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Diabetes care tips
Blood glucose monitoring - Section 2          Print this Article

Section 2
Blood glucose meters:  features and how they work
Alternative testing sites
Why should I test my blood glucose?
When should I test my blood glucose?
Meter Accuracy
Non-invasive blood glucose monitoring devices

Meters offer a wide range of features.  Be sure to talk with your doctor about which meter is best for your needs.

Blood glucose meters:  How they work and various meter features

In the United States, blood glucose is measured in mg/dL (milligrams per deciliter).  In most other countries, blood glucose is measured in mmol/L.  Many meters can be set for either measurement so it is very important that you always make sure your meter is set to the measurement you are familiar with.

Blood glucose meters measure how much glucose is in a given blood sample.  Where the sample comes from (fingers, toes, forearms, ear lobes) will affect test results.  Although alternative site testing is enticing (claiming to be less painful) I personally do not feel it is accurate enough for use on young children that cannot sense their own symptoms of hypoglycemia.  Readings from forearm (or toe) blood lag behind fingertip blood and cannot accurately reflect rapidly changing glucose levels in the rest of the body.  This means that a forearm sample could come back higher than is actually true and a person could really be suffering from hypoglycemia (low blood glucose) and need treatment.

A test strip is placed inside a blood glucose meter (you can ONLY use test strips designed for your particular meter). A drop of blood is then placed on the strip.  Blood reacts with chemicals in the test strip causing a small electronic pulse to be sent into the meter which then calculates the level of glucose in the blood sample and shows the result in a digital display.

Different makes of meters offer a variety of functions and data management capabilities to match individual testing needs. Newer models of meters feature one-step sampling and readout. The chemically-treated strip is placed in the device and a drop of blood is placed on the test strip which acts like a straw drawing the blood sample into the strip. and the meter then gives a reading.

Newer meters offer on screen averages for week or months at a time, can hold hundreds of previous test results with time and date, and may even have graphing capability. The new generation of meters may also have upload features so that a person or doctor can upload meter readings directly from the meter and into a computer for analysis and review. Some meters allow entry of additional data throughout the day, such as insulin dose, amount of carbohydrates eaten, stress, illness, or exercise data.   Not all meters have these feature so be sure to talk with your doctor about what features are available and suit your needs best.

Alternative testing sites

Some meters offer alternate site testing, which allows a consumer to test on sites other than the fingertips, which can become sore and callused over time.  However, alternate site testing is not as accurate as finger pricks.  If your reading is low from an alternate test site (i.e., forearm), or you feel like you may be low, you still have to repeat the test on a fingertip.  (I personally find finger sticks not only more accurate, but also less painful when done properly.  

The main drawback of alternative site testing is that they are not consistently reliable.    A person may test higher on the forearm than if they had tested on their fingers.  This can be a real problem if a person (especially child) is already on the low side.  A false higher reading may prompt a parent to give a correction bolus when in fact, the child is already, or heading into, a state of hypoglycemia (low blood glucose).

IOH Health Tip:  When traveling, it is better to use fingertips.  Rapid changes in blood sugar, especially on airplanes, may not be detected in alternative testing sites.  See more Traveling Tips for persons with diabetes.

 For now, finger tip testing is still more reliable than alternate site testing. 


Why should I test my blood glucose?

The glucose meter is a key element of home blood glucose monitoring (HBGM, also called "self glucose monitoring") by people with diabetes mellitus and other disorders that require blood glucose monitoring. Since approximately 1980, a primary goal of the management of type 1 diabetes has been the achievement of closer-to-normal levels of glucose in the blood for as much of the time as possible, guided by HBGM several times a day. This has greatly increased the trouble and time spent in the daily care of this disease but has also reduced rates of long-term complications and improved the management of short-term complications such as hypoglycemia.  It is estimated that approximately 85% of all diabetics do not adequately keep their blood glucose levels at, or near, their target ranges.  This may be one reason for the high rate of associated diabetes complications. The best way to manage your blood glucose levels is simple: test regularly, and often.  You cannot correct highs and lows if you don't know about them.

Glucose monitoring is also an important tool for helping your doctor determine your best care plan approach.  Readings can help determine what your basal (background) insulin should be, as well as your insulin-to-carb ration, correction factor, and how your blood glucose levels are faring during the night.

Meter use For determining or monitoring hypoglycemia

Although the apparent value of immediate measurement of blood glucose might seem to be higher for hypoglycemia than hyperglycemia, meters have been less useful. The primary problems are precision and ratio of false positive and negative results. An imprecision of ±15% is less of a problem for high glucose levels than low. There is little difference in the management of a glucose of 200 mg/dl compared with 260 (i.e., a "true" glucose of 230±15%), but the difference between 70 mg/dl and 55 (i.e., 67±15%) represents a more unsatisfactory uncertainty. The imprecision is compounded by the relative likelihood of false positives and negatives in populations of people with diabetes and those without diabetes. People with type 1 diabetes usually have glucose levels above normal, often ranging from 40 to 500 mg/dl, and when a meter reading of 50 or 70 is accompanied by their usual hypoglycemic symptoms, there is little uncertainty about the reading representing a "true positive" and little harm done if it is a "false positive."

In contrast, people who do not have diabetes but periodically have hypoglycemic symptoms, will have a much higher rate of false positives to true, and a meter is not accurate enough to base a diagnosis of hypoglycemia upon. Hypoglycemia (unrelated to diabetes) needs to be diagnosed by a medical professional.  This is especially important because hypoglycemia unrelated to diabetes can be attributed to other health problems.  A C-peptide test may be conducted to rule out an over production of insulin due to insulinomas (tumors on the pancreas).  Other tests can also help determine why a person with diabetes has hypoglycemic episodes -- something a home blood glucose test meter cannot determine.

A meter can occasionally be useful in the monitoring of severe types of hypoglycemia (e.g., congenital hyperinsulinemia), to ensure that the average blood sugar (when fasting) remains above 70 mg/dl.  


When should I test my blood glucose?

Your doctor will help you decide when to test and how often.  However, the more often you test the better your chances of catching lows and avoiding highs; both important factors in reducing your risk of long-term complications of diabetes.

Generally, most physicians will advise that you test:

  • Morning fasting blood glucose, before breakfast
  • Two hours post-prandial (after eating all your meals)
  • At lunch time
  • At dinner time
  • Before bedtime
  • Once, or more during the night (especially if you wake with symptoms of the Dawn Effect (Phenomenon) or Simogyi Effect.
  • Any time you have any of the symptoms of hypoglycemia (low) blood glucose, of hyperglycemia (high blood glucose).
  • Your doctor may also advise you to test more often during illness, or stressful events.
  • Before, during, and after (excessive) exercise.
  • It is also a good idea to test more frequently when driving for long periods because it is important to keep your glucose above 70 mg/dL while driving.
  • Anytime there is a change in your care plan (increase or decrease in insulin, or when it is administered, or change in insulin, or addition of medications) you will need to test more often to see how your body reacts.
  • You will also need to test more frequently if you are on an insulin pump, especially at first.  But the benefits of pumping far outweigh the inconvenience of extra finger sticks!

Also, you need to RECHECK blood glucose levels anytime you have suffered a hypoglycemia episode (70 mg/dL or lower) that required treatment.  Keep rechecking blood glucose until your levels have stabilized.  See "Hypoglycemia Treatment."


Glucose meter accuracy

Accuracy of glucose meters is a common topic of concern. Nearly all of the meters have similar accuracy (±10-15%) when used correctly. However, a variety of factors can affect the accuracy of a test including calibration of meter, ambient temperature, pressure use to wipe off strip, size of blood sample, high levels of certain drugs in blood, hematocrit (blood iron levels), dirt on meter, aging of test strips, and of course, wet fingers or fingers that are dirty or contaminated with food. Models vary in their susceptibility to these factors, and in their ability to prevent or warn of inaccurate results with error messages.

IOH Health Tip:  High altitude, pressurized airplan cabins, humidty and heat can all affect test strips as well as meter performance.  Be aware of these things when traveling and recheck if your blood glucose reading does not seem to match how you are feeling.  It is important to check your blood glucose levels more frequently when traveling!

You can learn more about how to test accurately in our "Blood Glucose Testing Tips for Accuracy" section.


Article disclaimer

Article by Lahle Wolfe, 02/24/2006. Ms. Wolfe is a diabetic, as well as the parent of a child diagnosed at age 4 with type 1 diabetes.


Family Doctor; Diabetes: Monitoring your blood glucose

Canadian Diabetes Assn.; How to monitor your blood glucose; Blood glucose monitoring

What are normal and target blood glucose ranges?

Print fast-reference guide

Diabetes Care Tips
Blood Glucose Monitoring
Section 1

About Glucose Monitoring and Glucose Meters

Terms to Be Familiar With

Lancet & Device Tips

"Less-Pain" Finger Sticks

Quick Summary of Glucose Testing Tips

Tips for Accuracy

Special Care Tips for Children

blood sugar blues
Blood Sugar Blues:  Williamson (The Fibromyalgia Relief Book), a medical writer, offers an overview of various medical conditions diabetes, heart disease, high cholesterol related to insulin resistance, a phenomenon caused by diet whereby people become immune to their own insulin, causing their blood sugar levels to fluctuate. While diabetics are usually aware of their dietary restrictions, Williamson believes that millions of other people are suffering, sometimes needlessly, because they eat the wrong foods, especially carbohydrates. With anecdotal and scientific evidence, she advocates a low carbohydrate/high protein diet for everyone. Much of the book focuses on several medical conditions (rather than on one sensationalized condition like "Syndrome X") and various treatments. Williamson reviews several popular diets, offering pros and cons, but lets readers decide which is for them. In clean, engaging prose, Williamson ably explains complicated jargon.

More Information

Testing for Ketones

Blood Glucose Monitors
Insulin Injection Devices
Symptoms, Causes, Treatment & Prevention of
Symptoms, Causes, Treatment & Prevention of
FDA Glucose Meter Warnings

Links for More Information
Meter Comparison

Non-Invasive Blood Glucose Monitors

glucowatch non-invasive glucose monitor     

The GlucoWatch offers non-invasive (no needle) glucose testing.  It is not a replacement for traditional glucose monitoring, but serves as a complementary approach to glucose monitoring.  For more information about the GlucoWatch visit GlucoWatch.

GlucoWatch is not intended to replace finger sticks.  It is meant to help trend glucose patterns and alert patients to hyperglycemia and hypoglycemia episodes. When GlucoWatch detects a high or low blood glucose reading you need to confirm the reading with a finger stick before taking action.  Be sure to replace the pad and allow for "warm up time" before relying on the GlucoWatch to alert your to abnormal sugars.

You can obtain more information about GlucoWatch on their site, or in our section on Glucose Meters.


Some paragraphs on this page contain excerpt, edited for content and style, from

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Page Updated 07/24/2006