just read this on POAS-
What's an evaporation line? Evaporation
("evap") lines result with the test's antibody strip just looks slightly
different than the space around it. There is a line of antibodies
(usually made from mouse cells) in the Control and Test section. The
Control line binds with any liquid and turns pink (or blue, in tests
using blue dye.) The Test/Result line turns pink only if pregnancy
hormone is detected. If not, the moisture passes over this strip and
does not turn pink. It may, however, become more visible when the light
hits the moisture on the strip-- it may appear gray, colorless, like a
"dent" in the test, or like a "ghost line." It may appear at any time--
as soon as the urine hits it, after a few minutes as the test absorbs
the moisture, or after the 10-minute time limit. It may appear when the
test is drying, or after it has dried. It may disappear as the test is
drying, or after the test has dried, or not disappear at all.
The simple fact is that there is always "something there" that is slightly visible-- it's simply the antibodies on the test that would
turn pink in the presence of hCG. When the test becomes wet, or as it
dries, or after it dries, the antibody strip may become more visible.
Therefore, all tests may have them. It is not a defect; it's just how tests are made.
A real positive is identified by its color (pink or blue, whatever the color of the test's dye is) and its
appearance within 10 minutes of urinating on the stick. A line that
appears after 10 minutes, regardless of color, must be considered an
evap line and is caused by the test's chemicals changing. HPT's are rapid assay diagnostics, which means any results appearing after the "rapid" time limit of 10 minutes are invalid.