How to Read a Lab Report: A Plain-English Guide
The parts of a lab report
Most reports, from a hospital or a diagnostic chain, follow the same layout. Knowing the columns removes most of the confusion.
| Column | What it tells you |
|---|---|
| Test name | What was measured, for example “Hemoglobin” or “LDL Cholesterol”. Some tests have several names (HbA1c, A1c, glycated hemoglobin). |
| Result | Your value from this sample. |
| Unit | How the value is measured, such as mg/dL, mmol/L, g/dL or ×10³/µL. The same number means different things in different units. |
| Reference range | The interval in which most healthy people in the lab’s comparison group fall. |
| Flag (H, L, HH, LL) | High, low, or critically high or low compared with that range. |
| Method and notes | How the test was run and special instructions, such as fasting status. |
What “reference range” really means
A reference range is usually set so that about 95% of healthy people fall inside it. That means roughly 1 in 20 healthy people will sit just outside the range for any single test. A result slightly outside the range, with no symptoms and a normal repeat, is often not a problem. Ranges also differ by lab method, age and sex, which is why you should always compare against the range printed on your report. Our guide on what a normal range is goes deeper.
Check the units before the number
US labs usually report glucose and cholesterol in mg/dL, while the UK, Europe, Australia and Canada often use mmol/L. A glucose of 5.5 is normal in mmol/L and impossibly low in mg/dL. Use the free unit converter or read our mg/dL vs mmol/L guide when comparing reports from different countries.
How to read your results in five steps
- Look at the unit and range first. Make sure you are comparing like with like.
- Find the flagged results. Note which are slightly out of range and which are far out.
- Group related tests. Cholesterol, LDL, HDL and triglycerides belong together, as do creatinine and eGFR, or hemoglobin and ferritin.
- Compare with your previous results. A value that has moved steadily over years tells you more than one snapshot. See how to track results over time.
- Write down questions for your clinician: what changed, what could explain it, and when to retest.
Common reasons a result is out of range
- Not fasting when fasting was required, or recent heavy exercise.
- Dehydration, which concentrates many blood values.
- Medicines and supplements, such as biotin, steroids or high-dose vitamins.
- Normal variation: many values change by 5–20% between two draws.
- A real change that needs follow-up, which is why trends matter.
Looking up a specific test
Our blood test library explains typical ranges, common causes of high and low results and how to prepare, for the tests people ask about most.
Frequently asked questions
What does H or L mean on a lab report?
H means the result is above the lab’s reference range and L means below it. A flag shows a result is outside the range, not that you have a disease.
Why do different labs have different normal ranges?
Labs use different instruments and methods and compare against different populations, so ranges differ slightly. Always use the range printed on your own report.
Can I compare results from different labs?
Yes, if you convert them to the same unit and read each against its own range. Large changes matter more than small differences between methods.