According to Torah law, a woman only becomes niddah if she sees blood through a normal uterine discharge, in the way of a regular period. However, the Sages decreed that if a woman finds blood on her skin or clothing the woman is niddah. It is important to note that blood found on a bedikah does not have the status of a ketem, it is considered to be a regular niddah. If the stain on clothing or skin is roughly the size of half a dollar bill, or larger, one should consult a rabbi. This is because such a large stain is usually indicative of the beginning of a period, and may not have the status of a ketem.
If a woman finds a ketem that did not make her niddah, it is proper to abstain from marital relations for that night. Other forms of physical intimacy are permitted. This will prevent accidentally having relations while niddah.
There are a few factors that affect whether a woman will become niddah through a ketem. The size of the ketem, the color of the clothing it was found on, and the type of material it was found on. Further, at times a ketem may be attributed to a source other than uterine blood, and the ketem will not make the woman niddah.