What is the most accurate pregnancy test to use?

Serum beta–human chorionic gonadotropin (hCG) is the hormone produced by the syncytiotrophoblast beginning on the day of implantation, and it rises in both the maternal blood stream and the maternal urine fairly quickly. The serum hCG test is the most sensitive and specific, and the hormone can be detected in both blood and urine by about 8-9 days after conception. This test can be performed quantitatively or qualitatively. Urine pregnancy tests differ in their sensitivity and specificity, which are based on the hCG units set as the cutoff for a positive test result, usually 2-5 mIU/mL.

Urine pregnancy tests can produce positive results at the level of 20 mIU/mL, which is 2-3 days before most women expect the next menstrual period. The kits are very accurate and widely available. The test can be completed in about 3-5 minutes. The kits all use the same technique—recognition by an antibody of the beta subunit of hCG. Falsely high readings of the hCG hormone can occur in cases of hydatidiform molar pregnancy or other placental abnormalities. Also, test results can remain positive for pregnancy weeks after a pregnancy termination, miscarriage, or birth. On the other hand, false-negative test results can occur from incorrect test preparation, urine that is too dilute, or interference by several medications.

Serum pregnancy tests can be performed by a variety of methods. The enzyme-linked immunosorbent assay (ELISA) is the most popular in many clinical laboratories. This test is a determination of total beta-hCG levels. It is performed using a monoclonal antibody to bind to the hCG; a second antibody is added that also interacts with hCG and emits color when doing so. This form of ELISA is commonly called a "sandwich" of the sample hCG. Radioimmunoassay (RIA) is still used by some laboratories. This test adds radiolabeled anti-hCG antibody to nonlabeled hCG of the blood sample. The count is then essentially determined by the amount of displacement of the radiolabeled sample.

The hCG level doubles approximately every 2 days in early pregnancy. However, it should be noted that even increases of only 33% can be consistent with healthy pregnancies. These values increase until about 60-70 days and then decrease to very low levels by about 100-130 days and never decrease any further until the pregnancy is over.