How to Read a Lab Report: What Every Column Means

A lab report shows your test result next to a reference range so you can see whether your value is typical. You read it by checking your observed value against the biological reference interval and looking for H or L flags. Each column gives you a different piece of context about your result.

What do the columns on a lab report mean?

Every blood test report organises your results into distinct columns. Each column tells you something different. Here is what a typical row looks like:

Column	Example	What it means
Test Description	Haemoglobin	The name of what was measured in your sample
Observed Value	13.5	The exact number the lab measured in your blood
Units	g/dL	The scale used to express the number
Biological Reference Interval	12.0 – 15.0	The expected range for most healthy people at this lab
Flag	H or L or blank	Marks results outside the reference interval
Method	Cyanmethemoglobin	The scientific technique the machine used

What does biological reference interval mean on a lab report?

The biological reference interval is the range of values seen in most healthy people. Labs calculate it by testing a large group of healthy volunteers and taking the middle 95 per cent of results [CLSI, C28-A3].

This means 5 per cent of perfectly healthy people will have a result outside the interval by chance. A flag does not automatically mean something is wrong. It means your value fell outside the range the lab considers typical.

Some examples of common reference intervals:

Test	Reference interval	Population	Source
Fasting blood glucose	70 – 99 mg/dL	Adults	ADA, 2024
Haemoglobin	12.0 – 15.0 g/dL	Non-pregnant women	WHO, 2011
Haemoglobin	13.0 – 17.0 g/dL	Adult men	WHO, 2011
Total cholesterol	Below 200 mg/dL	Adults	NCEP/ATP III

What does H or L mean on a blood test?

H stands for High. Your result sits above the upper limit of the reference interval.

L stands for Low. Your result sits below the lower limit.

A blank flag means your value sits within the interval. No flag was triggered.

Many labs print flagged values in bold or red text. Red text simply makes the number easier to spot. It does not mean the result is dangerous. A flagged value is worth discussing with your doctor — it is not automatically a cause for alarm.

Why do reference ranges differ between labs?

Different labs use different testing machines and chemical reagents. The machine's method changes the numerical output. The Clinical and Laboratory Standards Institute publishes guidelines explaining how method-dependent intervals are set [CLSI, C28-A3].

A result that appears slightly outside the range at one lab might fall inside the range at another — even for the same blood sample. This is why you should always compare your result against the reference interval printed on your own report, not a range you found online.

How do common lab units compare?

The same substance can be measured in different units depending on the lab and country. Here are the most common units and how they relate:

Unit	Full name	Typically used for	Conversion note
mg/dL	Milligrams per decilitre	Glucose, cholesterol, triglycerides, urea	Used widely in India and the US
mmol/L	Millimoles per litre	Glucose, cholesterol, electrolytes	Used in UK, Australia, Canada. Glucose: mmol/L × 18 = mg/dL
g/dL	Grams per decilitre	Haemoglobin, total protein, albumin	Standard for blood protein measurements
µg/L	Micrograms per litre	Ferritin, some vitamins, trace elements	Same numeric value as ng/mL for most tests
µIU/mL	Micro international units per millilitre	TSH (thyroid), insulin	Same as mIU/L
U/L	Units per litre	Liver enzymes (ALT, AST, ALP, GGT)	Activity-based measurement

What does the Method column mean?

The Method column names the scientific technique the laboratory machine used. Different methods produce slightly different numerical results for the same test.

For example, haemoglobin can be measured by the cyanmethemoglobin method or the sodium lauryl sulphate method. Both are valid. But the reference interval is set for the specific method your lab uses [CLSI, C28-A3]. This is another reason why comparing values across labs requires care.

Frequently asked questions

What does biological reference interval mean on a lab report?

It is the range of values seen in the middle 95 per cent of healthy people tested at that lab [CLSI, C28-A3]. Your observed value is compared against this interval to identify values that sit outside the typical range.

Why is my blood test value printed in red?

Labs use red text or bold print to highlight results that sit outside the reference interval. It makes flagged values easier to spot. Red text does not mean a result is dangerous — it means it is worth discussing with your doctor.

What does a blank flag column mean?

A blank flag means your result falls inside the reference interval. No H or L was triggered. The result is within the range the lab considers typical for the population it tested.

What is the difference between mg/dL and mmol/L?

They are different units for measuring concentration. For glucose: 100 mg/dL equals 5.6 mmol/L [ADA, 2024]. To convert glucose from mg/dL to mmol/L, divide by 18. For cholesterol: 200 mg/dL equals 5.2 mmol/L. To convert cholesterol, divide mg/dL by 38.7.

Why do two different laboratories show different reference ranges?

Reference ranges depend on the testing equipment and chemical methods used by each laboratory [CLSI, C28-A3]. Different machines produce slightly different numerical outputs for the same sample. Always use the range printed on your own report.

Can I use a reference range I found online to interpret my results?

No. Online ranges are general guidelines. Your lab's printed range is the correct one to use for your result. It reflects the specific machine and method your lab used [CLSI, C28-A3].

Sources

• American Diabetes Association (ADA). Standards of Care in Diabetes, 2024.
• Clinical and Laboratory Standards Institute (CLSI). Defining, Establishing, and Verifying Reference Intervals in the Clinical Laboratory; Approved Guideline — Third Edition (C28-A3).
• National Cholesterol Education Program (NCEP) Expert Panel on Detection, Evaluation, and Treatment of High Blood Cholesterol in Adults — Third Report (ATP III).
• World Health Organization (WHO). Haemoglobin concentrations for the diagnosis of anaemia and assessment of severity. Vitamin and Mineral Nutrition Information System. Geneva, 2011.

Last reviewed: August 2026

Upload your report to ReadMyReport.com to see what your own numbers mean.