The principle for the qualitative point of care immunoassay for detection of ?-hCG
when specimen is added to the sample well, it migrates via capillary action along the cassette membrane. The membrane contains an antibody-dye conjugate specific to ?-hCG that is mobilized when sample is added. Specimens that are positive for ?-hCG react with this antibody-dye conjugate to form a complex. In the test zone of the membrane, antibodies against hCG are immobilized. These immobilized antibodies act to capture hCG-antibody/dye complexes present in positive specimens. In positive samples for hCG a colored line will form in the test zone.
The changes in ?-hCG throughout a normal pregnancy
begin with a surge in secretion of ?-hCG 6 days after conception. During the first 8 weeks of pregnancy, hCG levels double every 1.3-2 days to reach a peak during the 8th – 10th week of pregnancy. Levels decline to about 90% reduction from the peak value by the end of the second trimester. After delivery, the hCG levels decrease quickly to return to normal within days.
?-hCG levels can be useful in identifying abnormal pregnancies because
excretion of hCG is often decreased in ectopic pregnancies, toxemia of pregnancy, or threatened abortion. Ectopic pregnancies will demonstrate an abnormally slow rate of hCG increase as the pregnancy progresses.
Three other conditions associated with increased production of hCG