It can sometimes be hard to tell whether that subtle, extra line on a pregnancy test is a positive result. An evaporation line (or evap line) typically appears as a faded streak, while a faint positive pregnancy test result looks like a lighter blue or pink line, depending on the test dye color.
Because interpreting your at-home pregnancy test can occasionally be tricky, this article covers evaporation lines and faint positives, tips to get accurate results, and what to do if you don't.
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How Pregnancy Tests Work
At-home pregnancy tests check your urine for human chorionic gonadotropin (HCG), a hormone released by your body when a fertilized egg implants in the lining of your uterus. An at-home test typically involves either peeing on a test stick or dipping it into a cup of urine and waiting several minutes to read the results: a plus or minus sign, one or two lines, or the words “pregnant” or “not pregnant.”
If one line (the control line, which confirms the test is working) appears on a non-digital test, it most often means you are not pregnant. If your test shows both the control line and a line in the test result window, this typically indicates pregnancy.
Pregnancy tests are most accurate when taken after you have missed a period. Some at-home early pregnancy tests can be used to detect a pregnancy up to 10 days before a missed period, but these results are less reliable.
Differences Between Evaporation Lines vs. Faint Positives
An evaporation line is a faint, colorless line that appears in the results window of an at-home pregnancy test. This happens if the urine being tested has dried beyond the reaction time of the test.
Sometimes evaporation lines and faint positive pregnancy test lines can appear similar, especially if you don't know exactly what you're looking for. Because of this, you may mistakenly assume that you are pregnant when you are not.
Does an evap line mean pregnancy?
An evaporation line does not indicate pregnancy. The safest bet is to take another test and read it within the specified time frame for the most accurate reading. Check with a healthcare provider if you're still unsure about the results.
Color
It's helpful to take a closer look at the line's color. Is there a faint color to the line that's the same shade as the control line (usually blue or pink)? This is probably a faint positive result.
If the line appears more "colorless," like gray, white, or shadowy, this is likely an evaporation line.
Thickness or Thinness of Line
Consider whether the line in question is roughly the same size as the control line. Is it the same thickness and does it run from top to bottom?
If the answer to these questions is yes, even though the line may not be as dark as the control line, it's likely a faint positive.
If the line you're looking at is a different size and shape or doesn't expand across the entire length of the test window, an evaporation line might be to blame.
How Long It Takes to Appear
There's a reason why it's important to read your pregnancy test within the time frame that's noted on the product's instructions (usually somewhere from three to 10 minutes). When urine sits on the test for too long past the reaction time, it'll dilute and dry out—causing an evaporation line.
If you're checking the result and see two lines during the manufacturer's stated time frame, it's more likely to be a faint positive result. But if you wait too long after the reaction time to interpret the results (tests are meant to be read while the strip is still damp), it's probably an evap line.
How Dye Works in a Pregnancy Test
Home pregnancy tests (dye-based and digital) check your urine for human chorionic gonadotropin (hCG). The dye-based tests work by chemically reacting—and changing color—when hCG is present.
How Common Are Evaporation Lines?
Evaporation lines can be common on home pregnancy tests, but they’re not often discussed.
Cause
An evaporation line happens when the pregnancy test is read after the time frame specified in the instructions.
Many at-home pregnancy tests suggest reading the results within two, five, or up to 10 minutes after urinating on the stick. Checking results after that window means your urine has dried, potentially causing that streaky evap line.
Like pregnancy tests, reading at-home COVID-19 tests can also prompt confusion. Does it matter how faint or thin the test result line is? Experts say the faintness or darkness of the test line—and how long it takes to appear—indicate how sick you are and how likely the virus is to be transmitted to other people.
Getting the Most Accurate Pregnancy Test Results
When taking at-home pregnancy tests, several factors go into getting the most accurate result and avoiding confusion. Experts recommend the following tips:
- Check your results within the recommended reaction time window, which will vary slightly by pregnancy test brand.
- Try a digital at-home pregnancy test instead of a dye-based one if you're concerned about evaporation lines.
- Test first thing in the morning because your urine is more likely to have higher levels of hCG at that time.
- Wait until a day after your missed period to test, even though some pregnancy tests are sensitive enough to detectpregnancy earlier than a missed period.
Though many pregnancy test manufacturers market their products as extremely accurate, research suggests that real-life situations (e.g., errors in the date of an expected period, certain medical conditions, or being peri- and post-menopausal) affect the accuracy rate.
Next Steps After a Faint Positive Result
You might consider retaking the at-home test to ensure the result if you think it may be a faint positive.
After that, the next step is to check with a healthcare provider to schedule an in-office appointment. They can confirm pregnancy via ultrasound or blood test, if needed, during this visit.
Summary
An evaporation (or evap) line is a slight, typically colorless streak on an at-home pregnancy test that can cause confusion. These lines usually show up when you've read the results outside the suggested window and the urine has dried. On the other hand, a faint positive test result will be the same color as the control line—just not as bold or dark. This is more likely to be the case if you look at the test results within the suggested time frame.
If unsure about your at-home pregnancy test results, try testing again the next day. After that, call a healthcare provider if your test is positive or if you need more help getting an accurate reading.
