Final answer:
The initial office lab test for pregnancy is an immunochromatographic assay (lateral flow test) that detects hCG in urine through a test line, with a control line ensuring the test's validity.
Step-by-step explanation:
The initial lab test used in an office setting to test for pregnancy is an immunochromatographic assay, commonly referred to as a lateral flow test. This test detects the presence of the hormone human chorionic gonadotropin (hCG) in urine. As the urine flows through the test strip, it rehydrates the reagents allowing the hCG to bind to antibodies conjugated to particles. These particles then bind to a second antibody on the test strip, creating a visible test line if hCG is present. A separate control line ensures the test is working correctly. This test is preferred due to its convenience, rapid results, and because it is non-invasive compared to blood-based tests such as ELISA.