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Table 4 Serum screening among pregnant women and number of women needed to be tested and treated with aspirin to prevent one case of PET.

From: Serum screening with Down's syndrome markers to predict pre-eclampsia and small for gestational age: Systematic review and meta-analysis

Test result

Prevalence PET (%)

Probability of PET after testing positive (%)

Risk of PET after treatment*

Probability of PET after treatment

NNTest1

NNTreat2

No test, no treatment3

3.0

3.0

-

3.0

-

-

 

10.0

10.0

 

10.0

  

No test, treat all3

3.0

-

0.9

2.8

-

323

 

10.0

-

0.9

9.0

-

90

Alpha feto-protein>2.0MoM: Sensitivity 7%; Specificity 96%

Test all, treat test positives

3.0

7.3

0.9

6.1

4762

147

 

10.0

26.2

0.9

18.6

1429

48

Human chorionic gonadotrophin>2.0MoM, second trimester: Sensitivity 19%; Specificity 93%

Test all, treat test positives

3.0

7.5

0.9

6.3

1754

142

 

10.0

27.2

0.9

19.3

526

47

Unconjugated estriol<0.5MoM: Sensitvity 6%; Specifcitiy 96%

Test all, treat test positives

3.0

4.6

0.9

4.0

5556

226

 

10.0

16.7

0.9

12.8

1667

70

Pregnancy associated plasma protein A (PAPP-A)<5th centile: Sensitivity 9%; Specificity 95%

Test all, treat test positives

3.0

6.5

0.9

5.5

3704

167

 

10.0

23.3

0.9

17.0

1111

53

Inhibin A>2.79MoM: Sensitivity 71%; Specificity 96%.

Test all, treat test positives

3.0

6.0

0.9

3.4

469

27

 

10.0

216.9

0.9

61.2

141

15

  1. * RR 0.90 (95% CI 0.81–1.01) Askie et al. Antiplatelet agents for prevention of pre-eclampsia: a meta-analysis of individual patient data. Lancet 2007;369:1791–9811.
  2. 1 NNTest is number needed to test and treat with aspirin to prevent one case of FGR calculated by 1/(proportion true positives (TP) – (proportion TP * RR)).
  3. 2 NNTreat is number need to treat if only treat test positives with aspirin calculated by 1/(probability after testing positive – probability after treatment).
  4. 3 Numbers are equal for all tests regardless of threshold, sensitivity and specificity.
  5. MoM multiples of median
  6. PET pre-eclampsia