What the letters beside your results mean

Laboratory reports use short flags beside results. They are not a diagnosis and not a severity score. They are a signal that a result sits outside the printed reference range, or far enough outside it to warrant urgency.

Flags are generated automatically by comparing a number against a stored interval. No person looks at each one, which is both why they are reliable as a mechanical check and why they carry no judgement about what the result means.

The common ones

H and L mean above and below the reference range printed on that report. They say nothing about how far outside, or whether it matters. A haemoglobin one unit below the range and one thirty units below carry the same L.

HH and LL, double asterisks, or the words critical or panic indicate a result the laboratory treats as needing attention straight away. These use a second, much wider set of limits, chosen so that crossing one is uncommon and consequential.

A means abnormal, used by some laboratories where a result is not a simple number — a qualitative result that came back positive when negative was expected, for instance.

Some reports use arrows or a change in typeface instead of letters. The meaning is the same: a mechanical comparison against a stored interval.

What a critical flag actually triggers

Critical limits exist because some results need acting on within hours, and a report sitting unread in a portal is not a reliable way to achieve that.

When a result crosses a critical limit, most laboratories telephone the requesting clinician directly rather than relying on the report being read. The call is usually logged, and many laboratories require the person taking it to read the result back to confirm it was heard correctly.

This means that if a report carries a critical flag, there is a good chance a conversation has already happened that you were not part of. It also means that if a flag is present and nobody has been in touch, that gap is worth closing.

Why a critical result is sometimes repeated first

Some critical results are checked on a fresh sample before anyone acts on them, because a small number of them are artefacts of how the sample was collected or handled.

Potassium is the classic example. A difficult blood draw, a tight tourniquet, or a delay before the sample reaches the analyser can all release potassium from cells into the surrounding fluid, raising the measured value without anything having changed in the person.

A request to repeat a test is therefore not a suggestion that the first result was ignored. It is often the fastest way to find out whether the number reflects the person or the sample.

What to do about a critical flag

A result flagged critical needs attention from a healthcare professional now, regardless of how anyone feels and regardless of what any general explanation says about that marker.

If a report carries such a flag and nobody has been in touch, contacting the requesting clinician or an urgent care service is reasonable. If symptoms are present, emergency care is the right route rather than a phone call.

The absence of a flag is not the same as reassurance. Flags only compare a number against a stored interval; they cannot see a pattern across markers, a change from a previous result, or anything about the person.