Blood Test Results Flagged as Abnormal? What They Usually Mean and When a Doctor Should Review Them
A blood test result marked high or low is common, and most of the time it is not a sign of disease. Laboratory reference ranges are usually set so that 95% of healthy people fall inside them, which means 1 healthy person in 20 will be outside the range for any single test. On a panel of 20 tests, around 2 in 3 healthy people would have at least one result flagged purely by chance. Many flags have simple explanations. A finger-prick sample that has broken down can falsely raise potassium. Hard exercise in the days before the test raises muscle enzymes. A recent cold can raise ferritin and inflammation markers. High-dose biotin, found in many hair and nail supplements, can make thyroid results look overactive, according to guidance from the Association for Diagnostics and Laboratory Medicine. Low vitamin D is expected in most people in a British winter. Some flags do matter, and the skill lies in telling them apart, by reading each result alongside the others, your symptoms, your history and any earlier results. At The Wellness a doctor reviews your results with you by video the same day, from £150, explains every flag, and arranges a repeat or further test only where it would change what happens next. Fees appear further down this page.
Reviewed by the medical team at The Wellness. Last updated 9 October 2026.
Have your results reviewed today on WhatsApp or email team@thewellnesslondon.com or call 020 3951 3429.
Key facts about flagged blood results
Reference ranges cover about 95% of healthy people, so 1 in 20 healthy results falls outside them.
The more tests on a panel, the more likely at least one flag appears by chance.
Sample handling, timing, exercise, recent illness and supplements all change results.
A single slightly abnormal result is usually repeated before any conclusion is drawn.
Trends over time are more informative than any one result.
Why are so many results flagged
A reference range is not a line between healthy and unhealthy. It is the range in which most healthy people’s results sit, typically the middle 95%. That leaves 2.5% of healthy people above and 2.5% below for every test.
Panels sold with dozens of markers therefore produce flags in many healthy people. That is not a reason to avoid testing. It is a reason to have results read by a doctor who knows which flags matter.
Which flags are most often harmless
Potassium slightly high. Often caused by red cells breaking down in the sample, especially after a finger-prick or a delay before analysis. A repeat venous sample usually settles it.
Vitamin D low. Very common between October and March in the UK, when sunlight is too weak to make enough vitamin D in the skin. It usually needs a supplement rather than further tests.
Bilirubin slightly high with otherwise normal liver tests. Often due to Gilbert’s syndrome, a common, harmless inherited condition that becomes more noticeable after fasting or illness.
Creatine kinase or liver enzymes raised after exercise. Hard training, particularly weights, can raise these for several days.
Ferritin or CRP raised after a recent infection. Both rise with inflammation and settle once you recover.
Send your results for review on WhatsApp or email team@thewellnesslondon.com.
Which flags should not be ignored
Haemoglobin low, particularly with low ferritin, which means iron deficiency anaemia. In men and in women after the menopause, iron deficiency needs to be explained, because blood loss from the gut is one of the causes.
HbA1c of 42 mmol/mol or above, which indicates prediabetes, or 48 and above, which is in the diabetes range and needs confirming.
A raised ALT that persists, which is often due to fatty liver and deserves a fibrosis assessment.
Falling kidney function, a raised PSA, very high cholesterol, a high calcium, a raised platelet count with other symptoms, or abnormal thyroid results with symptoms.
Some combinations of results need same-day attention, and a doctor reviewing them will tell you if yours do.
What can make results misleading
Timing. Testosterone and cortisol are highest in the morning, so afternoon samples can read low.
Supplements. Biotin taken at high doses, often 5 mg or more, can falsely lower TSH and falsely raise thyroid hormone results. Laboratory guidance advises stopping high-dose biotin before testing.
Exercise, alcohol and illness in the days before the test. Each can move liver, muscle and inflammation markers.
Sample type. Finger-prick samples are convenient but more prone to errors for some tests than a venous sample taken in a laboratory.
Fasting is not usually needed for cholesterol, according to NICE, though it is still needed for some tests.
What happens in a results review
A doctor goes through every result with you, flagged or not, and explains what each means for you. Your symptoms, medicines, supplements and history are taken into account, and earlier results are compared where you have them.
You leave with a clear plan. That may be reassurance, a repeat test taken properly at a laboratory, a further test that answers a specific question, treatment, or a referral to a specialist with the appointment arranged.
What does a blood test review cost in London
At the top of the London market, advanced biomarker panels commonly cost around £1,000, and comprehensive health programmes with imaging run from several thousand pounds upwards.
At The Wellness every blood test is explained by a doctor. All figures are from prices.
Executive Health Programme, the most comprehensive single assessment, from £11,995.
Signature Membership, with ongoing access to a doctor, from £6,950 a year or £610 a month.
Complete Biomarker Assessment, over 100 markers with consultation, from £1,400.
Comprehensive Blood Panel with consultation and interpretation, from £495.
Targeted Blood Panel, from £295. Extended 45-minute consultation for complex results, from £395.
Results review by video with a doctor, the same day, from £150.
Appointments are available 7 days a week by video, or at our London clinics, including Marylebone and Belgravia.
Why patients choose The Wellness to review their results
Because every result is explained, not just the flags.
Because results are read together, with your symptoms, history and earlier results, which is how harmless flags are told apart from important ones.
Because a repeat or further test is arranged only when it would change what happens next.
Because you can be seen the same day by video, wherever you had the test.
And because a finding leads somewhere, with treatment started or a referral arranged with a named consultant.
Related reading
The best blood tests in London explains which markers are worth paying for, comprehensive blood test and biomarkers covers the Complete Biomarker Assessment, thyroid blood tests explained covers thyroid results in detail, and fatty liver covers raised liver enzymes.
Book a results review on WhatsApp or call 020 3951 3429.
Frequently asked questions
Should I worry if my blood test results are flagged
Usually not. Reference ranges cover about 95% of healthy people, so flags are common in healthy people, especially on large panels. A doctor can tell you which flags matter for you.
Why is my potassium high on a home blood test
Finger-prick samples can break down before analysis, which releases potassium from red cells and falsely raises the result. A repeat venous sample usually settles the question.
Can supplements affect blood test results
Yes. High-dose biotin can falsely lower TSH and raise thyroid hormone results, and other supplements, exercise, alcohol and recent illness can move results too. Tell the doctor everything you take.
Do I need to fast before a blood test
Not for most cholesterol tests, according to NICE. Some tests still need fasting or a morning sample, such as fasting glucose, testosterone and cortisol.
Can a doctor review blood results from another provider
Yes. A doctor can review results from any accredited laboratory or home test service by video, explain them and arrange any repeat or further test.
How much does a blood test results review cost
At The Wellness a same-day video results review with a doctor is from £150, and an extended 45-minute consultation for complex results from £395.
The Wellness is a doctor-led private healthcare group providing medical care from our London clinics, including Marylebone and Belgravia. All doctors are GMC-registered. Blood analysis is performed by accredited laboratories. In an emergency call 999. This article is general information and not a substitute for medical advice about your own health.
Enquire now on WhatsApp or email team@thewellnesslondon.com or call 020 3951 3429.
References
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