It may be one of the scariest tests a young woman can ever take: a pregnancy test.
According to the claims on the back of the most popular brands of tests, the devices deliver a result that is 99 percent accurate.
But do they?
A worrying clip on TikTok suggests that what you think is positive might not be, especially with one brand.
In a video which has racked up 2.5 million views to date, an American woman named Andie describes her experience with false positive results with ClearBlue brand devices.
A false positive result means that a woman is told by a blue cross on the screen that she is pregnant, while this is not the case. A false negative would mean that one line appears to indicate no pregnancy, when in fact the user is pregnant.
In the video, Andie explains that she asked her husband to take a pregnancy test after realizing her period was shorter than normal.
He bought a test for hair that uses blue dye to display a result, made by the well-known brand ClearBlue.
The mother of two explains that she ‘never’ buys blue dye tests because ‘they are known for false positives’.
The ClearBlue test told her she was pregnant and showed a small cross instead of a single line.
TikToker Andie shows off the negative result from her favorite pregnancy test, which she says has “never grossed me out.”
However, the next day she bought another brand, First Response, which uses pink food coloring, and told her that she was indeed not pregnant.
“I always buy this,” she says, wagging the pink test at the camera. “I tell you, they’ve never done me dirty. I’ll never take a test with blue dye again in my life because I almost had a heart attack last night.’
More than 2,000 users commented on Andie’s post and shared similar experiences with blue dye tests.
Another said: ‘Same thing happened to me on the first comment! I got a crazy evap line a month after getting an IUD and called my OB. *sobs* blood test came back negative.”
Meanwhile, TikTok user Chloe said: ‘Clear blue needs to be taken off the shelves. I got so many false positives.”
According to the series of comments on social media, it appears that the confusion lies in what is known as the ‘evaporation line’: a faint, colorless line that can appear in the same position as a colored line indicating a positive result.
Evaporation lines are simply a spot of urine that dries and can appear if the test user waits longer than the suggested time to read the result.
The color of the evaporation line may appear slightly bluish, making it easy to mistake for a positive ‘cross’.
It also means that women may mistake a positive result for an evaporation line and assume they are not pregnant when they are.
Market research firms estimate that more than 20 million pregnancy tests are sold annually in the United States, compared to about three million live births per year.
“I did a test with blue dye and thought, ohhh yeah, that’s an Evap line,” said a woman named Kal, “…guess what wasn’t an Evap line.”
Pregnancy tests work by responding to the amount of a hormone called human chorionic gonadotropin, or HCG, in the urine.
HCG begins to be produced about six days after fertilization. The chemicals in the paper in the stick react with the hormone, causing a reaction that turns the dye a certain color.
The accuracy of the results depends on when you take the test; A pregnancy can be diagnosed from the first day of your missed period, or 21 days after you last had unprotected sex.
The results may also be more accurate if you take the test first thing in the morning, when your urine is more concentrated.