Most accurate pregnancy test
What is the most accurate pregnancy test to use?
@indeevar All pregnancy tests work by detecting human chorionic gonadotropin (hCG). This hormone is 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. It can be detected in both the blood and urine by about 8-9 days after conception.
There are a few types of pregnancy tests that include professional quantitative serum hCG tests, point-of-care qualitative serum hCG tests, and urine tests for hCG. The serum hCG test is the most sensitive and specific, with laboratory published sensitivities of 1, 2 or 5 mIU/mL. 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 testing kits can produce positive results at the level of 20 mIU/mL, which is 2-3 days before most women expect their next menstrual period. The kits are very accurate, widely available, and 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.
However, falsely high readings of the hCG hormone can occur in cases of hydatidiform molar pregnancies or other placental abnormalities. Also, test results can remain positive for pregnancy weeks after a pregnancy termination, miscarriage, or birth. In addition, false-negative test results can also occur from incorrect test preparation, urine that is too dilute, or interference by several medications.
Further complicating the science of pregnancy detection is that pregnancies which fail to properly implant can lead to brief increases in hCG levels, creating a false-positive result. Many over-the-counter (OTC) tests make claims such as “99% accurate on the first day of your missed periods.” It is important to advise women that these early results should not be considered definitive; when using home pregnancy tests, it is best to wait 1 week after a missed period for a more accurate result. 
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.