Our expert says:
Not a blood test, but a needle biopsy of the lump will be conclusive. If the lump is "loose" and slips between the skin if you take it between your fingers, and becomes bigger around menstruation and smaller at other times it is most likely a fibro adenoma, which is not cancerous and will not become cancerous. Also do self breast examination after your menstruation:http://www.breastcancer.org/symptoms/testing/types/self_exam/bse_steps.jsp
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