How accurate are EVAP lines?
An evaporation line on a pregnancy test is basically a false positive. While true false positives and false negatives are fairly rare,1 the evaporation line is the closest most women will come to having an error on their pregnancy tests.
Evaporation lines are common and can occur with any pregnancy test. An evaporation line is a line that appears in the results window of a pregnancy test as the urine dries. It can leave a faint, colorless line. If you're not familiar with evaporation lines, you might see this line and think you're pregnant.
Evaporation lines are colorless streaks, not faint lines. They typically appear if a person waits for longer than the suggested time to read the test result. An evaporation line can also appear if the test gets wet. An evaporation line does not indicate pregnancy.
An evaporation line is also often thinner than the control line. In the image above, you can see that the horizontal line is one thickness, while the vertical evaporation line is much thinner and of uneven thickness.
Evaporation lines are actually streaks and not faint lines. They are called "squinters," meaning you have to look closely to see them. Often, these lines will not go entirely across the test. Depending on the type of test, the evaporation line may look more gray than pink.
Evaporation Lines
The easiest way to differentiate between a positive test result and evaporation line is to check your test within the allotted time. A true positive will appear within two to 10 minutes, while urine won't dry and form streaks until some time later.
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 for getting a true positive pregnancy test is: do not read them after the allotted time!
Evaporation (Evap) Lines
Evaporation lines are considered false-positive results. Unlike a faint positive result, which will result in a light pink or blue line, an evaporation line does not activate the dye in the pregnancy test, so it may show up as a colorless or translucent line.
On a true positive pregnancy test result, the test line will be the same thickness as the control line. On the other hand, evaporation lines will usually be thinner than the control line.
Evaporation lines on pregnancy tests are generally colorless, but not always. There are some evaporation lines that do have color—either a light blue or light pink. The color, however, is very faint and not as dark as the color of the control line.
How likely is an evaporation line on a pregnancy test?
It's not uncommon to see evaporation lines in any pregnancy test. Pregnancy tests display an evaporation line in the results window as the urine dries. Colorless traces can be left behind. If you haven't heard of evaporation lines before, you might mistake this line for a pregnancy sign.
Should Pregnancy Test Lines Get Darker? Although hCG levels increase exponentially during early pregnancy, that doesn't mean that the pregnancy test line will necessarily get darker as each day passes.
While an evaporation line can appear on a pink or blue dye test, many frequent testers on popular online pregnancy and fertility forums adamantly argue that blue tests are more prone to these deceptive shadows.
If you let the test sit too long the the test may show a false positive result. A false positive is when the tests shows you are pregnant when you actually are not. Pregnancy tests work by detecting a hormone called human chorionic gonadotropin, HCG.
The key way to tell the difference between an evaporation line vs blue dye positive lines is to see whether the line gets thinner. If it stays thick, it's more likely to be positive, but if it gets thinner, you could be looking at an evap line.
Evaporation (Evap) Lines
The result appears after this three-to-five minute window. However, if you wait too long after the reaction time to interpret the result, the urine in the pregnancy test can dry up, resulting in an evaporation line. Evaporation lines are considered false-positive results.
What is an evap line and how is it different from a positive pregnancy test? 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.
If the pregnancy test kit is kept for too long and the urine begins to dry out, the evaporation line will appear as faint pink or blue. It may appear if the test is performed too soon or if the hCG levels are quite low, resulting in a false positive.