Why you should NEVER use a blue-dye pregnancy test, according to TikToker who says they're notorious for misleading results

It is perhaps one of the most anxiety-inducing 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 provide a result that is 99 percent accurate. 

But do they?

A concerning clip posted to TikTok suggests that what you think you see as a positive may in fact not be - especially with one brand. 

In a video that has so far garnered 2.5million views, a US woman named Andie details her experience of false positive results with devices made by the brand ClearBlue. 

A false positive result means a woman is told she is pregnant via a blue cross on the screen, when she is not. A false negative would involve one line appearing to indicate no pregnancy, when in fact the user is pregnant.

In the video, Andie explains that she asked her husband to pick up a pregnancy test after realizing her period had been shorter than usual.

He bought her a test 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 having false positives'. 

The ClearBlue test told her she was pregnant - displaying a small bue cross, as opposed to a single line.

TikToker Andie shows off the negative result of her favorite brand of pregnancy test - which she says has 'never done me dirty'.

However, the following day she purchased a different brand, First Response, which uses pink dye, and told her she was not in fact pregnant. 

'I always buy these,' she says wagging the pink test at the camera. 'I'm telling you, they've never done me dirty. I'm never taking a blue dye test again in my life because I almost had a f**king heart attack last night.'

More than 2,000 users have commented on Andie's post, sharing similar experiences with blue dye tests. 

Another said: 'Same thing happened to me with first response! Got a crazy evap line a month after getting an IUD and called my OBGYN *sobbing* blood test was negative.' 

Meanwhile, TikTok user Chloe said: 'Clear blue should be pulled of the shelves. I got so many false positives.'

According to the series of social media comments, it appears that the confusion lies in what's known as the 'evaporation' line - a faint, colorless streak that can appear in the same position as a colored line that indicates a positive result.

Evaporation lines are simply a patch of urine drying, and can appear if the user of the tests waits longer than the suggested time to read the result.

The color of the evaporation line can appear slightly blue-toned, making it easily mistaken for a positive 'cross'. 

It also means women may mistake an positive result for an evaporation line, and assume they are not pregnant when they are. 

Market research firms estimate that over 20 million pregnancy tests are sold annually in the United States - compared to about three million live births every year.

'I took a blue dye test and was like ohhh yeah that’s an evap line,' said a woman named Kal,'….. guess what wasn’t an evap line.'

Pregnancy tests work by reacting to the amount of a hormone called human chorionic gonadotrophin, or HCG, in the urine.

HCG begins to be produced roughly six days after fertilisation. Chemicals within the paper in the stick react to the hormone, producing a reaction that turns the dye a certain color.

The accuracy of the results will depend on when you take the test; pregnancy can be detected from the first day of your missed period, or 21 days after you last had unprotected sex. 

Results may also be more accurate if you do the test first thing in the morning when your urine is more concentrated.