All pregnancy diagnostic tests are based on the presence of HCG in urine, which can be detected 14 days after conception. The accuracy of these tests is of the order of 99%. The biological tests are tabulated below. Although very sensitive, they have been abandoned as they are quite cumbersome.
The immunological tests (e.g., the Gravindex test) are cheap, reliable, and accurate enough to replace biological methods. The kit consists of Gravindex antigen (latex particles coated with HCG), Gravindex antibody (serum containing HCG antibodies), dark slide, sticks and a pipette.
If a serum containing antibodies to HCG is allowed to react with HCG-coated red cells or latex particles, agglutination of the red cells or latex particles occurs. But if the serum is allowed to react with urine containing HCG prior to the reaction with red cells or latex particles, the agglutination of red cells or latex particles is inhibited, since the antibodies are used up by HCG of the urine. Hence inhibition of agglutination confirms pregnancy.