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Question 717
Obstetric infection → Viral
Classification
Question
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Interpretation of amniotic-fluid VZV PCR
Question Stem
A woman develops chickenpox at 15 weeks of gestation. After recovery, she asks about amniocentesis for VZV DNA PCR to determine whether the fetus will develop fetal varicella syndrome. Which statement most accurately describes this investigation?
Options
A
A positive result reliably predicts fetal varicella syndrome
B
A negative result has poor value for excluding fetal infection
C
Amniocentesis should be performed while maternal vesicles remain active
D
The test has strong negative predictive value but poor positive predictive value for fetal damage
E
PCR is useful only if the detailed ultrasound is already abnormal
Explanation
VZV DNA can be detected in amniotic fluid by PCR. Amniocentesis has a strong negative predictive value but a poor positive predictive value for fetal damage that cannot be detected by non-invasive methods. A positive PCR therefore does not establish that fetal varicella syndrome will develop. Amniocentesis should not be performed before maternal skin lesions have completely healed.
Option Validity
A) the positive predictive value for fetal damage is poor.
B) the negative predictive value is strong.
C) amniocentesis should not be performed before the skin lesions have completely healed.
E) counselling about PCR is not restricted to women with an already abnormal ultrasound.
Further Reading
RCOG Green-top Guideline No. 13, Chickenpox in Pregnancy, section 6.2.