Genetika 2011 Volume 43, Issue 2, Pages: 285-296
https://doi.org/10.2298/GENSR1102285L
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Credibility of the combined test in prenatal diagnostics
Lončar Dragan (Klinika za Ginekologiju i akušerstvo, Klinički centar Kragujevac, Kragujevac)
Congenital anomalies are the cause of perinatal death in 20-25% of the cases,
while 3% of children are born with malformation of varying size. The
objective of this study was to examine the predictive value and define the
credibility ratio of the combined test results. Of 317 examined pregnant
women, we had sixteen (5.05%) with the result of pathological karyotype after
amniocentesis including: nine (2.84%) with fetal numerical aberrations and
seven (2.21%) with fetal structural aberrations. While determining the
ultrasonographic parameters of the combined test we used the standards of the
Fetal Medicine Foundation. We carried out the quantitative settings of free
β-HCG and PAPP-A from vein blood of patients by applying commercial tests of
firm DPC-USA. Tests were based on the analytical immunochemiluminescence
assay and were realized by using the automated analyzer IMMULITE 2000.
Manufacturer of the analyzer is also the firm DPC-USA. Sensitivity of the
test is 94%, and specificity is 99%. Positive likelihood ratio [likelihood
ratio test (LR+)] is 94.00, a negative likelihood ratio is [likelihood ratio
test (LR-)] 12:06. Pretest probability that pregnant women carries fetus with
chromosomal abnormality is 1:250 or 0004. Posttest odds after the combined
test to discover this abnormality is 0.3760, and probability of the same case
is 0.2732 if it happens that the test result is positive. The result of our
study confirms the justification of combined test usage in routine clinical
practice, since the posttest odds rate in the case of a positive screening
increases several times over (almost 90 times), the probability of detecting
a chromosomal abnormality was about 70 times. Combined screening test if used
methodologically correct, has a high predictive value in detecting fetal
congenital anomalies.
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