Test BackgroundThe percentage free hCG β-subunit test is only suitable in selected rare trophoblastic disease variants and available as a specialist service. Please request tumour marker hCG for all oncology applications, including initial investigation of trophoblastic disease.
hCG is composed of two subunits, hCGα and hCGβ. It is the intact αβ-heterodimer (termed hCG) that is biologically active and regulates sex steroid production through a 7 transmembrane G-protein-coupled chimeric receptor. hCGβ free subunit does not activate this receptor and is therefore frequently described as ‘biologically inactive’. Several recent studies, however, suggest that hCG β-subunit may exert a growth-promoting activity in cancer.
In early pregnancy the proportion of total serum hCG immunoreactivity present as free hCGβ-subunit (hCGβ/hCG + hCGβ) may reach a peak of around 4 % within the first 4 weeks of pregnancy, but then quickly drops to <1 %. Pregnancies in which the fetus exhibits trisomy 21 have higher values. Samples for Down’s risk screening should be sent to a specialist laboratory providing this service.
Assays measuring ‘total hCG’ are used to monitor patients with gestational trophoblastic neoplasia (GTN); however the use of separate total hCG and free hCGβ may aid differentiation of variant disease forms. A percentage free βsubunit of hCG around 3-5 % is typical of invasive molar disease, while patients with placental site trophoblastic tumour (PSTT), a particularly aggressive form of GTN, show a greater proportion at around 45 %. hCGβ is cleared from the circulation more slowly than intact hCG and the proportion of hCGβ should, therefore, always be evaluated prior to commencement of treatment. Elevated serum free hCGβ has also been identified in a variable proportion of other malignancies, including: bladder, renal, biliary, pancreatic, gastic, liver and colorectal. Most studies demonstrate that either tissue expression or elevated serum hCGβ is associated with adverse prognosis.
Sample Required SST (gold top) preferred, serum (red top) acceptable
Sample Volume 1 mL
Turnaround Time 4 weeks
NotesBrahms Kryptor assay
The percentage free hCGβ-subunit test is only available as a specialist service.