Medicine is full of numbers that describe the health of a human body. Most basically, there are numbers like body temperature, an indication of fever and infection, and blood pressure, a measure of the circulatory system’s health. The mathematics of medicine is particularly crucial when it comes to diabetes. If you regularly test your blood glucose with a meter, you’re familiar with some of the most important numbers for managing this condition. There are also different types of laboratory tests for blood glucose, each with its own numerical language. Here’s how they all add up.

Too High
Having too much glucose in the blood is basically the definition of diabetes. In all humans, glucose, a sugar, travels through the bloodstream to provide the cells with energy. The body usually tightly governs the amount of glucose that is circulating, but in people with diabetes that regulatory framework is broken and glucose accumulates in the blood. Diabetes is diagnosed using results from one of these blood glucose tests:

• The A1C test measures a person’s average blood glucose level over the previous two to three months; people with values that are 6.5 percent or higher have diabetes. (Prediabetes, higher than normal blood glucose that indicates a heightened risk for developing type 2 diabetes, is diagnosed at A1Cs between 5.7 and 6.4 percent.)
• The fasting plasma glucose (FPG) test measures glucose levels in the blood after a person hasn’t eaten for at least eight hours; diabetes is diagnosed if the result is 126 mg/dl or above. (Prediabetes is diagnosed when the FPG result is between 100 and 126.)
• The oral glucose tolerance test (OGTT) is taken two hours after drinking a measured amount of a glucose solution. If the result is 200 mg/dl or higher, the diagnosis is diabetes. (Prediabetes is diagnosed at OGTTs between 140 and 200.)

Diabetes can also be diagnosed if a person has classic symptoms of high blood glucose, like continual thirst and excessive urination, combined with the result of a random blood glucose test that is 200 mg/dl or more.

Aiming at a Target
The goal of treatment is to bring blood glucose down to near normal levels or to a target worked out with your doctor. This is believed to be the best way to avoid the complications of diabetes. If your blood glucose has been high for some time, you may also find that you feel a lot better when it gets lower—less tired and sluggish.

After diagnosis and during treatment, the A1C test is used to give doctors and patients a look at the big picture. This number can help answer questions like: Is medication working? Is blood glucose getting better or worse? Has weight loss or gain affected blood glucose levels? A1C tests should be done at least twice a year, but your doctor may order them more often.

The American Diabetes Association recommends that most people with diabetes keep their A1C at or below 7 percent, but you and your doctor may come up with a somewhat different target. So what does that 7 percent mean? It’s clearer if you think of it in terms of another diabetes number: the estimated average glucose, or eAG. An A1C of 7 translates to an average glucose of 154 mg/dl, which is expressed in the same units used by your blood glucose meter (table, left).

The A1C and eAG represent averages over time, but people with diabetes also need to know what their blood glucose is doing day to day and hour by hour. Information from this kind of testing lets you gauge the effects of certain foods and activities, like eating a plate of pasta or going for a jog. A standard handheld blood glucose meter can provide an on-the-spot test whenever you want it. If it’s important to see trends over the course of a day, you may be prescribed a continuous glucose monitor (CGM), which measures glucose levels in fluid just under the skin every few minutes. Both meters and monitors measure the concentration of glucose in an amount of fluid: Milligrams per deciliter, or mg/dl, is the unit used in the United States. (In most of the rest of the world, glucose is measured in millimoles per liter, or mmol/L.)

If you’re using the blood glucose meter alone, how often and when to test your blood glucose should be part of a plan you make with your doctor. People who take insulin typically test several times a day while others may test less. Testing frequency also depends on how well treatment is going. People with high blood glucose may test more often because they are looking for ways to bring it down. Changing medications is also a reason to check blood glucose more often than usual.

Since blood glucose levels fluctuate over the course of a day, it’s difficult to put a hard number on what is too high at any given point. For people aiming for a target A1C of less than 7 percent, levels should generally be under 130 mg/dl before meals and under 180 mg/dl after meals. Levels that are persistently over 200 mg/dl are cause for concern, though, and should prompt a call to your doctor.

Your target eAG will give you an idea of the range of numbers you should be shooting for to reach your goal. For example, with a target A1C of 7 percent, the corresponding eAG is 154 mg/dl. So, over a 24-hour period, values coming in from meters or monitors should fall on either side of that target so that the average is around 154 mg/dl.

Too Low
There is a hard number, however, for blood glucose that is too low: Generally, anything 70 mg/dl or less as measured by a meter or CGM needs immediate treatment with 15 grams of a fast-acting carbohydrate, like three or four glucose tablets, or half a cup of sugary (not diet) soda or juice. Low blood glucose, which is also called hypoglycemia, is a common side effect of some diabetes medications, including insulin and sulfonylureas. Skipping a meal, exercising, or drinking alcohol can also result in hypoglycemia.

In severe cases, hypoglycemia can lead to unconsciousness, coma, and even death. More frequently, though, low blood glucose is just uncomfortable, causing nervousness, shakiness, sweating, and other symptoms. These sensations can be the first clue that blood glucose is dipping and it’s time to test. Some people have a condition called hypoglycemia unawareness and don’t experience these symptoms. This makes testing very important so that dangerous lows may be avoided.

No one likes to feel reduced to a string of digits. And, at first, a steady stream of glucose readings from meters, CGMs, and laboratory A1C tests may seem overwhelming. But mastering those numbers and what they mean is key to getting control of your diabetes. That’s an equation for health.