<qa>  What is the hCG beta level in serum?Gestational trophoblastic disease comprises a spectrum of diseases with different propensity for local invasion and metastasis, that is, partial and complete hydatidiform mole, choriocarcinoma, and placental site trophoblastic tumor. Persistent trophoblastic disease may develop both from partial and complete moles. All trophoblastic tumors produce human chorionic gonadotropin hCG, and monitoring of therapy is largely based on the determination of hCG in serum. The more malignant forms also express excessive amounts of hCG beta, and simultaneous determination of hCG and hCG beta can be used to differentiate between molar disease and trophoblastic cancer, which is associated with a proportion of hCG beta exceeding 5. Over glycosylated hCG has also been reported to be a characteristic of trophoblastic cancer, but its clinical utility remains to be established. The hCG beta level in serum is also used to evaluate prognosis with not only very high but also very low levels in relation to tumor burden being indicative of adverse prognosis. Because of that: 1. more malignant forms also express excessive amounts of hCG beta; 2. the half-life of hCG beta is longer than that of hCG; 3. the hCG beta hCG ratio will increase when the levels decrease after successful therapy; and 4. the ratio has been found to increase during development of therapy-resistant disease, monitoring of gestational trophoblastic disease usually relies on human chorionic gonadotropin beta measurement. Treatment of choriocarcinoma is most often monitored by serial assay of hCG beta in serum until the levels are undetectable, and simultaneous determination of hCG beta can sometimes reveal a relapse earlier than hCG. Therefore, measurement of hCG beta is important in treatment of gestational trophoblastic tumors. 