PREGNANCY TESTS - A REVIEW

Pregnancy tests are widely used both by the public and by healthcare professionals. All tests depend on the measurement of human chorionic gonadotrophin (HCG) in urine. Other pregnancy-specific materials have been proposed as pregnancy tests but none can better the sensitivity and convenience offered by immunoassay of HCG. Ultrasound detection is also not as sensitive as HCG measurement. The current generation of tests is based on monoclonal antibodies to the beta-subunit of HCG; these virtually eliminate the possibility of cross-reaction with pituitary luteinizing hormone (LH) and it is this feature which permits the high sensitivity. However, it is important to recognize that the 'beta-subunit' antibody reacts with both intact HCG, which is the major component in pregnancy serum, and with fragments of the beta-subunit (beta-core), which are the major form in urine. Both the blood and urine of non-pregnant subjects contain small amounts of HCG. HCG from the implanting blastocyst first appears in maternal blood around 6-8 days following fertilization; the levels rise rapidly to reach a peak at 7-10 weeks. With most current pregnancy test kits (sensitivity 25 units per litre) urine may reveal positive results 3-4 days after implantation; by 7 days (the time of the expected period) 98% will be positive. A negative result 1 week after the missed period virtually guarantees that the woman is not pregnant. With the present generation of test kits, false positive results due to interfering materials are extremely unlikely. Pregnancy tests have now reached a level of sensitivity and specificity which is unlikely to be surpassed either by better tests or alternative technology.