Multiple marker screening uses a combination of maternal age and 2 or more biochemical
tests, with or without an USS, to produce a single result for risk of Down syndrome,
trisomy 18, and open neural tube defects (ONTDs).
• A screen is positive when the risk of one or more of the screened disorders falls above a
designated risk cut-off.
• A risk cut-off – The risk of the condition being present in the fetus at term or at midtrimester.
The risk for the latter will be higher, because 23% of fetuses with Down
syndrome are lost between mid-trimester and term (risk cut-off of 1:350 at term would be
similar to 1:280 at mid-trimester).

Detection rate (DR) or sensitivity: The proportion of affected individuals with positive
screening results.
• False-positive rate (FPR): The proportion of unaffected individuals with positive screening
results. It is the complement of the specificity.
• As screening performance improves, the FPR decreases and/or the DR increases.
• Multiples of the median (MoM): The absolute value of the assayed marker (serum or
NT) divided by the gestation-specific median value of the serum marker in the measuring
laboratory or by using standard or sonographer-specific curves for NT. This allows direct
comparison of results between programmes.

Factors potentially affecting screening performance
Gestational dating – USS improves the precision of gestational age estimation, and reduces
the error for each screening marker. This effect is greater for markers whose concentrations
change most with gestational age. For all marker combinations, the FPR is lower by about 2%
when gestational age is estimated using a scan.
Insulin-dependent diabetes mellitus – Some second trimester serum markers tend to be lower
in women with IDDM. After weight correction, AFP is ~10% lower and uE3 is ~5% lower in
diabetic women. NT measurement, free β-hCG, and PAPP-A are not affected.
Ethnic origin – Adjusting for ethnic origin slightly increases the DR for a given FPR.
Statistically significant differences in NT measurement have been found between ethnic
groups. However, these differences may be too small to warrant correction.
Maternal weight – There is a negative association between the levels of maternal serum
markers and maternal weight. With second trimester screening, maternal weight adjustment
increases DR by about 1% for a given FPR.
• Weight adjustment is beneficial if there is a marginally elevated AFP when screening for
ONTD. Weight adjustment does not appear to be necessary for NT risk adjustment,
because it increases by only a clinically insignificant amount with increasing maternal
Assisted reproduction – In the first trimester, a lower value of PAPP-A has been reported in IVF
pregnancies, but data on NT and first trimester free β-hCG remain inconsistent.

First trimester screening

Nuchal translucency (NT)

Maternal age + NT + hCG + PAPP-A

Nasal bone

Second trimester screening

Triple marker testing----Quadruple testing