Master the board step 3

Master the Boards: USMLE Step 3
Maternal serum alpha
fetoprotein (MS-AFP)
increases with gestational
age, and is expressed in
multiples of the median
(MoM).
• > 2.2 MoM is considered
elevated.
• < 2.5 MoM is considered
normal.
Inhibin A is made by the
placenta during pregnancy
and normally remains
constant during 15th–18th
week of pregnancy. The level
of inhibin A is increased
in the blood of mothers of
fetuses with Down syndrome.
Second Trimester
A 23-year-old woman (G3 P1 Abortion 1) is seen at 17 weeks gestation. She recently
underwent a triple marker screen with the maternal serum alpha fetoprotein (normal
< 2.2 MoM). Her test showed an elevation in maternal serum alpha fetoprotein. On
examination, her uterus is at the umbilicus. What is the next step in management?
a. Amniocentesis
b. Chorionic villus sampling
c. Inhibin A
d. Recommendation of termination of pregnancy
e. Ultrasound
Answer: E. The most common cause of an abnormal maternal serum alpha fetoprotein
(MS-AFP) is gestational dating error. The first step in evaluating any pregnancy with an
abnormal MS-AFP is to get an obstetric ultrasound to confirm the gestational date.
Screening Test Diagnostic Significance Next Step in Management
SECOND TRIMESTER OPTIONAL TESTS
Triple Marker
Screen
Testing window
is 15–20 weeks
gestation.
1. MS-AFP
2. β-hCG
3. Estriol
4. Add Inhibin
A in high-risk
women (↑
sens to 80%).
• Never test MS-AFP alone:
Only 20% sensitivity → ↑ to
70% sensitivity with triple screen.
• ↑ MS-AFP:
Neural tube defect (NTD), ventral
wall defect, twin pregnancy,
placental bleeding, renal disease,
sacrococcygeal teratoma
• ↓ MS-AFP:
– Trisomy 21 (Down syndrome)
° ↓ MS-AFP
° ↓ Estriol
° ↑ β-hCG
– Trisomy 18
° ↓ MS-AFP
° ↓ Estriol
° ↓ β-hCG
1) Abnormal MS-AFP:
First step in management:
• Perform ultrasound to confirm
dating.
• If dating error, repeat MS-AFP.
• A normal repeat MS-AFP is
reassuring.
Note: Accurate gestational dating
is needed for interpretation of
results.
2) Dates confirmed by ultrasound:
Next step in management:
For ↑ MS-AFP:
a) Amniocentesis for amniotic
fluid alpha fetoprotein (AF-AFP)
level and acetylcholinesterase
activity
For ↓ MS-AFP:
b) Amniocentesis for karyotyping
Note: Elevated levels of amniotic
fluid-acetylcholinesterase activity
are specific to open NTD.
AF-AFP = amniotic fluid alpha fetoprotein; MS-AFP = maternal serum alpha fetoprotein
420