B1 Pregnancy testing

Question 1

What proteins are detectable during pregnancy and what is detectable first?

Answer 1

HCG (Human Chorionic Gonadotropin) is first detected (peaks ~8 wks), also oestrogen and progesterone

Question 2

Name the 3 main types of assays which detect HCG and where are they most frequently used

Answer 2

1. Inhibition of latex agglutination- clinical
2. Radioimmunoassay- clinical
3. Dip-stick- home; clinical

Question 3

Explain latex agglutination if person IS pregnant

Answer 3

(sample can be urine or serum)
Sample contains HCG, therefore binds rabbit anti-HCG when mixed with the sample. The latex HCG coated beads are added and the rabbit anti-HCG sample is mixed. Result = no agglutination
(as HCG has bound rabbit anti-HCG removing it from further reactions)

Question 4

Explain latex agglutination if person Is NOT pregnant

Answer 4

No HCG in sample, rabbit anti-HCG has nothing to bind.

Latex HCG coated beads are added & sample is mixed. Result = Agglutination as anti-HCG is free to react with sample

Question 5

Explain Radio immunoassay (RIA) if person IS pregnant

Answer 5

Sample (contains HCG) is added to filter paper rabbit anti-HCG
HCG is bound by Ab (rabbit anti-HCG)
Radiolabelled HCG is added however can not be bound by Ab, wash step removes radioHCG
NO radioactivity = sample contains HCG & woman is pregnant

Question 6

Explain Radio immunoassay (RIA) if person Is NOT pregnant

Answer 6

No HCG in added sample (doesnt bind filter with rabbit anti-HCG)
Radiolabelled HCG is added and binds Ab on filter paper, only excess radioHCG is washed away
High reactivity - No HCG in sample therefore no pregnancy

Question 7

Explain dip stick if person IS pregnant

Answer 7

Urine containing HCG is added
sample binds anti-HCG coloured latex beads
Beads move up filter HCG beads are bound by immobilised anti-HCG
As latex beads are bound a coloured line forms & other beads pass by
IgA protein binds immobilised anti-IgA beads (control)
2 lines = pregnancy

Question 8

Explain dip stick if person Is NOT pregnant

Answer 8

NO HCG in urine

Beads move past immobilised antiHCG and bind immobilised anti-IgA & coloured line forms (one line = not pregnant)

Question 9

Advantages and disadvantages of

1. Inhibition of latex agglutination
2. RIA
3. Dip-stick

Answer 9

1. Time consuming & relies on visual interpretation
   Not absolutely quantitative
2. Can be easily automated & Is easily quantified
   Produces radioactive waste
3. Very simple
   Not quantitative

Question 10

How does a digital pregnancy test work

Answer 10

Same as lateral flow but spectrophotometer measures absorbance
(computer program calcs HCG conc)

Question 11

Name some problems of measuring HCG

Answer 11

Can be raised in other rare conditions such as tumours
Also raised if incomplete abortion
Is not present in ectopic pregnancy