Sometimes HCG can't be detected by a pregnancy test however that is usually around .1% of the time. If unsure then you could get a blood test which show give you a straight answer.
The hormone human chorionic gonadotropin (better known as hCG) is produced during pregnancy. It is made by cells that form the placenta, which nourishes the egg after it has been fertilized and becomes attached to the uterine wall. Levels can first be detected by a blood test about 11 days after conception and about 12 - 14 days after conception by a urine test. In general the hCG levels will double every 72 hours. The level will reach its peak in the first 8 - 11 weeks of pregnancy and then will decline and level off for the remainder of the pregnancy.
Can anything interfere with my hCG levels?
Nothing should interfere with an hCG level except medications that contain hCG. These medications are often used in fertility treatments, and your health care provider should advise you on how they may affect a test. All other medications such as antibiotics, pain relievers, contraception or other hormone medications should not have any effect on a test that measures hCG.