Congenital abnormalities & their identification

Question 1

Define Antenatal Screening Test

Answer 1

• Available for all women
• Gives measure of risk of fetus being affected by particular disorder
• Higher risk patients offered diagnostic tests

Question 2

Define Antenatal Diagnostic Test

Answer 2

Performed on women with high risk to confirm/ refute possibility

Question 3

Define the ‘Sensitivity’ of Screening Test

Answer 3

Proportion of subjects with condition screened positive for condition

Question 4

Define ‘Negative Predictive Value (NPV)’ of Screening Test

Answer 4

Probability that a subject who is negative will not have the condition

Question 5

Define ‘Specificity’ of Screening Test

Answer 5

Proportion of subjects without the condition who are negative

Question 6

Define ‘Screen Positive Rate’ of Screening Test

Answer 6

Proportion of subjects who are classified as high risk by test

Question 7

Define ‘Positive Predictive Value’ of Screening Test

Answer 7

Probability that a subject who is screened positive will have the condition OFTEN LOW

Question 8

Define False Positive Rate of Screening Test

Answer 8

Number classified as positive who do NOT have the condition

Question 9

How are Neural Tube Defects screened?

Answer 9

Alpha fetoprotein (AFP) - produced by fetal liver

Question 10

How are Chromosomal abnormalities screened?

Answer 10

Raised blood results;

• Human chorionic gonadotrophin beta-subunit (B-hCG)
• Pregnancy associated plasma protein A (PAPP-A)
• AFP
• Oestriol
• Inhibin A
• • other risk facotrs: maternal age, ultrasound measurements

Question 11

Outline the uses of Ultrasound

Answer 11

• Confirm dates
• Screening test
  
  • Nuchal translucency [11-14wks]
  • Structural abnormalities [50% of trisomies]
• Aid diagnostic tests
  
  • Amniocentesis
  • Chorionic villus sampling (CVS)
• Diagnostic tests
  
  • Structural abnormalities [18-21wks, operator dependant]

Question 12

Outline Amniocentesis

• Procedure
• Time performed
• Some uses
• Complications

Answer 12

Diagnostic test

• Involves removal of amniotic fluid using fine-gauge needle under USS
• Safest from 15wks, can be performed later
• Prenatal diagnosis via PCR & FISH;
  
  • Chromosomal abnormalities eg Downs
  • Infections eg Cytomegalovirus (CMV), Toxoplasmosis
  • Inherited eg Sickle-cell anaemia, Thalassaemia, CF
• 1% miscarry

FISH: Fluorescence in situ hybridization

Question 13

Outline Chorionic Villus Sampling

Answer 13

Diagnostic test

• Involves biopsy of the trophoblast via fine-gauge needled through abdo/ cervix into placenta
• After 11 weeks [earlier than Amnio]
• Prenatal diagnosis via PCR & FISH;
  
  • Chromosomal abnormalities eg Downs
  • Inherited eg Sickle-cell anaemia, Thalassaemia, CF
• Slighly above 1% miscarry

FISH: Fluorescence in situ hybridization