Current pregnancy tests are based on measurement of the beta subunit of hCG in urine or serum . In each of these, the alpha subunit is identical and the special characteristics of the molecule are conferred by the beta subunit . In urine, some of these tests also measure free beta subunits and beta subunit fragments while other tests are more specific for the intact molecule. Pregancy tests typically detect increased hCG levels 1-2 weeks after conception. Urine tests may give different results depending on the degree of concentration or dilution of the urine.
Some tumors can produce hCG or free beta subunits.
Renal failure leads to accumulation of interfering substances that may produce incorrect results in some tests. Some patients have a low level of circulating hCG-like material. In most patients, the level of this material is not high enough to produce a positive result, but it can cause confusion in rare cases. In cases where there is a question, qualitative tests may be followed by quatitative tests several days apart to demonstrate the expected rising hCG levels. Ultrasound to demonstrate a gestational sac is also used to confirm pregnancy.