Understanding your blood test results

What a results report shows

Each line of a report gives the name of the test and your result with its unit, set beside the laboratory’s reference range. A result outside the range is usually flagged high or low, often with a letter or a colour.

The flag is a prompt to look, and a doctor decides what it means. The same number can be expected in one person and important in another, depending on age, sex, medicines, pregnancy and recent illness.

What a reference range means

A reference range usually covers the middle 95% of results from a group of healthy people, with the lower limit at the 2.5th percentile and the upper limit at the 97.5th [1]. By that design, about 5 in every 100 healthy people will have a result outside the range for any one test.

Some results are judged against a decision limit instead, a threshold above or below which a specific medical decision is recommended [1]. A total cholesterol above 9.0 mmol/L, for example, prompts a specialist assessment even without a family history of early heart disease [2]. For someone taking levothyroxine, the aim is a TSH inside the reference range [3].

Units, and why the same result can look different

Laboratories in the UK report most results in SI units. HbA1c is given in mmol/mol, so 48 mmol/mol is the same as 6.5% in the older percentage units still used in some countries [4]. Vitamin D is reported in nmol/L, and the UK advisory committee on nutrition treats 25 nmol/L as the level below which the risk of poor bone health rises [5].

Results from abroad may use different units, such as mg/dL for cholesterol or ng/mL for vitamin D, so the numbers look very different for the same amount. Ranges also vary between laboratories. Compare a result only with the range printed on the same report, and bring the original reports when you see a doctor.

Why one result is rarely the whole story

Results move from day to day, and preparation matters. NICE advises not eating meat in the 12 hours before a blood test for kidney function, and notes that very high or very low muscle mass can make the estimate misleading [6]. A new low result is confirmed by repeating the test within 2 weeks before anything else is decided [6].

The same caution applies elsewhere. A raised triglyceride level between 10 and 20 mmol/L is repeated as a fasting test before going further [2], and a thyroid dose is judged on 2 similar TSH results 3 months apart [3]. The trend across several tests tells you more than any single number.

Limits and risks of reading results alone

Reading results without context can mislead in both directions. A borderline flag can cause worry and further tests that were never needed, while a result inside the range can reassure when symptoms still need a doctor’s attention.

A report is not suited to diagnosing yourself, and an automated score cannot weigh your history the way a doctor does. If a laboratory or doctor calls you urgently about a result, follow their advice. If you have chest pain, difficulty breathing or sudden confusion, or feel seriously unwell, call 999 in the UK or the local emergency number [7].

Questions to ask your doctor

These questions turn a page of numbers into a plan you understand.

• Which of my results matter, and which are expected for me?
• Is this result new, or has it been like this before?
• Should we repeat anything before acting on it?
• Could my medicines, supplements, exercise or a recent illness explain it?
• When should I test again, and which tests?

How The Wellness can help

Every result we arrange is explained by a doctor, and a telephone consultation is a convenient way to go through a report from us or from elsewhere. You can see your own results on our blood results page, and our guide to biomarkers explains each marker in plain English.

If you would like a fresh baseline, the Core Wellness blood test includes a full blood count, liver tests, lipids, HbA1c, TSH, ferritin and vitamin D, with samples analysed by accredited laboratories. Every doctor is GMC registered, and our clinics on Baker Street in Marylebone and Eccleston Street in Belgravia are open every day.