Blood Glucose 4 tested - mmol/L or mg/dL?

Most meters outside the US show mmol/L. A reading of 4 is far more likely to be 4 mmol/L than 4 mg/dL.

If it is 4 mmol/L

= 72 mg/dL. Fasting without diabetes: Normal · With diabetes before a meal: Below target

If it really is 4 mg/dL

That is 0.2 mmol/L — severely low, below the level 2 hypoglycemia threshold of 54 mg/dL (3.0 mmol/L). Most meters show "LO" below 20 mg/dL. If the person is shaky, sweaty, confused or drowsy, treat it as a severe low: 15–20 g of fast-acting carbohydrate if they can swallow safely; if not, glucagon if available, recovery position and call 911 (US) or your local emergency number.

Full explanation for 4 mg/dL →

No immediate action needed

Note the time and whether you had eaten — the same number means different things fasting and after a meal.
Looking at the pattern over days (or a CGM report) tells you more than one reading.
With diabetes, on insulin or a sulfonylurea: this is below your pre-meal target — have a small snack before driving or exercise.

If you don't have diabetes — reading 4 mmol/L (72 mg/dL)

4 mmol/L (72 mg/dL) is a normal blood sugar both fasting and after a meal. Nothing to do.

Fasting prediabetes cut-off: the ADA uses 5.6 mmol/L (100 mg/dL); the WHO uses 6.1 mmol/L (110 mg/dL). This site follows the ADA.

Fasting (8+ hours without food)

Normal

2 hours after eating (or glucose tolerance test)

Normal

If you have diabetes — reading 4 mmol/L (72 mg/dL)

4 mmol/L (72 mg/dL) is not a low, but it is below the usual pre-meal target of 4.4–7.2 mmol/L = 80–130 mg/dL. If you use insulin or a sulfonylurea and are about to drive or exercise, have a small snack first.

Before a meal / fasting

Below target

1–2 hours after a meal

In target

Targets for most non-pregnant adults (American Diabetes Association (ADA) Standards of Care). Your own targets may be different — for example in pregnancy, for children, older adults or people prone to lows. See pregnancy targets.

72 mg/dL as an average: HbA1c 4.1%

If 72 mg/dL were your average glucose over two to three months, it would match an HbA1c of about 4.1% (normal range). A single reading says little about your HbA1c — it is the long-term average that counts. Learn more in how to measure blood sugar.

Make sure the number is right

Wash and dry your hands before a finger-stick: fruit juice or sugar on the skin can raise the result a lot.
Home meters may differ from a lab value by up to about 15%. At 72 mg/dL that means roughly 61–83 mg/dL.
CGM sensors measure tissue fluid and lag 5–15 minutes behind blood when glucose changes quickly — confirm lows with a finger-stick.

Check another reading

Nearby values

Sources: ADA Standards of Care · ADAG study (HbA1c and average glucose) · WHO: Definition and diagnosis of diabetes · MedlinePlus: Blood glucose test

This page explains one reading in general terms. It is not a diagnosis or a treatment instruction. Follow the plan agreed with your doctor or diabetes team.