H, L and CRIT: how to read your own report
What the reference interval really claims, why one value outside it is not automatically a problem, and the one number you should compare it against.
Your report opens on a phone now, which means most people read it before their doctor does. That is a good thing, and it is worth knowing what you are looking at.
The markers
Every result that falls outside its reference interval is marked with a letter, not only a colour:
- H means above the interval.
- L means below it.
- CRIT means a critical value, and you will not be finding that one out by opening a link. We telephone.
The letter is there because colour alone is not a signal. Reports get printed on black and white printers, they get photocopied for insurers, and roughly one man in twelve cannot reliably separate red from green. A marker that only works for some readers on some paper is not a marker.
What “normal” actually claims
A reference interval is not a boundary between healthy and ill. It is the range that covers about 95 percent of a healthy reference population.
Read that again, because the consequence is unintuitive: if you measure twenty analytes on a perfectly healthy person, on average one of them will come back flagged. A single H on a twenty line report, on its own, with no symptoms, is usually the arithmetic rather than a disease.
That is not a reason to ignore it. It is a reason to take the whole page to the person who ordered it rather than searching one line of it on the internet at eleven at night.
Why your interval is not your neighbour’s
Intervals are keyed to sex and age, because the underlying biology is. A haemoglobin that is comfortably normal for a man is anaemia in the making for nobody, and the same figure in a woman sits differently against her interval. Creatinine tracks muscle mass. Alkaline phosphatase in a growing thirteen year old is high for an adult and entirely normal for them.
This is also why the interval is printed beside your result and kept with it permanently. Reference intervals get revised when reagents change. If we reprinted your 2024 report against today’s intervals, the numbers would stay the same and the flags might not, which would make a document you are holding disagree with itself.
The comparison that matters most
The single most useful number on your report is often not on your report at all. It is your own previous result.
A creatinine of 1.1 is unremarkable. A creatinine of 1.1 in somebody who was 0.7 eighteen months ago is a conversation. Neither the interval nor the H marker can see that, and a doctor with both reports in front of them can.
So keep your old reports, or let us keep them: bring your report number to the counter and we will find anything we have ever issued you.
What the report will not do
It will not diagnose you. A result is one input into a judgement that also involves your history, your symptoms and an examination, and that judgement is the doctor’s work rather than the laboratory’s.
What we are responsible for is that the number is right, that it is yours, and that you can read what it says. If any part of your report is unclear, ring us and ask. That is not a nuisance call, it is the job.