What other pregnancy tests are available?

The other type of immunoassay, the radio-immunoassay, can also be used to test for pregnane}’. Although based on the same principle as the agglutination immunoassay, it is more expensive and more complicated. A radioimmunoassay takes about four hours to perform and, unlike the agglutination immunoassay, which can be performed in an outpatients’ clinic, it has to be done in a laboratory’ with specialized equipment It is, however, an extremely sensitive test capable of detecting the veiy low levels of HCG produced in early pregnancy. Two factors make this test extremely precise. One involves the use of antibodies found only in a fractional part of HCG. A molecule of HCG can be split into two parts- the alpha unit, whose chemical structure is identical to some other hormones, particularly the luteinising hormone (LH] produced prior to, and just after, ovulation; ana the beta unit, which is structurally unique. When antibodies to the whole HCG molecule are used, an early radio-immunoassay pregnancy test may also detect LH and so provide inaccurate results. If antibodies specific to the beta sub-unit are used, the test immediately becomes more precise since it is only measuring for the presence of HCG.

The other factor that makes a radioimmuno-assay an extremely sensitive test is the use of HCG which has been ‘tagged’ with radioactive iodine, in place of HCG coated latex particles or red blood cells.

The test itself consists of incubating a sample of serum or urine with a pre-determined amount of antibody to the beta sub-unit The radioactive HCG is then added, and a gamma ray counter is used to measure how much radioactive HCG has been bound to the antibodies. If the woman snot pregnant all the unneutralized antibodies will have adhered to the radioactive HCG and the reading on the gamma ray counter will be high. If she is pregnant however, much less radioactive HCG will bind itself to the antibodies, and the reading on the gamma ray counter will be low. Radioimmunoassays are particularly valuable in detecting an ectopic pregnancy. If a doctor suspects that this may be the case he will arrange for a radioimmunoassay to be carried out, in order to test for the extremely low levels of HCG produced by this type of pregnancy.

A test called a radioreceptor assay may also be used to detect pregnancy. It involves the use of what are known as receptor proteins to attract the HCG instead of antibodies. The test only takes about an hour to perform, but because the method also measures LH it can only be used after the time of the missed menstrual period when this hormone will no longer be present.

The most recent development in the detection of pregnancy has been the use of ultrasonography. With an ultrasound scan – which has the great merit of taking less than five minutes to perform – it is now possible to see the developing embryo in the uterus as early as six weeks after LMP. rk- – 4& ~