If you’re hoping for a positive pregnancy test, you might squint at the test stick, hoping to see two solid lines. And if you wait long enough, you might actually see both lines, but that doesn’t necessarily mean you’re pregnant. In some cases, what looks like a positive result might actually be an evaporation line (or lines).
So what’s an evaporation line? Dr. Robert Seton, a reproductive endocrinologist and infertility specialist at Shady Grove Fertility in New York City, says it’s a sign that shows up on a home pregnancy test after the sample dries up and evaporates.
An evaporation line “appears as a faint, colorless line where the ‘test’ line is,” says Dr. Seton. “It’s basically a test product that only shows up after the urine has dried.” Evaporation lines aren’t a sign of pregnancy; rather, they usually mean that you waited too long to read your test results.
Here’s how to tell the difference between a positive pregnancy test and an evaporation line, as well as tips to avoid blue or pink evaporation lines on a pregnancy test.
What is the evaporation pathway?
Evaporation lines may appear on pregnancy tests after the urine sample dries up. Evaporation lines are usually colorless and indicate that the test took too long to interpret the result.
How do home pregnancy tests work?
In a successful pregnancy, your body starts producing a hormone called human chorionic gonadotropin (hCG) as soon as the fertilized egg implants in the uterine wall. Home pregnancy test kits are designed to detect hCG in a urine sample.
“HCG levels rise during early pregnancy, and many brands of pregnancy tests use an immunoassay to detect hCG in your urine when your period is late,” explains Lizbeth Chan, MD, obstetrician-gynecologist at Dignity Health Medical Group: Northridge.
With dye pregnancy tests, your result is determined by the number of colored lines that appear in the testing window immediately after urinating onto the test strip, says Dr Chan. A positive result will usually show two lines, while a negative result will only show one line (the control line). Alternatively, you can also use a digital pregnancy test, which will say “Pregnant” or “Not Pregnant.”
Home pregnancy test kits come with very specific instructions on how and when to test, and how and when to read the results to avoid mistakes. Refer to the pregnancy test instructions for more specific instructions.
Why do evaporation lines appear?
Evaporation lines are very common and can appear on a pregnancy test when urine dries up. They usually appearwhen you’re instructed to read your result, which is 2-5 minutes after you urinate on the test strip . In other words, if you wait too long to read the pregnancy test, the line may disappear.
Dr Chan adds that evaporation lines can also appear due to other chemicals in urine: “When the urine dries, these chemicals can cause a line on the test strip,” she says.
The problem with evaporation test strips, Dr Chan explains, is that they can be mistaken for a positive test result, leading people to believe they are pregnant when in fact they are not.
How do we determine the evaporation path?
Evaporation lines are usually very thin and often colorless (or pale gray). No dye is detectable in them. Evaporation lines may be thinner than the control line and may not extend to the end of the track.
Meanwhile, “if your pregnancy test is positive, you’ll see a line that’s the exact same color as the control line,” says Dr. Seton. Depending on the particular test, it may be blue or pink.
But evaporation lines can be very difficult to distinguish from the rare, faint positive pregnancy test result: “If you test shortly after implantation and your hCG levels are still low, or your urine is very dilute, a faint positive pregnancy test line can still appear when you’re actually pregnant,” says Dr. Chan.
What is the evaporation pathway?
Evaporation lines typically have the following characteristics:
- Pale, colorless or light gray
- Sometimes thinner than the control line
- You may not be able to drive the entire length of the track.
- Follow the pregnancy test instructions and the “Results” window will appear after
How to avoid evaporation lines on pregnancy tests
It’s important to follow the instructions on your pregnancy test kit closely to avoid the disappointment (or fear) that can come from a false positive evaporation line. Remember, not all tests are the same, so don’t assume that just because you’ve taken one pregnancy test before, you know how to do them all.
“It’s important to check your pregnancy test results within the time frame recommended by the pregnancy test kit,” Dr. Chan explains. “If you take a pregnancy test too early, especially after the recommended time frame, an evaporation line may show up as a false positive.”
Dr. Setton adds that most home pregnancy tests require you to read your results within two to five minutes of urinating. “Follow the instructions on any home pregnancy test kit you purchase to avoid confusion with an evaporation line.”
Testing first thing in the morning when your urine is at its most dilute will give you the most accurate results. Also, avoid using expired test strips or strips that have been stored in hot or cold temperatures.
Still not sure? Wait a few days, test again, and check your results. Testing after your period is late will give you a more accurate result. “If you have any doubts, contact your doctor and get a blood test to confirm,” says Dr. Setton.
