Oh, the dreaded evap line. If you’ve ever tossed a negative pregnancy test in the trash, only to dig it out hours later and be shocked by that second line, you know what we’re talking about. In most cases, it’s not a true positive, but rather a very cruel and confusing phenomenon called an evap line.
To understand why evap lines happen and how to avoid them, let’s first go over the anatomy of a pregnancy test.
How do pregnancy tests work?
There are three types of home pregnancy tests (HPT): mid-stream tests, dip tests, and dropper tests.
- Mid-stream tests: These are held in the stream of urine for a few seconds and then set aside for a few minutes to wait for the results.
- Dip tests: Pee in a cup, then dip the test (usually in the form of a strip) into the cup.
- Dropper tests: With these types of test, urine is dropped inside the testing well using a dropper that has collected fresh urine from a cup.
For all of these tests, the most important step is reading the instructions first. Different tests contain different instructions, and sometimes, a single box of tests contains different types, where the instructions are slightly different for each test. Be sure to read the instructions thoroughly so you don’t waste your pee!
When the wait time has passed, you look into the results window to determine whether the test is positive or negative. Positive tests carry a plus sign, two lines, or an explicit message like “pregnant” or “yes.”
The tests that show two line results have two hidden “indent” lines where the ink pools: one for the control line, and one for a potential positive result. As the ink moves across the result window, it collects in these one or both of these indents.
What is an evap line?
An evap line (evaporation line) is a faint line that appears after a negative test has dried. Because tests are typically meant to be read three to five minutes after they are taken, the tests are still damp at the time they are read. After this time frame passes, the urine in the test dries. As the ink makes its way across the screen, the result is displayed. Sometimes, as the ink moves across the screen, a little bit of ink will accidentally get caught in the indent line. As the test dries and the urine evaporates, this caught ink is pulled to the surface and becomes darker.
This is why the most important rule of pregnancy tests is: do not read them after the allotted time! The results are only accurate in the time frame specified in the instructions.
How to distinguish a faint line from an evap
Okay, so now you know how to spot a false positive. But what about when you see a faint line within the time frame allotted by the test? How do you know it’s really positive?
- Don’t test too early. The earlier you test, the lowest your hCG levels will be. If you have a real squinter, test again in a day or two. As hCG rises, a true positive will darken.
- Take a more sensitive test. Different test types and brands have different hCG thresholds in order to display a positive result. Because of this, cheap tests that you order online (such as Wondfo) are more reliable when you test early than digital tests. Keep in mind that the downside of testing early is that you are more likely to find out if you have a chemical pregnancy. These very early miscarriages are quite common, they usually do not signify any kind of underlying fertility problem, and before the advent of highly sensitive pregnancy tests, women might have had chemical pregnancies without even realizing it.
- Don’t over-hydrate. If you have been drinking a lot of water and/or urinating frequently, your urine may be too dilute for an accurate reading. It’s best to hold your urine for two or three hours to ensure it is not too diluted. First morning urine is usually more concentrated than urine from random hours during the day.
What does it mean if there is a line with no color?
On a dip test, a positive result is two lines of the same color. If there is a very faint second line with no color at all, it’s possible for it to be an indent line.
Remember the discussion above about indent lines and the way pregnancy tests work? In the same way that ink may accidentally pool in the indent line, the indent line itself may sometimes become barely visible. If your second line appears gray or colorless, it is probably an indent and not a true positive.
(H/T to the always excellent and informative TFAB for much of the content in this post.)