How to Measure Blood Sugar: Meter, CGM, Lab
There are three ways to see your blood sugar: a finger-stick meter, a continuous glucose monitor (CGM), and lab tests at the doctor's office. Each answers a different question.
Finger-stick testing with a meter
- Wash your hands with warm water and dry them. Sugar on the skin (fruit, juice) falsifies the result; alcohol wipes are not needed and can dry the skin.
- Insert a test strip into the meter.
- Prick the side of a fingertip with the lancing device — less painful than the pad. Rotate fingers.
- Gently squeeze the finger from the base to form a drop. Don't press hard right at the tip — tissue fluid dilutes the sample.
- Touch the strip to the drop. The result appears in seconds.
- Note the value, time and context (fasting, before or after a meal, exercise).
Check that strips are within their expiry date and stored as directed. Home meters must meet accuracy standards but can still differ from a lab result by up to about 15%.
Continuous glucose monitors (CGM)
A small sensor on the upper arm or abdomen measures glucose in the tissue fluid every few minutes, for 10–15 days, and sends it to a phone or reader. Advantages: trend arrows, alarms for highs and lows, and reports such as time in range (70–180 mg/dL). Because tissue glucose lags 5–15 minutes behind blood glucose, confirm a low or a reading that doesn't match your symptoms with a finger-stick. Over-the-counter CGMs are now also available in the US for adults not on insulin.
When to test
| Situation | Typical testing |
|---|---|
| Multiple daily insulin or a pump | Before meals and snacks, at bedtime, before exercise and driving, when you suspect a low — or a CGM |
| Basal insulin or sulfonylureas | Fasting every day, plus when you feel low |
| Type 2 diabetes on diet or metformin | As agreed — often occasional pairs before and 2 hours after a meal to learn your responses |
| Gestational diabetes | Usually fasting and 1 or 2 hours after each meal |
Lab tests
- Fasting plasma glucose: after at least 8 hours without food. 100–125 mg/dL = prediabetes, 126 or more = diabetes.
- Oral glucose tolerance test (OGTT): glucose measured 2 hours after drinking 75 g of glucose. 140–199 = prediabetes, 200 or more = diabetes. Also used in pregnancy.
- HbA1c: the share of hemoglobin carrying glucose — reflects the average of 2–3 months, no fasting needed. 5.7–6.4% = prediabetes, 6.5% or more = diabetes. Can be misleading with anemia, some hemoglobin variants, kidney disease or recent blood loss.
- Random glucose: 200 mg/dL or more with typical symptoms = diabetes.
Making sense of the number
Enter any reading in the checker — it shows the meaning fasting and after meals, with and without diabetes. For the overall picture see blood glucose levels chart.
Sources: ADA: Checking your blood sugar · NIDDK: Continuous glucose monitoring · ADAG study (HbA1c and average glucose) · MedlinePlus: Blood glucose test
Check a blood glucose reading
Related guides
Educational content only; not medical advice. Follow the treatment plan you have agreed with your doctor or diabetes team. In an emergency, call 911 (US) or your local emergency number.