What is an Evaporation Line?
An evaporation line, also called an evap line, is a faint line that can appear on a home pregnancy test as the urine sample dries. When taking a home pregnancy test, a colored line will appear if the test detects the pregnancy hormone hCG (human chorionic gonadotropin) in the urine. An evaporation line occurs when urine creeps up the test strip as the sample dries, causing a faint, colorless line to appear where the positive line would be. This faint line is caused by the antibodies and dye in the test, not by a positive pregnancy result.
An evaporation line is very common and does not indicate pregnancy. It occurs when taking any type of home pregnancy test. The evaporation line will appear after the testing window time has passed, usually within 10 minutes or more after taking the test. It may look gray, colorless, and very faint. In contrast, a positive pregnancy test will feature a clearly colored line appearing within the manufacturer’s specified testing time.
How Often Do Evaporation Lines Occur?
Evaporation lines are extremely common on home pregnancy tests. Some sources estimate that as many as 50% of home pregnancy tests can develop evaporation lines after the testing period. This is because the urine sample continues to evaporate and creep up the absorbent strip, causing the antibodies to collect and create a faint line. The frequency of evaporation lines also depends on the quality of the pregnancy test. Tests that use inferior antibodies are more likely to develop evap lines. High quality pregnancy tests will help reduce the occurrences of evaporation lines.
Here is a table estimating the frequency of evaporation lines on home pregnancy tests:
| Source | Estimated Frequency of Evaporation Lines |
|---|---|
| Princeton University Health Services | Up to 50% of tests |
| Mayo Clinic | Common |
| American Pregnancy Association | Very common |
As the table demonstrates, evaporation lines are a very common occurrence, with some sources estimating they occur in up to half of all home pregnancy tests.
What Factors Increase the Chance of an Evap Line?
Certain factors can increase the likelihood of getting an evaporation line on a home pregnancy test:
- Reading the test after the manufacturer’s recommended testing window – Waiting too long to check the test allows more time for evaporation to cause a faint line.
- Using a low quality pregnancy test – Cheaper tests often have inferior antibodies that are more likely to bind non-specifically and create evap lines.
- Testing in a very humid environment – Humidity speeds up the evaporation process of the urine sample on the test.
- Having high levels of hCG from a recent pregnancy – Evap lines can be more likely for those with residual hCG leftover from a previous pregnancy.
- Diluting the urine sample too much – Very dilute urine evaporates faster, increasing the chances of an evap line.
- Allowing the test to dry laying flat – Flat tests allow for optimal evaporation and wicking of urine up the strip.
Being aware of these factors can help minimize the chances of getting an evaporation line versus an accurate result.
How Can You Tell an Evap Line From a Positive Result?
Telling the difference between an evaporation line and a positive pregnancy test line can be tricky. Here are some tips:
- Check within the testing window – A positive line will show up quickly within the time specified on the test instructions, usually 3-5 minutes. An evap line shows up after 10 minutes or longer.
- Compare line color – An evap line will typically be colorless or gray, while a positive line will be pink, blue, or other colored.
- Compare line thickness – Positive lines are usually solid and definite in color. Evap lines look faint, skinny and almost blurred.
- Note line appearance – A positive line will appear smooth and consistent across the test window. Evap lines tend to look irregular.
- Consider test quality – Premium brand tests yield fewer evap lines than cheaply made strips.
- Re-test if uncertain – If a faint line appears, test again 1-2 days later to see if the line darkens as hCG rises.
When uncertain, it’s best to re-test with first morning urine 1-2 days later. If the line was an evaporation line, it will not likely appear on the new test. A real positive line will become darker.
How to Prevent and Troubleshoot Evaporation Lines
Here are some tips to prevent evaporation lines when taking a home pregnancy test:
- Read results within the specified testing window – Don’t check more than 5 minutes after starting the test.
- Store tests properly – Avoid extreme temperatures and humidity during storage.
- Check expiration date – Use a fresh, non-expired test for optimal results.
- Limit fluid intake – Take the test using concentrated morning urine.
- Follow all test instructions – Carefully apply urine sample to the strip or cup.
- Lay the test flat – This prevents urine from running down the strip early.
- Use premium tests – First Response and Clearblue tests tend to get fewer evap lines.
If an evaporation line appears, the best thing to do is purchase a new test and re-test 1-2 days later, ideally using concentrated morning urine. Drink normally before the re-test and apply urine only as instructed. Be sure to check results within the specified testing window for the most accurate outcome.
Conclusion
In summary, evaporation lines are extremely common on home pregnancy tests, occurring in up to 50% of tests taken. They are caused by drying urine and antibodies binding nonspecifically along the test strip. Certain factors like test quality, environment, and urine concentration can increase their occurrence. To tell an evap line from a true positive, check the color, thickness, appearance and timing of the line. Preventing evap lines involves proper test storage, expiration checks, timing, and following instructions. When in doubt, re-test in 1-2 days for confirmation. Being aware of the prevalence and causes of evaporation lines can help ensure accurate home pregnancy test results.