H and L flags, mmol/L versus g/L, reference ranges that vary by lab. A practical guide to making sense of what is on the page.
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.
The first column lists the name of each marker. Some are written in full, for example haemoglobin. Others appear as abbreviations, such as TSH for thyroid stimulating hormone or 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.
Your result is a number followed by units, and the units tell you the scale of measurement. The same marker can look very different depending on the units used. Vitamin D, for example, is sometimes reported in nmol/L, the UK standard, and sometimes in ng/mL, the US standard. 50 nmol/L and 20 ng/mL are the same value.
If you are comparing results from a UK lab and a private provider, check that both are using the same units before comparing numbers. The reference range will usually tell you which unit is in use.
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, for example 12 to 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 that 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.
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 high is different from significantly high. |
| L | Below the lower limit of the reference range. | The same: context matters more than the flag alone. |
| HH · LL | Significantly above or below range. | More likely to require clinical attention. |
| * or ! | Critical value. The lab has likely already alerted your GP. | Contact your GP if you have not heard. |
| No flag | Within the reference range. | Does not necessarily mean optimal: see context below. |
Context transforms a number. A TSH of 4.5 mIU/L is within the NHS reference range of 0.4 to 4.5, but would be considered borderline by many endocrinologists, particularly in a patient with symptoms of an underactive thyroid. 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.
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 reads results from NHS and private labs, standardises the units, and shows you the trend for each marker over time.
Track your markers over time, not just today. biomarkr keeps every result in one place and shows you the direction each one is heading. Free for your first year.
Join early access →Reference ranges and guidance in this article are drawn from NHS sources and NICE Clinical Knowledge Summaries. Your own lab report ranges always take precedence.
Educational purposes only · not medical advice · always speak to your GP or a qualified clinician about your results