What does H or L mean on a blood test?

H usually means the result is higher than the lab’s reference range and L means it is lower. A flag is a prompt to look closer, not a diagnosis. Small flags are common in healthy people.

Which reference range should I use?

Use the range printed on your own report. Labs set their own ranges based on their methods and equipment, so a range from another lab or a website may not apply.

Why can I see my results before the doctor calls?

Federal rules mean many lab results are released to the patient portal soon after they are ready, sometimes before the doctor has reviewed them. If something worries you, contact the doctor’s office.

Is one abnormal result something to worry about?

Usually one slightly abnormal result is not a cause for alarm on its own. The doctor looks at how far out it is, related tests, earlier results and symptoms. For urgent symptoms, call 911.

How to read a blood test report, line by line

November 6, 2026 · 6 min read · Vita Health Coach

The short answer

A blood test report lists each test on its own line: the test name, the result, the unit it is measured in, the reference range the lab uses, and a flag if the result falls outside that range. Read each line from left to right, look at the flags, then compare with earlier results. One flagged number rarely tells the whole story, so go over the report with the doctor who ordered it.

What is on a blood test report?

Most blood test reports in the US follow the same pattern, even when they come from different labs. At the top are the details about the person and the sample. Below that, the tests are grouped into panels, and each test sits on its own row.

A panel is a group of tests ordered together. The complete blood count (CBC) and the comprehensive metabolic panel (CMP) are the two you will see most often. A lipid panel checks cholesterol and other blood fats. Each row in a panel usually has the same columns.

Test name: what was measured, such as hemoglobin or potassium.
Result: the number the lab measured in this sample.
Units: how the number is measured, such as mg/dL or mmol/L.
Reference range: the span of results the lab expects in most healthy people.
Flag: a letter or mark, often H for high or L for low, when the result is outside the range.
Comments: notes from the lab, for example that a sample was hard to test or that a result was phoned to the doctor.

Start with the details at the top

Before you look at any number, check that the report belongs to the right person. Look at the full name and the date of birth. Mix-ups are rare, but they happen, and it takes a few seconds to check.

Next, look at the dates. The collection date is when the blood was drawn. The reported date is when the lab finished the test. If you are comparing two reports, compare the collection dates, because that is the moment the results describe.

Some reports also say whether the person was fasting, which means they had nothing to eat or drink except water for several hours before the test. This matters for a few tests, such as glucose (blood sugar) and triglycerides (a type of blood fat). If the report says the sample was not fasting, the doctor will read those lines with that in mind.

Read each row from left to right

Take one row at a time. Say you see a line for potassium. Potassium is a mineral that helps nerves and muscles, including the heart, work properly. The row shows the result, then the unit, then the reference range, then a flag if there is one.

Ask three simple questions for each row. What was measured? Is the result inside the range printed on this report? Is there a flag? If the result is inside the range and there is no flag, the lab did not find it unusual. That is not the same as a diagnosis, but it is the usual starting point.

Always use the reference range printed on your own report. Ranges differ from lab to lab because labs use different machines and methods. A range you find online may not match the one your lab used.

What the flags mean

A flag means the result is outside the lab’s reference range. H usually means high and L usually means low. Some labs use an asterisk, bold type, color, or the letter A for abnormal. A few results may carry a stronger mark, such as the word critical, which means the lab treats the value as needing quick attention.

A flag does not mean something is wrong. Many reference ranges are built to include the middle 95 percent of results from a group of healthy people. That means about 1 in 20 healthy people will fall just outside the range on any single test. On a report with 20 or 30 tests, one or two small flags are common in people who are well.

What matters more is how far outside the range a result is, whether related tests move together, whether the result has changed since last time, and whether the person has symptoms.

Look at the whole picture, not one number

Tests that measure related things often move together. For example, hemoglobin (the protein in red blood cells that carries oxygen), hematocrit (the share of the blood made up of red cells) and the red blood cell count tend to rise and fall as a group. Kidney tests such as creatinine, BUN and eGFR are also read together.

Compare with earlier results if you have them. A value that has been slightly high for years means something different from a value that was normal last time and is now well outside the range. Doctors call this looking at the trend.

Context matters too. Recent illness, a hard workout, not drinking enough fluids, a recent meal, and some medicines can all shift results. The doctor knows the person’s history and can weigh these things. That is why it helps to read a report with the doctor, not alone.

What to do after you read it

Results often appear in a patient portal before the doctor has looked at them. Under federal rules, many results are released to patients quickly. So if you see a flag and have not heard from the office, it may simply mean the doctor has not reviewed it yet.

Write down your questions, note any symptoms, and contact the doctor’s office if something worries you. If the report marks a value as critical and no one has called, phone the office or its after-hours line the same day. If the person has chest pain, trouble breathing, signs of a stroke, fainting or sudden confusion, call 911 right away. Do not wait for a lab result.

How Vita Health Coach helps

Vita Health Coach is an iPhone and iPad app from VitaMedas, Inc. You can take a photo of a lab report, or connect a US hospital through its patient portal, and the results go into that person’s own file. Each new result is explained in plain words and compared with the last one, with the short answer first: can this wait, or is it time to see a doctor, and how soon. Vita explains health information and never diagnoses or prescribes. It is free to try, with paid plans in the App Store.

Questions people ask

What does H or L mean on a blood test?: H usually means the result is higher than the lab’s reference range and L means it is lower. A flag is a prompt to look closer, not a diagnosis. Small flags are common in healthy people.
Which reference range should I use?: Use the range printed on your own report. Labs set their own ranges based on their methods and equipment, so a range from another lab or a website may not apply.
Why can I see my results before the doctor calls?: Federal rules mean many lab results are released to the patient portal soon after they are ready, sometimes before the doctor has reviewed them. If something worries you, contact the doctor’s office.
Is one abnormal result something to worry about?: Usually one slightly abnormal result is not a cause for alarm on its own. The doctor looks at how far out it is, related tests, earlier results and symptoms. For urgent symptoms, call 911.

Sources

Vita Health Coach is an iPhone app that keeps a parent’s hospital results in their own file and explains each new one in plain words. Free to try; paid plans in the App Store.

Vita Health Coach on the App Store

General health information, not medical advice. Vita Health Coach explains; it never diagnoses. In an emergency, call 911 or your local emergency number. Read the medical disclaimer.