Intro
Knowing your blood glucose level at the right moment can prevent a hypoglycaemic emergency, stop a high from turning into DKA, and give you the data to adjust your insulin safely. Even people using a continuous glucose monitor (CGM) still need to understand fingerstick testing — and everyone benefits from knowing how to read their numbers.
This guide covers the practical basics: when to test, what equipment to use, how to interpret results, ketone testing, common mistakes, and when high or low readings need urgent attention.
Key Points
- Blood glucose testing is fundamental to diabetes management — it informs every insulin dose, helps identify patterns, and catches dangerous lows and highs early
- Fingerstick meters give instant point-in-time readings; CGMs provide continuous trends and alerts — both have a place in modern diabetes care
- Target ranges are individualised — general adult targets (ADA) are 4.4–7.2 mmol/L (80–130 mg/dL) pre-meal and <10.0 mmol/L (<180 mg/dL) post-meal
- Always confirm with a fingerstick when CGM readings conflict with how you feel
- Test for ketones if glucose is persistently elevated, you are unwell, or your team advises it — high ketones with high glucose requires urgent medical care
- Record and review your readings — patterns over time matter more than individual numbers
Fingerstick Testing vs CGM
Fingerstick Blood Glucose Meters
A traditional meter requires a small drop of blood from a fingertip (or approved alternate site) on a test strip. Results take seconds and are highly accurate when performed correctly.
Advantages:
- Inexpensive and widely available
- Accurate reference standard for confirming CGM readings
- Works independently of sensor life or connectivity
Limitations:
- Captures only a single moment in time
- Requires repeated finger punctures
- Cannot show trends or warn of rapidly changing glucose
Continuous Glucose Monitors (CGM)
A CGM sensor sits just under the skin (usually on the upper arm or abdomen) and measures glucose every few minutes. Most systems display readings on a phone or receiver and alert you when glucose is rising or falling quickly.
Advantages:
- Real-time glucose trends and directional arrows
- Alerts for lows and highs, including overnight
- Substantially reduces time spent in hypoglycaemia in people with Type 1 Diabetes
- Integrates with some insulin pumps (closed-loop systems)
Limitations:
- Lags behind blood glucose by 5–15 minutes during rapid changes
- Requires calibration and accurate sensor placement
- Higher cost; not universally covered
Fingerstick checks are still needed when:
- Your CGM reading doesn’t match your symptoms
- Your glucose is changing very rapidly (CGM lag matters most here)
- You are about to treat a low or give a corrective dose and want confirmation
- The sensor is near the end of its wear period
When to Test
Your diabetes team will recommend a personalised testing schedule. Common recommended times include:
| Situation | Why |
|---|---|
| Before each meal | Guides insulin dose selection |
| 2 hours after meals | Checks post-meal spike (target: <10.0 mmol/L / <180 mg/dL) |
| Before bed | Prevents overnight lows |
| Before driving | Safety — treat any low before getting behind the wheel |
| During and after exercise | Exercise lowers glucose; monitor to prevent hypos |
| During illness | Illness raises glucose and ketone risk |
| When symptoms suggest a low | Confirm before treating |
| When symptoms don’t match CGM | Verify with a fingerstick |
| After any insulin dose change | Track the effect of the adjustment |
People on multiple daily injections or insulin pumps typically test more frequently than those on oral medications alone. If you are using a CGM, you may test by fingerstick less often — but not never.
Understanding Your Results
Units: mmol/L vs mg/dL
Blood glucose is measured in two units depending on country:
- mmol/L — used in the UK, Australia, Europe, and most of the world
- mg/dL — used in the United States and some other countries
To convert: mg/dL ÷ 18 ≈ mmol/L (e.g. 180 mg/dL ÷ 18 = 10.0 mmol/L)
Common Target Ranges for Adults with Diabetes
These are general ADA-aligned targets. Your team will personalise yours based on age, medication, hypoglycaemia risk, and other factors.
| Timing | Target (mmol/L) | Target (mg/dL) |
|---|---|---|
| Fasting / before meals | 4.4–7.2 | 80–130 |
| 2 hours after meals | <10.0 | <180 |
| Before bed | 5.6–7.8 | 100–140 |
Targets may be less strict for older adults, people with frequent severe hypoglycaemia, or those with reduced hypoglycaemia awareness.
Targets are tighter during pregnancy (gestational or pre-existing diabetes) — always follow your obstetric team’s guidance. See Gestational Diabetes: Screening, Treatment, and Follow-Up for more on pregnancy blood glucose monitoring.
How HbA1c Relates
Your HbA1c reflects your average glucose over ~3 months — it is a different view of control to your daily readings. A typical adult target is HbA1c below 7% (53 mmol/mol), but this is also individualised. See Understanding HbA1c for a full explanation of targets, limitations, and the relationship between HbA1c and Time-in-Range.
Factors That Affect Readings
Many things alter blood glucose beyond what you ate:
Physiology:
- Illness and infection — glucose often rises significantly; insulin needs increase
- Stress — cortisol and adrenaline raise glucose
- Sleep deprivation — impairs insulin sensitivity
- Hormonal changes — menstrual cycle phases, puberty, pregnancy
- Alcohol — can cause delayed hypoglycaemia hours later, especially with insulin
Medications:
- Corticosteroids (e.g. prednisolone) often cause substantial glucose rises
- Some antipsychotics and immunosuppressants affect insulin sensitivity
Activity:
- Aerobic exercise usually lowers glucose (during and after)
- High-intensity or anaerobic exercise (sprints, heavy lifting) can temporarily raise glucose
- Prolonged activity increases hypoglycaemia risk for hours afterward
Equipment and technique:
- Cold hands or poor circulation can give falsely low fingerstick readings
- Dirty or wet fingers affect test strip results
- Expired strips or incorrect meter calibration
- CGM sensor placement near lipohypertrophy (lumpy skin from repeated injections)
Ketone Testing Basics
Ketones are produced when the body burns fat for fuel instead of glucose — which happens when there is insufficient insulin. In Type 1 Diabetes, this can escalate quickly to diabetic ketoacidosis (DKA), a life-threatening emergency.
When to Test for Ketones
- Blood glucose is persistently above 14 mmol/L (250 mg/dL)
- You are feeling unwell, nauseous, or vomiting
- You have missed or significantly delayed an insulin dose
- Your insulin pump has failed or been disconnected
- Your diabetes team advises it during a sick day
How to Test
- Blood ketone meters (e.g. FreeStyle Optium, Keto-Mojo) test directly from a fingerstick — more accurate and faster to reflect real-time changes
- Urine ketone strips are a cheaper alternative but lag behind blood ketones by 1–2 hours and may give false reassurance in DKA
What the Results Mean
| Blood Ketone Level | Interpretation |
|---|---|
| Below 0.6 mmol/L | Normal — no action needed |
| 0.6–1.5 mmol/L | Elevated — increase testing frequency; contact your team if rising or unwell |
| 1.5–3.0 mmol/L | High — contact your diabetes team or seek urgent care; DKA risk |
| Above 3.0 mmol/L | Medical emergency — seek emergency care immediately |
Never ignore high ketones. DKA can develop within hours in people with Type 1 Diabetes. See Diabetes Emergencies — Hypos, Highs, and Ketones for full sick-day guidance and DKA warning signs.
Common Mistakes
- Testing too infrequently — a single pre-meal reading gives limited information; patterns across days and circumstances matter
- Ignoring post-meal readings — post-meal spikes are a major driver of HbA1c and cardiovascular risk; testing 2 hours after eating reveals them
- Poor fingerstick technique — cold, dirty, or inadequately lanced fingers give unreliable results; wash hands with warm water before testing
- Trusting a CGM during rapid glucose changes — the 5–15 minute lag means CGM may show 6.0 mmol/L while blood glucose is actually 3.2 mmol/L and falling
- Not recording patterns — individual readings are less useful than patterns; review your logbook or app with your diabetes team regularly
- Skipping sick-day ketone checks — during illness, especially with vomiting, glucose and ketones can rise rapidly; test both and follow sick-day rules
- Storing strips incorrectly — heat, moisture, and expired strips all reduce accuracy; check expiry dates and store strips in their sealed container
When to Seek Urgent Help
Call emergency services (999 / 911) immediately if:
- Someone with diabetes is unconscious, having a seizure, or cannot be roused
- Someone cannot swallow safely and has a suspected low blood glucose
- Blood glucose is extremely high (>20 mmol/L / 360 mg/dL) and ketones are elevated, or the person feels very unwell (DKA suspected)
- Repeated vomiting is preventing fluids or insulin administration
Contact your diabetes team urgently if:
- Blood glucose is persistently above 14 mmol/L and not responding to corrections
- Ketones are above 1.5 mmol/L and rising
- You are vomiting and unable to keep insulin down
- Your insulin delivery system has failed and you cannot access insulin
Treat a low blood glucose (hypoglycaemia) immediately if:
- Blood glucose is below 4.0 mmol/L (70 mg/dL), whether or not you have symptoms
- Use the 15/15 rule: 15 g of fast-acting carbohydrate (3–4 glucose tablets, or 150 ml of juice or regular soda), wait 15 minutes, recheck
- If glucose remains below 4.0 mmol/L, repeat the treatment
- Once recovered, eat a small snack with complex carbohydrate if the next meal is more than 1 hour away
FAQ
When should I test my blood glucose? For people managing diabetes with insulin, key testing times include before meals, 2 hours after meals, before bed, before driving, during and after exercise, and whenever you feel unwell or suspect a high or low. Your diabetes team will recommend a personalised schedule.
What is a normal blood glucose level? For adults without diabetes, fasting blood glucose is typically 3.9–5.5 mmol/L (70–99 mg/dL). For people with diabetes, ADA targets for most adults are 4.4–7.2 mmol/L (80–130 mg/dL) before meals and below 10.0 mmol/L (180 mg/dL) two hours after eating. Your team will set your personal targets.
What is the difference between a fingerstick meter and a CGM? A fingerstick meter gives a precise point-in-time reading from a blood sample. A CGM measures glucose continuously via a skin sensor, showing trends, direction, and alerts for highs and lows. CGMs lag 5–15 minutes behind true blood glucose during rapid changes — always verify with a fingerstick when the reading conflicts with how you feel or before treating a borderline low.
When should I test for ketones? Test if blood glucose is persistently above 14 mmol/L (250 mg/dL), if you are unwell or vomiting, or if your team advises it during a sick day. High ketones (above 1.5 mmol/L) require urgent action. Above 3.0 mmol/L is a medical emergency.
What are the signs of low blood glucose? Shakiness, sweating, rapid heartbeat, hunger, dizziness, confusion, and weakness are common. Treat immediately with 15 g of fast-acting carbohydrate. If someone is unconscious or cannot swallow, call emergency services — do not attempt to give food or drink.
Why does my CGM reading sometimes differ from my fingerstick? CGMs measure glucose in the fluid between cells, not directly in blood. This causes a 5–15 minute lag during rapid glucose changes. Significant divergence at stable glucose levels may indicate a sensor accuracy issue. When in doubt, trust the fingerstick.
What factors affect my blood glucose beyond food? Exercise, illness, stress, sleep, alcohol, hormonal changes, and certain medications all alter glucose. Corticosteroids in particular cause significant rises. Recognising your personal patterns is as important as the numbers themselves.
Further Reading
- ADA — Blood Glucose Monitoring — American Diabetes Association patient guide to testing and targets
- NICE — Type 1 Diabetes in Adults: Diagnosis and Management (NG17) — UK clinical guideline covering monitoring standards
- Diabetes UK — Checking Your Blood Sugar — practical patient-facing guidance
- JDRF — CGM Overview — CGM types, benefits, and real-world use
Related Guides
- Understanding HbA1c — What It Means and How to Use It
- Continuous Glucose Monitors — CGM vs Fingerstick
- Type 1 Diabetes — An Overview
- Early Signs of Type 1 Diabetes
- Diabetes Emergencies — Hypos, Highs, and Ketones
- Gestational Diabetes: Screening, Treatment, and Follow-Up — blood glucose monitoring in pregnancy and what pregnancy-specific targets mean
Educational only — not a substitute for advice from your diabetes care team.