A faint positive reading on an EPT ("Early Pregnancy Test") is still a positive reading. However, many women are confused when a "very faint band" appears. With a little understanding and objectivity, you can differentiate a faint positive reading from an "evaporation line."
Store-bought (OTC) pregnancy tests come in different shapes and sizes, and use different color schemes. But most essentially work the same way. The pregnancy test will have a colored band already on it. This colored band is called a "control" band (Fig 1). Most clinical tests of any kind utilize a control. The control is a comparison, used to see how whatever is being tested measures up against something. In a pregnancy test, the control band allows the user to see what a "positive" result will look like. In other words, if the woman using the pregnancy test is, in fact, pregnant, her result should match the control band.
An OTC pregnancy test (often called an "EPT" for "Early Pregnancy Test") begins to work when urine is applied. Most tests recommend using "first morning urine." This is because the hormone being tested (called HCG) is most concentrated in the urine when the urine has remained in the body for some time. In short, the darker the urine, the more concentrated it is with the target hormone. The lighter the urine--caused by frequent urination or drinking large amounts of clear liquids--the less concentrated the hormone will be in the urine, and the less reliable the test results will be.
Read the packaging on pregnancy tests, since the sensitivity among different tests varies greatly. Tests with a lower number test sensitivity (this will be a number followed by the letters "MIU") will be able to detect lower concentrations of HCG, and thus detect pregnancy earlier than tests with a higher number test sensitivity. While specialized hospital tests can detect pregnancy in some women as early as 7 days after ovulation, OTC pregnancy tests begin to be reliable at 10 or 11 days after ovulation. The longer after ovulation the test is used, the more reliable it becomes.
Note that HCG levels, concentration and differences between women themselves are all variables. That is, while the test does not change in how it works, the factors being applied to the test do change; and so reliability may fluctuate, particularly in the first few weeks after ovulation.
Negative EPT Reading
If a test is "negative," the strip should look as it did when it came out of the package (Fig. 1). That is, no second band of color appears. A "negative" result in the 10--12 day range does not necessarily mean that the woman is not pregnant, only that the levels of HCG in the urine were not at levels which the test could read. When testing early and getting a negative reading, it is best to repeat testing again every few days until sufficiently past the number of days on the test's packaging instructions.
Positive EPT Reading
If the test is "positive," a second color band will appear. This color band may appear as dark as the control band. However, depending on the amount of HCG in the urine and the test sensitivity for the particular test, positive readings may also appear fainter in color than the control band (Fig. 2). These still represent "positive" readings, meaning that pregnancy is detected.
Evaporation Lines: "False Positives"
EPTs have a "reaction time"--a number of minutes during which the test results are considered reliable. If a band appears before the stated reaction time (typically 5 to 10 minutes), it is still likely a positive read. However, any results appearing after the upper limit of the stated reaction time are considered questionable or unreliable.
Sometimes people are confused when a second line does appear after the reaction time has elapsed. They see a line where it "should be," and think it is just a very faint positive reading (Fig. 3). Often, however, this line is what is called an "evaporation line." An evaporation line may appear on the EPT when urine has had time to evaporate from the strip. The line is generally not a lighter shade of the control band color. Often, it has no color to it at all, yet is still visible, as when a white paper towel is wet. An evaporation line is usually thinner than the width of the control band, as well.
Many times, when women are hopeful that they are pregnant--or fearing so--they lose objectivity and see the evaporation line as something it is not: a faint positive reading. EPTs are designed for people who "want to know now." Although the waiting is difficult, it is always best to wait a couple more days and then recheck any EPT result that is not a clear positive on the first attempt.
Final Word
A woman can get a clear positive reading on an EPT, but then get a report that she is not pregnant upon a first visit to the doctor. This does not mean that the test failed or malfunctioned. An EPT may have correctly read a positive pregnancy result, followed by an undetected miscarriage and loss of the pregnancy before the first doctor visit.
