Credibility of the combined test in prenatal diagnostics

Abstract

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 ?-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.