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False positive tests may occur when certain hormones are increased, such as after menopause or when taking hormone supplements. A false positive result may also result from "phantom HCG" (when there is an abnormal interaction between the test and the patient's blood sample). This can be confirmed by a urine HCG test, which will be negative. Phantom HCG is suspected when the blood HCG test continues to be positive despite there being no pregnancy or genital tract tumor.
A pregnancy test is considered to be about 98% accurate. When the test is negative but pregnancy is still suspected, the test should be repeated in 1 week.
Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.
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