What are Pregnancy Test Evaporation Lines?

As the name suggests, evaporation lines appear due to urine evaporation within the testing area of your pregnancy test. These lines show up in the result window of the test, exactly where a positive sign would be. An evaporation line develops when the urine on the test area begins to dry, leaving a faint line.

What Causes Evaporation Lines on a Pregnancy Test?

These lines show up due to the particular urine specimen's composition and may appear on any test regardless of brand. You may see a clear-ish line if you read the results after the time frame the instructions stated or if you took the test incorrectly.  

How Common are They?

Evaporation lines are pretty standard on all brands of pregnancy tests. If you are unaware of this, you may think you have a positive result when you don't. The best way to avoid confusion is to follow the manufacturer's instructions. Read the results of your pregnancy test within the time frame recommended.

What Do Evaporation Lines Look Like?

Sometimes, it is tough to tell. Here are some general guidelines to help you figure it out.

An evaporation line is usually a grayish color on pregnancy tests that use pink dye.

An evap line may be grayish or a very light blue on tests that use blue dye. The TTC community finds that the blue dye tests show evaporation lines more often than pink dye tests.

It might look like a colorless streak similar to an indentation in your pregnancy test result area.

Usually, an evaporation line will be thinner than the width of the control line. 

Is This an Evap Line Or a Positive Result?

Here are a couple of guidelines to help you determine what you see.

Did you read the results within the time frame of your test instructions?

Yes - Possible early positive result

No - It could be an evap line.

Is the result line thinner than the control line?

Yes - Possible evap line

No - Possible positive result

Is the result line very light in color or almost clear?

Yes - Likely an evaporation line

No - Positive result possible

Remember:  You will only see an evaporation line if the test is negative or if you tested too early. If the test were positive, the color of the dye would fill in the space where you see the evaporation line.

Ultimately, if you are confused, take another pregnancy test in the morning and see if the results have changed. If the line gets more colored, the suspected evaporation line was a very early positive result.

How Can You Stop Them From Forming?

Ultimately, you can't stop the formation of these types of lines. Any brand of pregnancy test can have them (except for the digital tests that say pregnant or not pregnant.)

You should always read the literature that comes with the test and look for instructions on interpreting your results. Usually, home pregnancy tests have a reaction time of three to five minutes. If in doubt, test again a few days later.

If you don't see a clear positive result in the amount of time that the manufacturer states, you should not rely on that test result. Letting your test sit and reading it later can be not very clear. The chances of an evaporation line forming increase when you do this.