bHCG Flashcards
List indications for testing bHCG?
Aim: to determine the presence and quantitiy of beta human chorionic gonadotropin (HCG) in the blood
Indications:
• Confirm and monitor normal or ectopic pregnancy (must be excluded in any fertile woman w abdo pain)
• Gestational trophoblastic disease
• Use as tumour marker in germ cell tumours (esp choriocarcinomas)
Describe bHCG structure and function?
Structure:
• bHCG is a glycoprotein (with alpha and beta subunits) produced by the human placenta by syncitiotrophoblasts (outer layer of trophoblasts invading endometrium)
• Beta subunit measures as there is less risk of cross reactivity with LH and FSH (similar alpha units)
Function: maintain corpus luteum during beginning of pregnancy (produces nourishing progesterone)
What happens to bHCG levels in pregnancy?
During pregnancy:
• Raised in 1st trimester
• Secreted by placental trophoblastic cells present from early pregnancy (1 week from implantation)
• bHCG functions to maintain pregnancy viability
• Diagnosis: >25 U/L, between 10-25 requires repeat
• Levels rise rapidly (doubles every 48 hrs), peaking 8-12 weeks, then falls and plateaus
When should you be suspicious of an ectopic pregnancy?
Suspect ectopic or non-viable pregnancy if:
• If bHCG doesn’t double every 48hrs
• >1500 U/L and no evidence of pregnancy on US (should be visible on transvaginal US at this level)
• Note: presumed ectopic until proven otherwise
Rx: surgical or pharmacological termination of pregnancy (anti-progestogen mifepristone)
Describe the use of bHCG as a tumour marker?
Tumour marker:
• Elevated in some germ cell tumours- seminomas and non-smeinomas (choriocarcinomas, embryonal tumours, teratomas)
• Read in conjunction with AFP
• Not useful in diagnosis (not specific or sensitive) but useful in monitoring growth and development
Describe the process of specimen analysis for bHCG testing?
Specimen collection:
• Aseptic venepuncture technique
• Venous blood collected in tube containing serum gel separator blood from serum -> precipitates fibrinogen -> coagulation
Specimen analysis:
• Sandwich assay
• Sample (1mL) added to reaction vessel w anti-bHCG antibodies attached to magnetic peptides and different anti-bHCG antigens (alkaline phosphatase conjugates and paramagnetic particles coated with rabbit monoclonal anti-bHCG complexes)
• bHCG binds to immobilised monoclonal anti-bHCG on solid phase, while the rabbit anti-bHCG alkaline phosphatase conjugate reacts with different antigenic sites on the bHCG
• Incubation -> materials bound to the magnetic particles are held in place while everything washed away
• Luminescent material added and reaction with antibody complexes creates light -> measured in luminometer
-> directly proportional to amount of bHCG present
What are expected levels of bHCG?
- Normal < 10U/L
* Suspect ectopic if >1500 U/L and no evidence of pregnancy on US
