6.22 - Pregnancy Testing

Monoclonal antibodies in pregnancy testing

• A monoclonal antibody can be made to bind to one specific chemical, so it can be used to detect that chemical in a sample.
• The everyday example is the pregnancy test. It detects HCG, a hormone found in a woman's urine only when she is pregnant.

How the test stick works

• The end that's urinated on carries antibodies to HCG attached to coloured (blue) beads - these antibodies and beads are free to move.
• Further along, a test line has more antibodies to HCG stuck firmly in place so they can't move.

If the woman is pregnant:

1. HCG in the urine binds to the antibodies on the beads.
2. The urine carries the beads and hormone along the stick to the test strip.
3. The hormone binds to the fixed antibodies, holding the beads there and turning the test line a colour.

If she is not pregnant:

There's no HCG, so the beads are washed past the fixed antibodies and nothing sticks - the strip stays unchanged.

The same idea detects other substances

This is a lateral-flow test. Using different antibodies, the same idea can detect other substances - such as the antigens on a pathogen.