Monitoring blood glucose

Glucose, the simplest of all sugars, enters the bloodstream from two main sources: food and drink consumed and the liver. Your blood glucose levels will fluctuate throughout the day. There are various ways that you can measure your blood glucose.

Finger-prick measurement using a glucometer. This is how you find out what your blood glucose level is at that moment in time. It’s a snapshot. You’ll need these things to do the measurement:

1. A glucometer (glucose monitor). There are several brands on the market so it might be best to ask your healthcare team for advice on a glucometer that will suit you. Some have a larger display for people with impaired sight and others have sophisticated technology allowing you to upload glucose readings to a smartphone or computer. Remember many medical practices will be able to supply a glucometer free of charge. Always make sure that you get the correct strips to match the glucometer you’re using.

2. A finger-prick or lancing device which comes with the glucometer. These pierce the skin with a needle (lancet) so that a drop of blood can be taken for measurement. The depth of piercing can be adjusted by dialling numbers 1 to 5, with 1 being a very shallow and 5 the deepest. The thickness of your skin will determine what number to set. There is no set time to change the lancets but if pricking your finger becomes particularly painful then a new lancet is indicated. People with diabetes soon work out how often they prefer to change the lancet. To change the lancet, remove the cap from your finger-prick device and put in a new lancet usually by twisting and pushing in. Then put the cap back on. 

3. Glucose strips. These strips usually come in batches of 50 and must work with the type of meter you’ve chosen.

How to measure your blood glucose

  • Wash your hands with soap and warm water. Don’t use wet wipes or sanitiser as products in them can affect the result. Ensure your hands are warm so it’s easier to get blood thus reducing discomfort.

  • Take a strip, keeping the side with the bar code facing up, slot the barcode   end into the glucometer. 

  • The glucometer will now switch on and a display, usually a flashing drop of blood, will indicate it’s ready for the test.

  • Load the plunger of the pricking device by pulling the top of it backwards until a click is heard.

  • Choose which finger to prick but avoid your thumb and index finger. Try not to prick the middle of the finger pad, or too close to a nail. Place the flat end of the device against the side of your finger and press the release button (often white or yellow). Use a different finger or area of the finger for the next measurement.

  • Take your meter with the strip and hold it against the drop of blood. It will tell you if the strip is filled, usually by beeping. 

  • Before you look at your reading, check your finger. Use a tissue to stop bleeding if necessary. If you get the depth of prick correct, then often there will be very little blood to wipe away.

  • Most meters show the result in five seconds. Usually, they have a memory you can access but if you’re keeping a diary then note it down.

  • You can use the same tissue to take  out the strip and throw that away too.    

  • Taking out the strip will usually turn the meter off.

  • If you’re still unsure then google your device, and invariably there will be a website, or video explaining. This is useful for resetting the time and date on the glucometer.

Electronic blood glucose monitor, called a flash glucose monitor or continuous glucose monitor (CGM).These devices don’t require regular skin pricking or needles. Instead, they use a sensor inserted into your skin. They measure the amount of sugar in the fluid surrounding your cells, called interstitial fluid rather than blood glucose. There will always be a lag between interstitial fluid glucose and blood glucose so they may not match exactly. Most CGM devices are so accurate that they don’t need to be calibrated against a finger-prick measurement. The CGM transmits the measurements to a reader or smartphone. 

Know your body and how it works

Measuring your blood glucose should always be done with a purpose. This can be to assess the effect of medication, including insulin, on your blood glucose. It’s also valuable to know what food, exercise, and other life events, such as illness, can do to your blood glucose.

To see how food affects your blood glucose, it’s best to measure before and two hours after a meal. This can be recorded in your machine to be downloaded later, or in a diary. Some meters are linked to apps which allow you to see the readings on your smartphone.  Alternatively, there are also diabetes apps you can manually enter your reading into and then see them in the form of graphs or tables.

The most important reason to check your blood glucose is to have information that will continue to improve your control. Your diabetes team will continue to help in the decision-making process, but it’s also valuable to keep learning how to act on blood glucose readings. 

How often should you test

This depends mainly on your current diabetes control and the type of treatment you’re receiving. As a rule, people on oral medications should measure about three times a week. Patients using insulin should measure as often as they inject. This means a person taking one long-acting insulin injection daily should check their fasting blood glucose daily. A person injecting a mixed insulin twice daily should measure before each injection, so two readings daily. Similarly, a person using a basal bolus regimen of one long-acting insulin daily 

and short-acting insulin before meals should check before each injection and before bed: four readings daily. However, there are people who check many times more than this, especially if using a CGM.


HbA1c Fasting blood glucose mmol/L Two-hour post meal blood glucose mmol/L
YOUNG < 6,5% 4 – 7 4,4 – 7,8
MOST < 7,0% 4 – 7 5 – 10
ELDERLY < 7,5% 4 – 7 < 12

Low blood glucose

If your blood glucose is too low, usually below 4 mmol/L, you may experience  hypoglycaemia. These need to be treated immediately by consuming quick-acting sugar, otherwise your blood glucose will drop further. If this happens, you may experience a severe hypo and need emergency treatment. Your healthcare team will advise on the amounts and types of sugar that can be used.

High blood glucose

The level at which symptoms begins to appear is different for everyone, but the common ones include passing more urine than normal, being very thirsty, having headaches and feeling tired and lethargic.

For people using insulin, a ketone test should be done if there is persistent or unexpected high readings. Ketones are acids produced when the body breaks down fat stores. The presence of ketones indicates a shortage of insulin, and is dangerous causing nausea, vomiting, dehydration and coma. People using insulin should be educated on how to manage ketones. However, if you have ketones always contact your healthcare team or go to the nearest emergency room.

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