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EspañolGuide 3 — results in hand
Blood test results explained: ranges, flags and GP comments
What the reference range on your results means, what H, L and ‘satisfactory’ are telling you, why two laboratories disagree, and what to do with a value outside its range.
Key figures
- 1 in 20
- healthy people fall outside a reference range on any single test[1]
- 2 in 3
- healthy people have at least one flagged value on a twenty-test panel[2]
- 3 in 1,000
- tests affected by an error, 62% of them before the analysis itself[8]
- 18
- studies in a review: results shown on a bar were understood better than numbers alone[10]
Start with the number that changes everything
On any single test, about one healthy person in twenty falls outside the reference range. That is how the ranges are built, not a fault in them. On a long set of results, at least one flag in a healthy person is more likely than none.
Every page in this guide follows from that. A flag is a comparison, and the real work is deciding whether this particular comparison matters for you.
How a range is built, and why your laboratory’s is the one that counts
To set a range, a laboratory tests healthy people of a given age and sex on its own analyser and keeps the band that contains 19 in every 20 of their results. The one in twenty left outside are healthy too. On a set of twenty tests, that arithmetic leaves about two healthy people in three with at least one flagged value.
Because the analyser, the method and the local population differ, so do the ranges. That is why the only range that applies to your value is the one printed beside it. In the UK, the Pathology Harmony project, supported by the Department of Health in England, has brought many routine chemistry ranges into line, but laboratories are not obliged to follow it, and haematology and hormone tests still differ more. For some lines, cholesterol and HbA1c among them, doctors use NICE thresholds rather than a reference range at all.
What your GP’s comment is telling you
In England, results ordered by your GP are reviewed by a healthcare professional and usually appear in the NHS App within a few days, often with a short comment. The wording varies between surgeries. One NHS practice explains its comments like this: ‘normal’ means within range; ‘satisfactory’ means outside the range but not a concern and needing no treatment; ‘borderline’ means very close to the edge and possibly worth repeating; ‘abnormal’ means it needs looking into.
The NHS says that if your results do not show anything, you may not need to do anything else, and that if you have heard nothing after a few weeks you should contact the GP surgery or specialist who ordered the test.
Two sets of results, two dates: is the change real?
Every result moves a little from one test to the next even when nothing has changed: the body varies around its own set point, and every measurement carries some imprecision. Laboratory medicine has a tool for exactly this question, the reference change value, which says how large a difference between two results must be before it is unlikely to be ordinary fluctuation.
In practice, compare results from the same laboratory, taken in similar conditions, and look at the direction over several sets of results rather than a single step. A surprising one-off value is usually repeated before anything is decided, partly because errors do happen: in a study of more than 51,000 hospital tests, about 3 in 1,000 were affected by an error, and 62% of those arose before the sample reached the analyser.
Why a flag worries more than it should
Results now often reach you before anyone explains them. A systematic review of 39 studies found that people value that access but struggle above all with borderline values and with knowing when follow-up is needed. Another review, of 18 studies, found that showing a value as a mark on a horizontal bar against its range improved understanding and reduced the urge to contact a doctor, compared with numbers alone.
That is why the pages in this guide place values on a bar rather than leaving them to a letter, and why the full report does the same.
The four questions
How far outside, in which direction, next to what, and compared with when. Those four settle most sets of results, and none of them is answered by the letter printed at the end of the line.
If you have your results in front of you now, the upload block on these pages sorts your own lines by what deserves attention, against the ranges your laboratory printed.
- How far outside?A value just past the limit and one far beyond it carry the same letter. The distance is most of the information.
- In which direction?High and low do not mean the same thing for the same line, and some lines only matter in one direction.
- Next to what?A line is read with its group: haemoglobin with MCV and ferritin, ALT with ALP and GGT.
- Compared with when?A value that is stable over several sets of results says something different from one that keeps drifting.
The articles in this guide
Your own results
Your blood test results, explained marker by marker
Upload your blood test results: in a few minutes, the full report sorts every marker by what needs attention, against the ranges printed by your own laboratory, explains each one, and ends with an action plan and questions for your GP.
- Your name is removed before the reading
- Never used to train an AI, never resold
- Delete a report, or your account, at any time
PDF or photo · several files accepted · See an example report
Sources
Guidelines, consensus statements and peer-reviewed studies. Every figure on this page comes from one of them. Preprints are not cited.
- Patient information
- Review
Interpretating Normal Values and Reference Ranges for Laboratory Tests
- Review
- Official guidance
- Official guidance
- Official guidance
- Review
- Observational study
Errors in a stat laboratory: types and frequencies 10 years later
- Systematic review
- Systematic review