My Missus looked at what she thought was a neg test and went off to get a bottle of wine, I took a look at it and could see a very faint line. I showed her the test, she didn't drink the wine and a week later tested again and it was positive. It's all based on hormones and the level of them in the urine, it can take some individuals a bit longer to produce enough hormone (protein, based on a protein based enzyme linked immunosorbent assay) to produce a result that yields a positive result