On a laboratory result sheet, you may come across the letter H or L next to a test result figure. These marks often appear automatically and sometimes make people feel anxious right away. In fact, understanding what they mean can help you approach your results more calmly.

What the Letters H and L Mean

In general, the letter H comes from the word high, meaning your result is above the upper limit of the reference range. Conversely, the letter L comes from the word low, meaning the result is below the lower limit of the reference range. In other words, these marks only show that your figure falls outside the comparison range printed on the result sheet.

Some laboratories mark this in other ways, for example a number printed in bold, given an asterisk, or shown in a different color. The point is the same: to flag a result that falls outside the reference range.

Do These Marks Mean You're Ill?

An H or L mark does not automatically mean you have a particular illness. The mark comes from comparing a figure with the reference range, and it only shows the position of the result, not its cause or its meaning. Many things can push a result slightly outside the range, from your body's condition when the sample was taken, to food or drink beforehand, to activity, to other factors that differ from person to person.

What a doctor usually pays more attention to is not simply whether a mark is present, but how far the result is from the range, whether there are any symptoms, how the other test results look, and the trend over time.

How to Approach It

  • Don't panic just because you see a single H or L mark.
  • Look at your results as a whole, not just one number.
  • Keep your test results so a doctor can compare them over time.
  • Mention any symptoms you're feeling when you consult a doctor.

This article is intended to help you understand the marks on laboratory results, not to diagnose a health condition. If you find an H or L mark and feel worried, it's best to discuss it with a doctor, including a 20FIT doctor, so that your results can be explained fully in light of your condition.