A pregnancy test may show a small streak where the positive line should be. This is called an evaporation line. It only appears on tests that show negative results. A positive test would change color in the same spot, blotting out the evaporation line.

A person's hands holding pregnancy test.Share on Pinterest
laflor/Getty Images

Taking a pregnancy test can be a nerve-racking experience, especially when it is difficult to understand what the lines on the test mean.

An evaporation line does not indicate pregnancy. If this type of streak appears, it usually means that a person has read the results too late or has otherwise misused the test.

For most people, getting an accurate result merely involves taking another test. Some people take pregnancy tests too early to receive accurate positive results. It may be a good idea to wait for a few days before taking another test.

An evaporation line is a slight streak that appears where the positive line on a pregnancy test should be.

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. These lines often occur when a person has checked the result too late or taken the test incorrectly.

When an evaporation line appears, it is best to take another test for an accurate result.

If a person sees an evaporation line, it means that the test is negative, or they took it too early in the pregnancy to show a positive result.

Pregnancy typically begins when a fertilized egg implants in the lining of the uterus. The only exception is an ectopic pregnancy when the embryo attaches outside of the womb’s inner lining.

After typical implantation, the body starts to produce a hormone called human chorionic gonadotropin (hCG).

The body’s hCG levels rapidly increase during the first trimester. rising by at least 49% every 24 hours in the first trimester. At the end of the first trimester, hCG levels fall.

Home pregnancy tests work by detecting hCG levels during the first trimester of pregnancy. Tests that can detect smaller concentrations of hCG in the urine are more likely to show positive results earlier.

The levels of hCG are very low in the days immediately before and after the first missed period. Also, these levels can vary from person to person in early pregnancy, as well as from pregnancy to pregnancy in the same person. Some people naturally have lower levels than others.

The compounds in home pregnancy tests interact with hCG, and a positive result will often look like a line, a dot, or a plus sign. Most tests also have lines that indicate that the tests are working.

Usually, only urine that contains a sufficient level of hCG can cause the test’s dye to stain in a way that shows a positive result.

There are, however, a number of rare conditions that can cause a test to come out positive in the absence of pregnancy. Examples include familial hCG syndrome, molar pregnancy, or conditions that affect the pituitary gland.

Learn more about the causes of a false pregnancy test.

How much HCG does a person need for a positive pregnancy test?

Some pregnancy tests promise to detect pregnancy 5 or more days before a missed period. While they often can, the effectiveness of the test depends on the level of hCG in the urine.

According to research published in 2014, the average hCG concentration in urine at 9 days after ovulation — around 5 days before a missed period — is 0.93 milli-international units per milliliter (mIU/ml).

Most early-result pregnancy tests can only detect hCG when it rises to levels of 25 mIU/ml or higher, which happens after ovulation around day 11

By day 14, hCG levels are typically around 137 mIU/ml. For some people, however, they can be as low as 45 mIU/ml.

Because hCG increases so rapidly in the early days of pregnancy, getting the day of ovulation wrong by 1–2 days can affect the result of a pregnancy test.

A person who believes that they are on day 11 can often expect an accurate result, but if they are actually on day 9 after ovulation, the test may not be able to detect pregnancy accurately.

False negatives are more common than false positives. If the result is negative, it may be a good idea to take another test in a few days, to give the hCG levels a chance to rise.

Very few medications, including fertility drugs, may cause false positives. Otherwise, a positive result followed by a negative result a few days later could mean a very early pregnancy loss.

In early pregnancy, there may be very little hCG in the urine, and the positive line may be faint. But even a faint positive line has color, and this distinguishes it from an evaporation line, which is colorless. Any coloration in a positive line indicates pregnancy.

A mark on a pregnancy test may be an evaporation line if:

  • More than 10 minutes have passed since taking the test.
  • The mark is faint and colorless, and it resembles a water spot.
  • The mark has no visible dye in it.

If the control line on the test does not change color, this means that the test has failed.

A line on a pregnancy test a positive result if:

  • There is a visible dye in the line, even if the color is faint.
  • The line appears within the period specified on the instructions, which is usually 3–5 minutes.
  • A person has taken an early-result test at least 11 days after ovulation.
  • A person has taken a regular test at least 14 days after ovulation.
  • A person has missed her period.

Evaporation lines are also likely to appear if a person has not followed the instructions on the test’s packaging.

To prevent evaporation lines:

  • Try urinating in a cup before taking the test, then dipping the test in the urine for the duration specified on the box. This technique prevents too much urine from splashing onto the test.
  • Do not use a pregnancy test that has passed its expiration date.
  • Avoid storing pregnancy tests in very hot or cold locations.
  • Check the results within the time frame specified by the test instructions. Evaporation lines and false positives can appear over time.

Urine hCG concentrations are often highest when a person wakes up. Taking a test first thing in the morning can increase the chances of an early positive result.

Waiting for the results of a pregnancy test can be agonizing. If the result shows a colored line, even a faint one, then the test is likely positive. If the results show a faint, colorless line, then this may be an evaporation line.

A simple way to help ensure accuracy is to take two tests and carefully follow the test instructions.

Anyone who is unsure of the results should give hCG levels time to rise and take another test in a few days. This can reduce the risk of false negatives.

A doctor can provide the most accurate results by testing the blood or the urine.