Inside the observation
How to read your lab results: reference ranges, units and flags
A plain-language guide to lab reports: why reference ranges differ between labs, what H and L flags mean, and what to do when results arrive before your clinician calls.
Why lab reports are confusing
A lab report is written for the clinician who ordered it, but most patients now see it first in a portal. Each line packs in a test name, a value, a unit, a reference range and sometimes a flag. Understanding those parts will not tell you what a result means for your health, but it will help you ask better questions.
This guide explains the structure. Interpreting results depends on your history, medications and symptoms, so discuss them with the clinician who ordered the test.
The parts of a typical result line
Most portals show the same five elements, even if the layout differs.
| Part | What it tells you | Watch for |
|---|---|---|
| Test name | What was measured, sometimes abbreviated | Similar names can mean different tests |
| Value | Your measured result | Check the unit before comparing with an older result |
| Unit | The scale used, such as mg/dL or mmol/L | Different labs may use different units for the same test |
| Reference range | The range seen in most healthy people, for that lab and method | Ranges differ between labs |
| Flag | H or L for high or low; some labs mark critical values | A flag is a prompt to review, not a diagnosis |
What a reference range really means
MedlinePlus describes a reference range as the high and low ends of the results considered normal. Ranges are usually set from a group of healthy people and cover the middle 95% of their results. By design, then, about 1 in 20 healthy people will fall slightly outside a range on any single test.
Ranges also depend on the lab’s equipment and method, and sometimes on age and sex. That is why the range printed beside your result, from the lab that ran it, is the one to use.
Sources: MedlinePlus: How to understand your lab results · Testing.com: Reference ranges
Units: the most common source of false alarms
The same substance can be reported in different units, for example milligrams per deciliter in one lab and micromoles per liter in another. A number that looks much higher than last year’s may simply be in a different unit. Always read the unit before comparing results over time.
Flags, trends and things that shift results
A high or low flag means the value is outside the reference range. A slightly out-of-range value can be normal for you, while a value that has moved within the range over time can still matter. Clinicians often look at the trend across several results rather than one number.
Food, recent exercise, medications, supplements, hydration and how the sample was collected can all affect a result. False positives and false negatives also happen, which is one reason a result may be repeated.
When results arrive before your clinician calls
Federal information-blocking rules mean many results are released to patient portals promptly, often before a clinician has reviewed them. In a 2023 JAMA Network Open survey of 8,139 patient-portal users, 95.7% preferred immediate release, although people with non-normal results reported more worry. A 2026 study in the same journal found that 68.6% of adults who had online record access and a recent test result viewed it before hearing from a clinician.
Federal guidance also says a provider can delay releasing your results if you ask them to, for no longer than necessary. If you would rather hear difficult news from a person, tell your clinic.
Sources: Steitz et al., JAMA Network Open (2023) · 2026 study summary (EurekAlert) · ASTP FAQ on delaying results at a patient’s request
Questions to bring to your appointment
Write these down next to any result that worries you.
- Is this result expected given my conditions and medications?
- How does it compare with my previous results, in the same units?
- Does anything I did before the test, such as eating or exercising, explain it?
- Does it need repeating, and when?
- What would change in my care if it stays the same?
Questions about this guide
Is a result just outside the reference range a problem?
Not necessarily. About 5% of healthy people fall outside a typical range on any single test. Your clinician will consider the trend and your circumstances.
Why is my range different from the one online?
Ranges depend on the lab’s method and equipment, and sometimes age and sex. Use the range printed on your own report.