How to read blood test results

A lab report is a table of numbers, units and ranges. Once you know what each column means, it is much easier to understand what your results say and what to ask your doctor.

Updated 2026-09-28

What is on a blood test report?

Most lab reports list the same things for every test:

• Test name: often an abbreviation, such as TSH, HbA1c or ALT. Some tests have several names; ALT is also called SGPT.
• Result: the value measured in your sample.
• Unit: how the value is expressed, for example mmol/L or mg/dL.
• Reference range: the range of results the lab considers typical.
• Flag: usually H (high) or L (low) when a result falls outside the reference range.
• Collection date: when the sample was taken, which matters when you compare tests.

What reference ranges mean

A reference range is usually built from the results of a large group of healthy people, and covers the middle 95% of them. That means about 1 in 20 healthy people will fall slightly outside the range on any single test, and a result just outside the range is often not meaningful on its own.

Ranges also differ between laboratories and can depend on age and sex. Always compare a result with the range printed on the same report, not with a number you found online.

What H and L flags mean

An H or L flag only says that a result is outside that lab's reference range. It is a prompt to look closer, not a diagnosis. Your doctor will consider how far outside the range it is, your other results, your symptoms and your history.

Why units differ between labs

The same test can be reported in different units. Glucose is reported in mmol/L in much of Europe and in mg/dL in the US: 5.5 mmol/L is the same as 99 mg/dL. Cholesterol works the same way, with 1 mmol/L equal to about 38.7 mg/dL.

Before you compare results from two labs, make sure they use the same unit. Each biomarker page lists the units labs use and how to convert between them.

One result versus a trend

A single result is a snapshot. Values naturally vary a little from test to test, so one number rarely tells the whole story. Looking at several results over months or years shows whether a value is stable, drifting in one direction, or responding to treatment.

This is why keeping your old reports matters: a Ferritin result that is still in range but has halved over two years tells your doctor more than either result alone.

What can affect your results

• Fasting: glucose and Triglycerides are affected by what you ate, which is why some tests are done fasting.
• Exercise: hard training in the days before a test can raise muscle markers such as creatine kinase (CK) and AST.
• Illness: a recent infection can raise inflammation markers like CRP and white blood cells.
• Medicines and supplements: some can change results or interfere with the test itself. High-dose biotin, for example, can interfere with some thyroid tests. Tell your doctor what you take.
• Timing and hydration: some hormones follow a daily rhythm, and dehydration can concentrate the blood.

Common blood test panels

Complete blood count (CBC)

Counts and describes your blood cells: red blood cells, Hemoglobin, Hematocrit, white blood cells and Platelets.

Metabolic panel

Blood sugar, electrolytes and kidney markers, such as Glucose, Sodium, Potassium, Creatinine and urea (BUN).

Lipid panel

Total cholesterol, LDL, HDL and Triglycerides.

Liver function tests

ALT, AST, ALP, GGT, bilirubin and Albumin.

Thyroid tests

TSH is usually tested first, often with free T4 and free T3, and TPO antibodies when an autoimmune cause is suspected.

Iron studies

Iron, Ferritin, Transferrin and TIBC together describe how much iron you have and how much is stored.

See all 55 biomarkers explained.

When to talk to your doctor

Always discuss your results with a doctor, especially if a result is far outside the range, the report marks it as critical, or you have symptoms. Do not start, stop or change a treatment based on a single number.

Frequently asked questions

What does H or L mean on a blood test?

H means your result is above the reference range printed on the report and L means it is below. A flag is a prompt to look closer, not a diagnosis: many flagged results are only slightly outside the range and have no medical significance.

Is a result outside the reference range always a problem?

No. Reference ranges usually describe the middle 95% of results from healthy people, so about 1 in 20 healthy people falls outside the range on any single test. Your doctor looks at how far outside it is, your other results and your history.

Why are my results different at another lab?

Labs use different equipment, methods and units, and set their own reference ranges. Compare each result with the range on the same report, and convert units before comparing numbers between labs.

Sources

• MedlinePlus: How to Understand Your Lab Results
• MedlinePlus: What You Need to Know About Blood Testing

Medical disclaimer: This page is general information, not medical advice. Reference ranges differ between laboratories, and only a doctor can interpret your results in the context of your health.