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DIABETES SERIES · ARTICLE 7 OF 17
Blood sugar testing
When, why, and what the numbers actually mean.
If you have diabetes, you will spend a lot of time looking at numbers. Numbers on a meter, numbers on a clinic report, numbers your nurse writes down with a serious face. Most people are never really told what these numbers mean, what's normal, what's worrying, and what to do about them.
This article unpacks blood sugar testing. By the end you'll know which tests exist, when to do them, what the results mean in plain language, and what to do if a number worries you.
Before you read on
This article uses mmol/L (millimoles per litre), which is the standard unit in South Africa. If you've seen mg/dL — that's the American unit. To convert: multiply mmol/L by 18 to get mg/dL. So 7.0 mmol/L is the same as 126 mg/dL.
What blood sugar actually means
Blood sugar — or blood glucose — is the amount of sugar circulating in your bloodstream at any given moment. It is the body's main fuel. Every cell in your body uses it.
In a person without diabetes, blood sugar stays in a tight range — usually between 4 and 7 mmol/L through the day. It rises gently after meals, then comes back down. The pancreas releases just enough insulin to keep this balance.
In a person with diabetes, this control is broken. Blood sugar can rise too high after meals and stay there. Sometimes — especially on insulin or some tablets — it can drop too low. Testing is how you see what's actually happening, instead of guessing.
The four main tests
There are four blood sugar tests you'll encounter. Each one measures something different. Knowing which is which helps you understand your own results.
1. Fasting blood glucose (FBG)
Measures your blood sugar after at least 8 hours without food. Usually done first thing in the morning, before breakfast. This is the most common screening test.
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What it tells you: whether your blood sugar is being well-controlled overnight and at rest.
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Where it's done: any clinic, a lab, or a home glucose meter. Quick — results in seconds with a finger-prick.
2. Random blood glucose (RBG)
A reading taken at any time of day, regardless of when you last ate. Often used in emergency situations or when someone is feeling unwell.
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What it tells you: what your blood sugar is right now. Less useful for tracking control because it depends so much on when and what you last ate.
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Where it's done: any clinic, emergency room, or home glucose meter.
3. Post-meal blood glucose (postprandial)
Measures blood sugar around 2 hours after the start of a meal. This is the test that tells you how your body handled the food you just ate.
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What it tells you: whether your medication, portion sizes, and food choices are keeping post-meal spikes under control. One of the most useful tests for fine-tuning what you eat.
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Where it's done: usually at home with a meter. Set a timer when you start eating, test 2 hours later.
4. HbA1c (glycated haemoglobin)
This is the “big picture” test. It measures your average blood sugar over the past 2 to 3 months. It works because sugar in your blood binds to your red blood cells, and red blood cells live about 90 days. The percentage of cells with sugar attached tells the story of the last few months.
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What it tells you: whether your overall control is on track. Doesn't depend on what you ate yesterday.
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Where it's done: a lab test ordered by the clinic. Usually every 3 to 6 months for diagnosed diabetics. Free at public clinics under chronic care.
5. Continuous Glucose Monitoring (CGM)
A wearable sensor (usually on the arm) that reads glucose levels every few minutes through the day and night. Brands you may see in SA include FreeStyle Libre and Dexcom. Hugely useful but more expensive, and usually self-funded.
Understanding the numbers
Here's what the typical ranges look like in mmol/L. These are general targets — your clinic may adjust them up or down for your specific situation, especially if you are elderly, pregnant, or have other conditions.
Test Normal Pre-diabetes Diabetes Diabetic target Fasting glucose Under 5.6 5.6–6.9 7.0 or higher 4.0–7.0 mmol/L Random / 2-hour post-meal Under 7.8 7.8–11.0 11.1 or higher Under 10.0 mmol/L HbA1c (%) Under 5.7% 5.7–6.4% 6.5% or higher Under 7.0%
The most useful number to know for yourself
HbA1c is the single most important number a person with diabetes can know about their own control. It's the one your clinic measures. Anything under 7% means you're broadly on track. Above 8% means meaningful change is needed. Above 10% is dangerous over time.
Each 1% drop in HbA1c reduces your risk of complications (eye, kidney, foot, heart) by around 20–25%. That's an unusually clear medical relationship — bigger reductions mean bigger benefits, for years to come.
When and how often to test at home
How often you should test depends on what kind of diabetes you have and how you're being treated.
Situation How often to test Type 1 diabetes (on insulin) 4–10 times a day. Before meals, before bed, before driving, when you feel unwell. CGM use is increasingly standard. Type 2 on insulin 1–4 times a day depending on insulin schedule. Mornings and before evening meal are most useful. Type 2 on tablets (e.g. metformin) Once a day or every few days. Pick varying times so you build a picture across the day. Type 2 controlled with food and exercise alone Every few days to once a week. Fasting reading on a notebook day is most useful. Pre-diabetes Once a week, fasting, same day each week. Track the trend over months. Newly diagnosed (any type) More often in the first month while you learn how your body responds to food, exercise, and medication.
A common mistake: testing at the same time every day forever. You'll get the same kind of reading every time. More useful is to vary — fasting one day, before lunch the next, 2 hours after dinner the third. This builds a picture of your blood sugar through a whole day across a couple of weeks.
Choosing a home glucose meter
South African pharmacies sell several brands. The meter itself is usually cheap — often subsidised or even free. The cost lives in the test strips, which can add up. Some meters cost R30 per strip, others under R5.
What to look for:
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Strip cost. By far the biggest factor in long-term cost. Compare per-strip prices, not the meter price.
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Strip availability. Make sure your local pharmacy stocks the strips for your meter. Some imported brands are hard to refill.
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Memory / log function. Useful for showing the clinic patterns. Most modern meters connect to a phone app.
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Coding requirement. Older meters need you to code each new batch of strips. Newer meters do this automatically — less to go wrong.
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Sample size. Modern meters need only a tiny drop of blood. Older ones need more. Smaller is better — less painful.
Free meters at the clinic
Many South African public clinics will give a Type 1 or insulin-using Type 2 patient a meter and strips at no cost. Ask your clinic nurse. For other patients, pharmacy promotions often include a free meter when you buy strips.
If you can only afford a meter when you're already diagnosed, a finger-prick test once a week (timed consistently) is still much better than no testing at all.
What to do when a number looks wrong
This is the section nobody is taught well. Here's what to do at each level.
Your reading What it means What to do Under 3.9 mmol/L Hypoglycaemia (low blood sugar). Common on insulin or some tablets. Eat fast sugar immediately — 3 glucose tablets, or 1/2 glass of juice, or 3 sweets. Re-test in 15 min. Repeat if still low. 3.9–10.0 mmol/L On target. Carry on as usual. 10.1–13.9 mmol/L Higher than target. Common after big meals or stress. Drink water, walk for 15 min if safe. Note what you ate. Re-test in 1–2 hours. 14.0–19.9 mmol/L High. Sustained at this level it does damage. Drink water, take medication as prescribed, walk if you feel okay. If high readings continue over several days, see the clinic. 20.0 mmol/L or higher Dangerously high. Especially in Type 1, could be heading toward diabetic ketoacidosis (DKA). Drink water. If you have ketone strips, check. If you feel nauseated, are vomiting, have fruity-smelling breath, or are confused — go to a hospital immediately.
Treat a number as a signal, not a verdict
One high or low number is information, not failure. Look at the pattern across several days, not single readings. Reach out to the clinic when you see consistent shifts.
And remember — blood sugar fluctuates naturally. A reading 2 mmol/L different from yesterday is not necessarily a problem.
Common confusion points
Why does my morning reading sometimes go UP overnight?
Two reasons. The “dawn phenomenon” is when natural hormones (cortisol, growth hormone) released in the early morning push blood sugar up — completely normal but more pronounced in diabetes. The “Somogyi effect” is when a low blood sugar overnight triggers a rebound high in the morning. If your fasting readings are consistently high, mention both to the clinic.
Why are my readings different on the same finger / different fingers?
Hand temperature, blood circulation, and how you obtained the drop all affect readings. Use the sides of fingertips (less nerve, more blood), warm the hand first, and squeeze gently from base to tip rather than the very tip. Differences of up to 1 mmol/L between immediate retests are normal.
Why doesn't my HbA1c match my daily readings?
HbA1c averages everything across 90 days, including times you weren't testing. If your daily readings are mostly good but you have big after-meal spikes you don't catch, your HbA1c will be higher than your fingerprick averages suggest. Testing 2 hours after meals reveals this pattern.
Should I test before exercise?
Yes, especially if you're on insulin. If you're under 5.5 mmol/L, eat a small snack first. If you're over 14 mmol/L, hold off and check for ketones. In between, exercise is fine.
Does illness affect my blood sugar?
Yes — usually pushes it higher. Flu, COVID, infections, surgery and stress all raise blood sugar. Test more often when you're sick. Drink water. If you can't keep food down, contact the clinic — your insulin or tablet dose may need adjusting.
The bigger picture
Blood sugar testing is one of those things that sounds harder than it is. The real skill is not the finger-prick — it's looking at the results across days and weeks and learning what they tell you. Your meter is a feedback tool, not a verdict machine. The clinic uses the patterns to fine-tune your treatment.
With practice, the numbers stop being scary. They become a conversation with your own body — a clear, fast signal of how your food, your movement, your sleep and your stress are landing today.
The next article in this series goes into what happens when blood sugar drops too low — hypoglycaemia. Who's at risk, how to recognise it, what to do, and how to prevent it. Critical reading for anyone on insulin or sulfonylureas.
Where to get more help
Diabetes South Africa — diabetessa.org.za · 011 792 9888
Your nearest public clinic — free HbA1c testing under chronic care; ask for it every 3–6 months
Phila Today Diabetes Series — next: hypoglycaemia — recognising and treating low blood sugar
Phila Today · Article 7 of 17 in the Diabetes Series