Evaporation lines are caused by evaporation of the urine in the result window of a pregnancy test. These are lines that shows in the result window of the test. An evaporation line develops when the urine on the test area begins to dry, leaving a faint, colorless line. The complete reaction time of our pregnancy tests is 5 minutes. You should not read tests after 5 minutes, as the tests have the possibility of developing evaporation lines.
Explanations for faint test lines:
Testing Too Early in Your Cycle: If you test too early, earlier than seven to ten days past ovulation, the HCG may not be at a sufficient level to produce more than a very faint line. If you do see a faint positive test result, then we always recommend a follow up test in a day or two using that first morning urine.
Diluted Urine Sample: Urine samples may experience “dilution” due to frequent urination or consumption of liquids. This is why first morning urine is recommended for pregnancy testing as it contains the most concentrated amount of the hormone, HCG.
Chemical pregnancy: A chemical pregnancy is indicated when implantation of a fertilized egg takes place and HCG is produced. A chemical pregnancy is, by definition, very shortly followed by miscarriage before any other pregnancy signs can be detected. Hence, a faint positive line on a pregnancy test may be detected, but there is a a miscarriage very quickly following implantation.
Related blog articles:
Pregmate Pregnancy Tests. How and when to do the test? 
Conception explained. Fertilization and Implantation.
What factors can give false positive results?
What if the testing line is very light in color, am I pregnant?
What are pregnancy test evaporation lines?
Fertility drugs and pregnancy testing
When to collect urine for the pregnancy test?
Reading pregnancy test results