How to Read a Blood Test: Reference Ranges, Thresholds and Why a Flag Is Not a Diagnosis

A lab report arrives as a column of numbers, and two or three carry a flag or an asterisk. A flag means one thing: this number sits outside the limits printed on your report. It does not say you are ill. Some of those limits describe where results of healthy people usually fall; others are thresholds a guideline chose because they predict disease or call for treatment. Knowing which kind you are looking at is most of what reading a blood test takes.

When to see a doctor

Right away, whatever the numbers say: chest pain, severe shortness of breath, new confusion or fainting; or signs of diabetic ketoacidosis — thirst and frequent urination together with vomiting, abdominal pain, drowsiness or fast, deep breathing (NHS, UK). Call 999 in the UK, 911 in the US, 112 in the EU. Also right away, even if you feel fine: the lab or a doctor has told you a result is critical, or the report marks a value as critical. Contact the doctor at once and do what they say, without waiting for symptoms. More on an abnormal test result: what next.

The same day: marked thirst, frequent urination, weight loss and tiredness without the danger signs above — possible type 1 diabetes. In the UK, an urgent GP appointment or 111 (NHS).

Routinely: a single moderate flag while you feel well. Talk it through with whoever ordered the test and repeat it on their plan rather than your own.

What is happening

Reference ranges describe healthy people. A typical reference range is built from a group of healthy volunteers: the lowest 2.5% and the highest 2.5% of their results are cut off, and the middle 95% becomes the printed range (JABFM). If your lab's range is built this way, about 1 healthy person in 20 lands outside it on any single test, by definition. Order 20 tests and — assuming, for simplicity, that they are independent — about 64% of healthy people get at least one flag. Real tests are linked, so the true share differs, but the direction holds: the more you test, the more flags.

Thresholds describe risk. Other limits are not taken from healthy people at all. They are decision points, set from data on who went on to develop disease (JABFM). For diabetes, the American Diabetes Association (ADA, US) uses fasting glucose of 126 mg/dL (7.0 mmol/L) or more, or HbA1c of 6.5% or more, and — unless blood sugar is obviously very high — two abnormal results before a diagnosis. Where "prediabetes" starts differs between guidelines: 100 mg/dL for the ADA, about 110 mg/dL (6.1 mmol/L) under the WHO-based criteria used in Russia, for example. The blood is the same; the line is a guideline's decision, and the "1 in 20" rule does not apply to it.

Targets depend on you. Cholesterol is judged against a target that depends on age, other conditions and overall cardiovascular risk; in the UK a GP estimates that risk with a tool called QRISK (NHS). The same LDL value can be fine for one person and too high for another.

Labs and units differ. Equipment and methods differ, so each lab sets its own ranges: compare your result with the range on your own report, not with a table online (MedlinePlus). For TSH, the thyroid test, limits used by different labs can differ by about a third (JABFM). Units differ too: glucose in mg/dL (US) is roughly mmol/L multiplied by 18, and HbA1c comes either as a percentage or in mmol/mol. Ranges also shift with age, sex and pregnancy: about 12% of people over 80 without thyroid disease have TSH above 4.5 mIU/L (USPSTF, US data).

One number wobbles. Your own values move from day to day: TSH in the same person can vary by up to half of its average (USPSTF). A trend across several results, taken the same way, says more than any single point.

What to do

• Keep your reports, paper or PDF, and compare like with like: same lab, similar time of day, same preparation. How preparation — fasting, coffee, a hard workout — moves the numbers is on the page about abnormal results.
• Ask one question about each flag: is this a reference range or a decision threshold? The answer changes what the flag means.
• Tell the doctor about medicines and supplements, biotin included: it can distort some tests. More in supplements without testing.
• Test for a reason. A yearly "panel of everything" without symptoms adds flags faster than answers: a Cochrane review of general health checks in adults found little or no effect on deaths (Cochrane, 2019). Checks recommended for your age and risk are different: in England, the NHS Health Check (blood pressure, cholesterol, a risk estimate) every five years at 40–74; in the US, blood-pressure screening from 18, yearly from 40 (USPSTF). See screenings on schedule.
• Bring your questions to the appointment: prepare questions for the doctor.

When this is not the case

• A normal result is not a clean bill of health. If symptoms continue, "everything is in range" does not close the question — go back.
• Pregnancy, children and older adults have their own ranges; read your report for your group and ask.
• Related questions have their own pages: tiredness and the thyroid test — check your thyroid if you are always tired; measuring blood pressure — know your blood pressure; percentages and risks in the news — what to do when you see a scary statistic; low iron — iron deficiency.

Sources

• Doles et al., J Am Board Fam Med 2025 — Interpreting normal values and reference ranges for laboratory tests
• MedlinePlus — How to understand your lab results
• American Diabetes Association — Standards of Care 2026: Diagnosis and classification of diabetes
• NHS — Cholesterol levels (QRISK)
• USPSTF — Hypertension in adults: screening
• NHS — Diabetic ketoacidosis
• USPSTF — Thyroid dysfunction: screening
• Krogsbøll et al., Cochrane 2019 — General health checks for reducing illness and mortality
• NHS Health Check

Checked: 24 September 2026