Box 19-1 How Does a Home Pregnancy Test Work

One of the most common home pregnancy tests is an immunoassay that detects the hormone human chorionic gonadotropin (hcG) in urine. HcG is a protein consisting of 244 amino acids in two subunits called a and p. The P subunit is unique to hcG, but the a subunit is found in some other proteins. The hormone begins to be secreted shortly after conception. 

The lateral flow home pregnancy immunoassay shown schematically in the figure is a qualitative test for hcG. Urine is applied to the sample pad at the left end of a horizontal test strip made of nitrocellulose that serves as a wick. Liquid flows from left to right by capillary action. Liquid first encounters detection reagent on the conjugate pad. The reagent is called a conjugate because it consists of hcG monoclonal antibody 

In a positive pregnancy test, both lines turn red. The test is negative if only the control line turns red. If the control line fails to turn red, the test is invalid. 

You should be able to use Equations 19-5 to analyze the spectrum of a mixture. 

Pq = radiant power of incident radiation c = concentration of fluorescing species 

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