There are a few different ways which may give a woman the status of niddah. The most common reason a woman becomes niddah is when she has her regular period. A woman who has a period is a niddah deoraita, whether she had a hargasha or not. A woman who finds a stain on her body or clothing, the blood is a ketem. There are many leniencies regarding ketamim, and the halachot of ketamim will be explained in the upcoming chapters.
When a woman is spotting, there may be uncertainty whether the stain is a ketem or the beginning of a period. If it were to be a period she would immediately be a niddah. However, if the spotting was merely a ketem, she would only become a niddah if the stain she found met the requirements of ketamim. The proper method to determine if the spotting is the start of a period or not is to put in a pad and measure the area of blood found on the pad after three or four hours. If the area of blood exceeds three fingers (about half of a dollar bill), a rabbi should be consulted. Less than that size may be treated as a ketem. One may change the pad twice throughout the day. However, if two of the pads have two fingers worth of blood in one day, a rabbi should be consulted to determine if the spotting is the start of a period or not.