Pregnancy test - Evaporation line

What is an evaporation line?

An evaporation line ("evap" line) is a term that describes any test line that appears on a pregnancy test after the time limit is up. It develops when the urine on the test strip begins to dry, often appearing as a faint, colorless line. Evaporation lines commonly appear as a result of the composition of the particular urine specimen – and therefore they can appear on any test regardless of the brand.

An "evap" line is considered a negative result.

Pregnancy tests typically have two lines that appear when used.

  • Control line: this should always appear and it lets you know that the test is working correctly
  • Test line: this will show up only when exposed to urine that contains the hCG (the pregnancy hormone)

The test line on a pregnancy test contains an antibody strip that binds to hCG and this is what causes the line to become visible and show a positive result if you are pregnant.

An evaporation line is a line caused by the evaporation of urine from the test. The evaporation line can either highlight the antibody strip and cause it to appear as a faint positive, or sometimes make the test line looks pale, gray or like an indentation. These results, after the time limit, should not be considered positive.



What if a faint line shows up within the recommended time limit?

If an "evap" line shows up in the time limit then it probably isn't an evaporation line as the process of evaporation has not begun yet. It is more likely a positive result! (Assuming that the testing procedure has been followed properly and the results have been interpreted within the time frame indicated by the manufacturer) Generally a pregnancy test won't show a positive test line if read within the time limit unless you are pregnant. If you get a faint result, it is usually a good idea to take another test in a few days. This allows time for the hCG to rise and will make a test result darker and easier to interpret.

Manufacturers recommend that you should not read tests after a designated time period, as most test brands have the possibility of developing evaporation lines. Be sure to read the instructions that come with your test.

How to avoid them

The best way to avoid an evaporation line is to discard the pregnancy test as soon as the time limit is up. Be sure to check the instructions for your particular pregnancy test brand and follow them exactly. Read the test result after the initial reaction time and before the limit. Once you have read the result, get rid of the test and don't be tempted to look at it later.