What does H mean on blood test results? H, L, A and other flags

What H and L actually say

On blood test results, H means the value is higher than the reference range printed beside it, and L means it is lower. The letter is produced by a computer comparing two numbers, your value and the range, and it says nothing else.

The HL7 standard used to exchange health data defines the letters exactly that way: H is a result above the upper limit of the reference range, L a result below the lower limit. Some reports spell the words out, others show the letter in a column of its own, and the order of reading stays the same: test, value, unit, range, flag.

That automation is the point and also the limit. The flag knows the value and the range. It does not know your medicines, whether you fasted, whether you smoke, the time of the draw or why the test was ordered. A clinician reading the same line brings everything the system did not have.

A, HH, LL and the other flags

H and L are the commonest letters, but not the only ones. The same standard defines a handful of others, and many reports use some of them.

Critical results are handled differently from ordinary flags. US federal rules require the laboratory to alert the clinician who ordered the test immediately when a result points to an imminently life-threatening condition, so a critical value is not supposed to wait for you to find it.

Asterisks, bold, color and arrows

Depending on the laboratory, the same comparison may appear as an asterisk, a value in bold or red, an arrow pointing up or down, the words high or low, or a separate column headed with a symbol. Some reports flag nothing at all and leave you to compare the value with the range yourself.

None of these conventions is more authoritative than another. They are typographic choices, and they all encode the same comparison. Color can even mislead: a 2026 systematic review of 39 studies found that visual cues on online results can make minor deviations look more alarming than they are, while failing to signal the findings that do matter.

H and H is not two flags

One common confusion comes from abbreviations, not flags. “H and H”, also written H&H, stands for hemoglobin and hematocrit, two related lines of the blood count, and US National Library of Medicine guidance lists it as another name for the hematocrit test, alongside Hct and PCV. None of these letters says anything about high or low; the flag, if there is one, sits further along the line.

The letter carries no distance

This is the most useful thing to understand about a flag. A value one unit outside the range and a value ten times outside it print the same letter.

A platelet count of 145 against a floor of 150 and a platelet count of 40 are both L. A ferritin of 28 against a floor of 30 and a ferritin of 4 are both L. The reports look identical in their flagging and describe entirely different situations.

So the first question on seeing a flag is never what the letter means. It is how far outside the value sits, which is a subtraction you can do yourself from the two numbers on the line, and whether the related lines beside it agree.

Why so many results carry a flag

Ranges are built so that about one healthy person in twenty falls outside them on any single test. On a panel of 20 tests, a 2025 review in the Journal of the American Board of Family Medicine calculates that about two healthy people in three will have at least one flagged result. Outside the range is not the same as unwell.

Borderline values are where people struggle most. The 2026 review found that patients value fast access to their results but often find it hard to judge the significance of borderline values or to know when follow-up is needed, and that many then contact their clinic to check whether anyone has reviewed them.

What to do when you see H or L

Many people now read results before their clinician does. In a US survey of 8,139 patients, 16.5% of those with abnormal results said reading them first increased their worry, yet almost all of them still preferred to receive results straight away.

Start with the distance, then the neighbours, then your own history: how far outside is the value, do related lines agree, and was it flagged last time? Note anything that could have moved it, from the time of the draw to a new medicine, and ask your doctor what the flag means in your case.

What a flag would be more useful saying

The 2025 review argues that reports should go further than a letter and state the significance in plain terms: no immediate concern, discuss at your next visit, or contact your doctor at your earliest convenience.

Showing the value on a bar against its range also helps. A 2024 review of 18 studies found that horizontal bars improved understanding and made people less likely to search online or contact their doctor than numbers with a range alone. Until reports do this, the letter only tells you a comparison was made; deciding whether it matters stays with you and your doctor.