What is a reference range?

On every lab report there are two other numbers next to yours. They are the low and high ends of the reference range. Most people only look to see whether their number falls between them, but the story is a little more complicated.

Where does the range come from?

A lab takes the results of a large group of healthy people and works out the range from them. One test may have separate ranges for different groups, such as children and adults.

So the word “normal” can mislead. The range says where most healthy people fall, not where the line between health and illness lies.

A number outside the range

A number above or below the range may or may not be a sign of a problem. Healthy people sometimes have results outside it. Your doctor uses what else is known about you to understand what affected the result.

A number inside the range

A result inside the range is no guarantee of health either. Some people with health problems have normal results. If you have a symptom that continues, you may need more tests despite a normal result.

Why do ranges differ?

Labs often use different methods, so their ranges differ. Units differ too. Two things follow:

• Judge your result against the range on the same report, not a number you saw elsewhere.
• Results from two labs cannot always be compared. To see a trend, it is best to test at one lab.

Positive and negative results

Some tests give no number and only say whether something was found. Negative means the test did not find what it was looking for, and positive means it did. An uncertain result is usually repeated. Wrong results are uncommon, but no test is perfect.

In short

Read your result, but do not make decisions from it alone. What a number means for you can only be told by someone who has also heard your history and examined you.