Blood test results look more complicated than they are. Once you understand the structure — what the columns mean, what the flags indicate, and why the ranges differ between labs — you can read any result with confidence. This guide covers the practical mechanics.
Step 1: Identify what was tested
The first column lists the name of each marker. Some are written in full (e.g. "Haemoglobin"), others as abbreviations (e.g. "TSH" for thyroid-stimulating hormone, "eGFR" for estimated glomerular filtration rate). If you do not recognise an abbreviation, it is worth looking it up — knowing what was and was not tested is the starting point for understanding your results.
Step 2: Read the result and its units
Your result is a number followed by units. The units tell you the scale of measurement. The same marker can look very different depending on the units used — for example, vitamin D is sometimes reported in nmol/L (UK standard) and sometimes in ng/mL (US standard). 50 nmol/L and 20 ng/mL are the same value.
Step 3: Compare to the reference range
The reference range is the interval considered normal for the population the lab has tested. It is usually expressed as a lower and upper bound (e.g. 12–16 g/L for haemoglobin in women). Your result is within range if it falls between these two numbers.
Reference ranges are not fixed medical thresholds — they are statistical constructs. They vary between labs, between age groups, and between sexes. A result at the very bottom of the normal range is not the same as one in the middle, even though both are technically "within range."
Step 4: Understand the flags
Most results include a flag column. The most common flags are:
| Flag | Meaning | What to do |
|---|---|---|
| H | Above the upper limit of the reference range | Note the degree — slightly H is different from significantly H |
| L | Below the lower limit of the reference range | Same — context matters more than the flag alone |
| HH or LL | Significantly above or below range | More likely to require clinical attention |
| * or ! | Critical value — lab has likely already alerted your GP | Contact your GP if you have not heard |
| (no flag) | Within reference range | Does not necessarily mean optimal — see context |
Why the same result can mean different things
Context transforms a number. A TSH of 4.5 mIU/L is within the NHS reference range (0.4–4.5) but would be considered borderline by many endocrinologists, particularly in a patient with symptoms of hypothyroidism. A ferritin of 15 µg/L is within range at most UK labs but is associated with symptoms of iron deficiency in many patients, particularly women.
This is why reading results in isolation — without knowing your history, your symptoms, and your previous results — can be misleading. The number on the page is the starting point, not the conclusion.
Tracking change over time
The most useful thing you can do with a blood test result is compare it to your previous results. A haemoglobin of 118 g/L that has dropped from 134 g/L over six months tells a very different story from one that has been stable at 118 g/L for three years. The direction of travel matters as much as the current value.
If you have results from multiple tests, keep them in one place. biomarkr is built specifically for this — it ingests results from NHS and private labs, standardises the units, and shows you the trend for each marker over time.
Questions worth asking your GP
- Which markers were flagged, and how far outside range are they?
- How do these results compare to my previous tests?
- Which result is most clinically significant, and why?
- Is there anything in these results that warrants further investigation?
Track this marker over time, not just today
biomarkr keeps every result in one place and shows you the direction each marker is heading — free for your first year.