What’s an evap line?
Evap lines, or evaporation lines, are a fairly common phenomenon that can happen on any kind of home pregnancy test that isn’t digital. They occur after urine dries, leaving a faint line behind that can easily be misinterpreted. What I didn’t realize was that an evap line can look exactly like a positive test result.
Why does an evaporation line occur on a pregnancy test?
Pregnancy tests are basically tiny chemistry experiments you run on your bathroom counter. Urine travels across a test strip where it has a chance to interact with both a control line, and a test line. This second line is where hCG (a pregnancy hormone) will interact with reagents on the test strip, resulting in anywhere from a faint line to a dark line depending on your hCG levels, indicating a positive pregnancy test. The tricky part, aside from not peeing on your fingers while you’re doing it, is that you can’t always control for every variable in this experiment. Here are a few scenarios that can lead to evap lines:
Indentations: An imperfection that causes an indent line in the test strip can cause urine to pool unevenly on a portion of the strip, leaving a faint line that could be misinterpreted.
Ink: Sometimes the ink used to signal a positive result is pulled into the results window by drying urine causing a very convincing second line that looks like a positive test line.
Dark urine: Sometimes urine can dry in a way that causes a faint gray, shadowy, or colorless line across the strip, giving the appearance of a faint positive pregnancy test.
These lines, no matter how convincing, are still negative results. It’s also important to note that evap lines are different from false positives, even though the rollercoaster you get sent on can be the same.
Are some tests better than others for avoiding evap lines?
The internet has strong feelings about what pregnancy test kits are better overall, but there’s no science to back up if blue dye tests or pink dye tests are better for avoiding evap lines specifically. Going digital in this case is probably best, or just following the instructions and not reading the results outside of the testing window. Seriously. Treat it like The Ring. After the allotted time, read the results, then take that test and get it out of your house. It’s too tempting to look and wonder if maybe it’s just a false negative (it isn’t - don’t do this to yourself).
But I want to look, because what if?!
A second line or a plus that shows up after the recommended time frame is over isn’t an accurate result and doesn’t count. You can always retest later on if you got a negative test result, and you’re thinking your levels of hCG just aren’t high enough to read yet. Just maybe wait until the morning since you’re going to want to use a first thing in the morning urine sample.
And if you’re like me and rocking a trusty IUD and NEED TO KNOW THIS VERY SECOND if that evap line is the first sign of a big problem, you can go to the doctor and get the blood test. But any result read outside of the testing window is invalid.
What do I do if I see an evap line that looks positive?
Retest. Either with a new, unexpired urine pregnancy test, or with a blood test at your ob/gyn if there’s a reason - like an IUD, or a suspected ectopic pregnancy. Or just because you need to know.
Regardless of if it’s a faint colorless streak, a faint positive line, or a dark line in the wrong spot, the best thing to do is throw away the test in question, and retest. This isn’t a valid pregnancy test result, and it’s not worth driving yourself nutty over. Trust me. Been there, dug that out of the trash… and regretted it.
Have you ever seen an evap line?
Help those of us who've fallen for this trick feel not so alone by telling us about it in the comments below.
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