The diagnosis of early pregnancy is primarily determined via detection and measurement of human chorionic gonadotropin (hCG). hCG assays vary considerably and include over-the-counter qualitative urine tests, point-of-care qualitative serum and urine tests, clinical laboratory qualitative serum assays, and clinical laboratory quantitative serum assays (Cole, 2009). Although urine pregnancy tests are quick, reliable, and easier for patient to provide specimen, serum pregnancy tests are the more sensitive and can typically detect pregnancy earlier.
Qualitative tests detect the presence of hCG, quantitative tests detect the level of hCG. The qualitative urine test detects hCG levels of 20-50 IU/L; results are reported as positive or negative. The qualitative serum test can detect hCG levels as low as 5-10 IU/L; results are also reported as positive or negative. Quantitative serum tests detect levels as low as 1-2 IU/L making this the test of choice for early pregnancy detection and monitoring.
Human chorionic gonadotropin (hCG) is composed of glycopeptide α- and β-subunits. The β-subunit is unique to hCG, whereas the α-subunit is essentially identical to that of luteinizing hormone (LH), follicle-stimulating hormone (FSH), and thyroid stimulating hormone (TSH). Assays for hCG specifically identify the beta subunit preventing cross-reactions.