you may need medication adjustments, says Wolf. On the other hand, if you check your blood sugar before you go to sleep and it’s on the lower end, you may want to eat a snack, adds Wolf. This applies for anyone taking insulin to control their blood sugar. Your physician will likely adjust your medication if your blood sugar is consistently low before bedtime.

“When we exercise, our muscles use glucose as a source of energy and this increased energy use persists after exercise,” says Celeste Thomas, M.D., assistant professor of medicine in the department of endocrinology, diabetes, and metabolism at the University of Chicago. “This puts some people with diabetes at risk for low blood sugar [hypoglycemia] during and after exercise.” For anyone taking insulin, checking your blood sugar both before and after exercising can help you keep tabs on any sudden drop. Adjusting your blood sugar-lowering medication prior to exercise and eating carbohydrates prior to or during a workout can help stabilize those readings, says Wolf.

When you’re sick, your body produces extra stress hormones (including cortisol), which can raise your blood sugar levels, says Betul Hatipoglu, M.D., professor of medicine at Case Western Reserve University School of Medicine in Cleveland, OH. As a result, “illness, especially infection, can put those with type 1 diabetes at risk for a diabetes emergency called diabetic ketoacidosis,” adds Dr. Thomas. This is a complication resulting from not enough insulin and needs to be treated immediately with fluids and additional insulin medication. Testing your blood glucose levels regularly can help you spot a spike like this and take action quickly.

Whether physical or psychological, any stress has the same effect. “Stress hormones such as cortisol push the liver to produce more sugars and make the body more resistant to insulin,” says Dr. Hatipoglu. The result: higher blood sugar levels. Of course, everyone gets stressed at times. But it’s chronic stress and its effect among people with diabetes—who are not able to process the additional sugar—that’s especially problematic. Your best strategy: “Monitor blood sugar levels frequently and, if needed, adjust the treatment to prevent any complications,” he says.

If you have type 1 diabetes or have type 2 diabetes and take insulin, it’s possible to develop low blood sugar while driving, causing difficulty concentrating, making decisions, and operating the vehicle. You may even pass out. Before you head out on the road, check your blood sugar levels, especially if you’re taking insulin or insulin-releasing med, such as Glucotrol (glipizide), says Dr. Thomas. Depending on the length of your journey, you should also pull over during your trip to check your levels again. The bottom line: Swings in glucose levels with diabetes can be controlled if you monitor them carefully.