An evaporation line on a pregnancy test can sometimes make the result of the test look like a positive. This can be avoided by following the test manufacturer's instructions when testing for a pregnancy.
Pregnancy tests that require urine may develop an evaporation line as the urine dries, leaving a shadowlike line in the result area. This can sometimes resemble a positive test result.
Evaporation lines appear after the recommended time to read the result has passed. Most pregnancy tests recommend reading the result within 30 minutes of taking the test to avoid an evaporation line false positive.
Evaporation lines resemble positive pregnancy test results. However, the second line will not be as visible as the first line. It may look more like a shadow than an actual line.
A faint line in the result area could also be the result of an early pregnancy. In this case, there may not be enough HCG (the pregnancy hormone that urine tests react to) to activate the second line fully. Testing again a few days later may provide a more definite result.
Avoiding Evaporation Lines
To avoid evaporation lines, check results within the recommended time frame. Alternatively, use a digital home pregnancy test, which do not use lines to show the result of the test.