How to Interpret the Results of an Evap Line on a Pregnancy Test
What Does an Evaporation Line Mean on a Pregnancy Test?
It can be confusing when a woman sees two lines on her pregnancy test—one dark and another very faint. If the line appears within five minutes of testing, then a faint line indicates a positive pregnancy. However, a faint line could also be an evaporation line, which, according to Dr. Peter Rizk, MD, "is caused by evaporation of the urine in the result window of a pregnancy test. It develops after the recommended reaction time frame (approx. five minutes) when the urine on the test area begins to dry, leaving a faint, colorless line. This may appear on any test regardless of brand." This line is not an indication of hCG hormone reacting with the reagent in the home pregnancy test strip and does not indicate a positive pregnancy. Unlike a faint positive line, which can be a light blue or light pink (depending on the brand of the test), an evaporation line is nearly colorless. Dr. Rizk says that "manufacturers recommend that you should not read tests after a designated time period, as most test brands have the possibility of developing evaporation lines."
What Causes an Evaporation Line?
An evaporation line appears when a woman waits too long and reads the result after the recommended reaction time period (usually five minutes after testing). This is why it's important to read the results right way. Some women keep the test strip after seeing a negative result and then check on it again after the recommend time and assume they're pregnant when they see a faint line. If the test turns out negative within 5 minutes of testing, then it is negative. Waiting a few more minutes will not make it a positive result!
Evaporation lines do not always appear for every test. This is because urine has very unique characteristics and composition among different individuals. Some women may see an evaporation line after five minutes while for others, this line may appear in an hour or so.
An evaporation line does not indicate pregnancy and is merely a result of evaporation. Do not make the mistake of considering this a faint positive line.
Faint Line vs. Evaporation Line
Faint Positive Line
Same thickness as the control line
Often thinner than the thickness of the control line
Time of appearance
Within the recommended reaction time. (This depends on the brand's recommendation, but it is typically 5 minutes.)
Anytime beyond the recommended reaction time (usually after 5 minutes or more).
Same color as the control line
Usually colorless, greyish, almost undetectable
So What Does a Faint Line Mean?
A faint line means a positive pregnancy if it appeared during the reaction time. If you see a faint line after 5 minutes of taking the test, then it is an evaporation line and IS NOT an indication of pregnancy.
The Difference Between an Evaporation Line and a Faint Positive
If you see a faint line within the reaction time period, it is not an evaporation line. No matter how faint it is, this line is considered a positive and is an indication that you took the test too early—most likely right after implantation when hCG levels are still low—or your urine was diluted. An evaporation line appears after the allotted reaction time and is virtually colorless.
What Color Are Evaporation Lines on Pregnancy Tests?
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. The evaporation line can also sometimes appear greyish.
Color of Evaporation Line on a Blue Dye Pregnancy Test
On blue dye tests, evaporation lines are very faint. This type of pregnancy test kit has two windows. The elongated window will show the control line, indicating that the test kit is working correctly. Right beside this is another round window where the test line will show up.
Negative Result: A single horizontal line appears for a negative pregnancy test. The horizontal line is always dark blue and solid.
Positive Result: If positive for pregnancy, there will be one horizontal line intersected with one vertical line, forming a plus sign. The evaporation line typically shows up as a very faint, thin and almost colorless vertical line. If you test early and have low levels of hCG, the vertical line may be faint. Again, if you read it after the recommended reaction time, the vertical line is most likely an evaporation line.
Types of Lines on a Pregnancy Test
A test line should typically appear at the center of the test window. If the line shows up anywhere but in the center (or not evenly spaced with the control line), it is called an indented line. Indent lines are completely colorless or may appear white in some cases. They can show up within the frame or outside.
An indent line is fairly common, but is sometimes misinterpreted as a positive pregnancy indicator. However, indent lines are simply the antibody strip of the test kit. It isn’t an evaporation line, a positive test result, or even a false positive one. If the indent line is grey or has color (however faint), then it is considered as an evap line.
Dye runs are smudged lines that show up in the area where a true test line should be. This is a false line. It may look like a promising indication of pregnancy, but it isn’t. It is also not an evap line. A true test line should be solid, not smudged. Dye runs are usually a result of the dye not being able to run smoothly across the test strip because the dye gets stuck and accumulates in one area of the strip. When this happens, the test results are unreliable. You should test again with a new test.
A disappearing line is a solid, colored line that disappears or fades after a few minutes or a couple of hours. This can be a huge downer for those who celebrate a positive result only to be disappointed when it disappears. This happens when the entire dye has not finished flowing over the test strip. The dye can briefly get stuck in one spot and then continue moving along later.
A disappearing line is not considered an evap line or a positive result. However, it is not a clear negative either. The pregnancy test is not reliable in this case, and it is recommended that you test again using a new test.
True Positive Test Result
A true, reliable positive result should appear as a solid line that is colored and does not go away. The color should be the same as the control line. The width should also be the same. It should be very visible and remain visible even after 48 hours. With most brands of home pregnancy tests, the positive test line never fades away.
A negative or a positive result on a home pregnancy test kit is not a definitive result. So many other factors may produce false positives or false negatives. If you think you may be pregnant, always get a checkup with a medical practitioner for confirmation.
Using and Interpreting a Home Pregnancy Test Kit
Most home pregnancy testing kits require a minimum of five minutes of reaction time. This is the time needed in order for the chemicals in the test kit to react with the hCG hormone in a woman's urine. hCG, or human chorionic gonadotropin, is one of the early hormones produced when a woman is pregnant. This is released by the chorion, one of the fetal membranes that ensure the survival of the embryo.
There Are Two Basic Types of Home Pregnancy Test Kits:
- The most common ones are dipsticks or test strips, which you either dip in a cup of urine or hold under a stream of urine. The urine will saturate the test strip. If there is hCG present, the hormone will react with antibodies in the strip. Dye will accumulate in this area, and a second line will appear, indicating a positive pregnancy. If no hCG is present, then there will be nothing on the antibody strip to react to the dye.
- The second type of home pregnancy test uses a test device and a urine collection cup. A urine sample is collected in the cup, and the test device is placed in it. Another version of this test kit has a small well on one end where you place a few drop of urine in using a dropper. If pregnancy is detected, the test device changes color.
How Accurate Are Home Pregnancy Tests?
Home pregnancy tests nowadays are very reliable, provided you use a concentrated urine sample and do the process correctly.
The best sample is the first urine in the morning. This will give the most accurate result. The day of urine collection also matters. Levels of hCG (if present) depend on how far along the pregnancy is. In the early stages (a few days before a missed period), you will have very low hCG levels because the main tissue that produces this hormone has yet to fully develop. As the pregnancy develops, hCG levels steadily increase.
Reading and Interpreting the Results
The results should be read between three to five minutes. Beyond that, urine will start to dry off, and an evaporation line will start to appear.
During the reaction time, bands will start to appear in the test window. These bands will show either a positive or a negative pregnancy. A single line means a negative pregnancy result, and two lines means the woman is pregnant. The first line that appears is the control band. The appearance of this line indicates that the test kit is working. The second line will only appear if the test kit has detected the presence of hCG in the urine sample.
Home Pregnancy Test Best Practices
It is important that a home test be performed properly to get the most accurate results. The instructions will differ slightly among various brands and types, but generally, you should follow these guidelines:
- Use only test kits that have not yet expired. The chemicals in the kit can expire and will not be reliable.
- Test kits must be stored properly prior to use. Exposure to direct sunlight and/or other degrading factors can cause the chemicals inside the test strip to be unstable and unreliable.
- Carefully read the instructions that come with the test kit. Manufacturers may have slightly different methods of doing the test. These differences may include the reaction time. Most manufacturers recommend reading results that appear within 5 minutes of testing. Others may require longer. It is important to know this because results that appear after the reaction time are invalid.
- Check to see what special preparations are required before a urine sample is obtained. For example, some test kits may be sensitive to certain food components or to changes in the body related to physical activity. Follow directions carefully to get the most accurate results.
- Use a clock if the test kit requires specific timing. Guessing the time will only yield inaccurate results.
Testing Time Frame
Many women wonder how soon they can take a home pregnancy test. As a general rule, you should wait at least a week after your missed period to test. If you want to take it sooner, you can use a pregnancy test with low sensitivity (less than 20 mIU/ml), however, these tests may not be accurate, so it is always better to wait.
Causes of a Faint Test Line Other Than Evaporation
Taking the Test Too Early
Some women use a home pregnancy test even before a missed period. The best time to test is at least seven to 10 days after ovulation, or at least three days after your missed period. Testing earlier than that can result in a false negative or a faint positive reading. If you get a faint positive line, repeat the test after a few days. Use the first urine in the morning. This urine sample is concentrated and will likely have a higher level of hCG, which will give you a more accurate result.
Home Pregnancy Test Sensitivity Is Low
Test sensitivity varies from brand to brand. Getting a test kit with higher sensitivity will give a more accurate result. Most tests are 20–25 miU/ml. The lower the number, the more sensitive the test will be. Typically, higher sensitivities are recommended for testing during the early stages of pregnancy. These test kits use a different formulation in the antibody strip so that no matter how low the concentration of hCG is, the antibodies will still pick it up. If you use a lower sensitivity test kit that is designed for testing during the later stages of pregnancy, the test will not be able to detect the low amounts of hCG.
Urine Sample Is Diluted
It might be that the urine sample used is diluted. If diluted, the concentrations of hCG in the urine are too low, and you will get a weak reaction on the test strip.
Urine dilution happens when there is frequent urination and/or you consume a large amount of liquids before taking a urine sample.
To avoid using a diluted urine sample, use the first urine in the morning. Generally, you can test at any time of the day, but the first urine in the morning is really concentrated. This is because urine has stayed in the urinary bladder for an entire night’s sleep. For a pregnant woman, this urine will contain the highest concentration of hCG. Hence, the first urine in the morning is the best sample to use to get more accurate results.
This is an early miscarriage that will still produce hCG hormones, resulting in a faint positive pregnancy test. During very early miscarriage, the fertilized egg gets implanted and produces littles amounts of hCG. It then detaches (miscarries) before the fetal membranes (i.e., chorion, etc.) forms. If the fetal membranes fail to form, hCG levels will remain low.
If a woman sees a faint line, there is a small possibility that hCG levels remained low due to an early miscarriage. Typically, women who experience a chemical pregnancy will bleed heavily about a week after their period was due. If you suspect a chemical pregnancy, please see a doctor immediately.
What Should I Do If I See an Evap Line?
An evap line on a pregnancy test means nothing, so it has to be discarded. It does not indicate pregnancy and is merely a result of evaporation. If you think the line is a faint positive, the best next step is to repeat the pregnancy test. hCG levels in the body typically double every 48 hours. As the levels of hCG rise, the positive test line should grow darker.
You can also consult a qualified medical expert for confirmatory tests ( i.e., an ultrasound). Seeking the aid of a medical expert for further tests is the better option. There are instances when false positive results happen with home pregnancy test kits. hCG levels are affected by several factors and are not limited to the presence of a fetus in the womb.