The general consensus among poskim is that disposable pantiliners are not susceptible to ritual impurity. Therefore, stains of any size found on a disposable pantiliner do not render a woman niddah, even during the seven clean days. However, there is a significant minority of poskim who rule that stains on pantiliners that are larger than a gris (the size of an Israeli shekel or American dime) will render a woman niddah based on various halachic arguments.
The position of this site is that disposable pantiliners should preferably not be worn during the seven clean days due to their disputed status. If a woman does wear a white disposable pantiliner during the seven clean days, and finds a stain larger than a gris on the pantiliner, it does not invalidate her clean days.
Ideally, a woman who experiences staining during her clean days should change her white underwear more frequently to prevent any stain from accumulating to the size of a gris. Where this is impractical or insufficient, she should ask an individual question about wearing white disposable pantiliners or colored underwear to avoid invalidating the clean days from the stains.
This response was updated on 20 January 2022.