How to Read a Blood Test Report

This page explains the format of a blood test report, not what your results mean. We give you access to your data — not medical advice. No clinician reviews or comments on your results before they reach you. If you have questions about your own results, take your report to your GP; in an emergency call 111 or 999.

What’s Actually on a Blood Test Report

Every blood test report — home finger-prick or clinic venous draw — follows roughly the same structure. For each marker tested, you’ll typically see four things side by side: the marker name, your value, the unit it’s measured in, and a reference range the lab compares it against. Some reports also show a flag — a symbol or letter next to any value that falls outside that range. This page walks through what each of those actually means as a piece of information — not what any specific number means for you, which is a conversation for a clinician, not a web page.

What a Reference Range Actually Is

A reference range is a statistical band, not a pass/fail line. Most UK labs build it from the middle 95% of results in a large sample of people believed to be healthy — which means, by definition, roughly 1 in 20 healthy people will have at least one result outside the “normal” range on any given test, for no meaningful reason at all. Ranges also vary between labs, between analysis methods, and sometimes between age bands or sexes for the same marker. A value flagged by one lab’s range might sit comfortably inside another’s.

None of that makes ranges useless — they’re a genuinely useful starting signal. It’s why a single flagged value, on its own, isn’t a diagnosis, and why context (your history, symptoms, and how a value has moved over time) matters more than any one number in isolation.

Understanding Flags — H, L, and Out-of-Range Markers

Most reports flag a value with an “H” (high) or “L” (low) when it falls outside the reference range, sometimes shown as an arrow or in bold instead. A flag is simply the report telling you “this number sits outside the range we compared it against” — nothing more. It doesn’t indicate severity, cause, or what (if anything) should happen next. A value can be flagged and be clinically irrelevant, and a value can sit inside its range and still be worth discussing with a GP if it’s moved a lot from a previous test, or alongside symptoms.

Measured vs. Calculated Values

Not every figure on a report comes from a direct measurement. Some — Free Testosterone and the Free Androgen Index are common examples — are calculated from other measured values using a standard formula, rather than measured directly in the sample. Where that applies on our own panels, we label it “(calculated)” so it’s clear which numbers are direct lab measurements and which are derived. Both are useful; they’re just different kinds of number.

Why the Units on Your Report Might Look Unfamiliar

UK labs report most hormones in molar units — nmol/L, pmol/L — while US labs and some online sources use mass-per-volume units like ng/dL or pg/mL. The two aren’t interchangeable without conversion, so if you’re comparing a UK report against a range or article written for a US audience, check the units line up before comparing numbers directly. Our free hormone unit converter converts between the two for 16 common hormones if you need to line up a US-format range against a UK report, or vice versa.

Getting the Most Out of a GP Conversation About Your Results

A blood test report is most useful as something you bring to a conversation, not something you read alone and act on. A few things that make that conversation more useful:

Bring the full report, not just the flagged lines — a clinician reading a value in isolation has less to work with than one seeing the whole panel.
If you’ve tested before, bring the previous report too — a trend across two or more tests tells a clinician more than a single snapshot.
Note any symptoms alongside the numbers — a value’s relevance often depends on what you’re actually experiencing, not the number alone.
If nothing is flagged but something still feels off, say so — normal-range results don’t rule out everything a symptom could be.

Frequently Asked Questions

Can I diagnose a condition from my blood test results alone?

No. A lab report gives you numbers and the ranges they’re compared against — it doesn’t diagnose anything. Diagnosis requires a clinician weighing your results alongside your history, symptoms and, often, more than one test over time.

What does it mean if one of my results is flagged?

It means that value fell outside the reference range the lab used for comparison — nothing else. It’s a signal worth discussing with a GP, not a conclusion. See “Understanding Flags” above.

Why might my result be flagged on one lab’s report but not another’s?

Reference ranges vary by lab, by testing method, and sometimes by age or sex bands, so the same value can sit inside one lab’s range and outside another’s. This is a property of ranges, not an error on either report.

Do you offer any interpretation of what my results mean?

No — our laboratory returns numbers only, with no clinical interpretation. No clinician reviews or comments on your results before they reach you. That’s a deliberate part of how the service works, not a gap in it.