Non-invasive screening: The probability of events

Abstract

Congenital anomalies are the cause of 20.0-25.0% of cases of perinatal death, while 3.0% of children are born with malformations of varying size. We examined the predictive values and defined the credibility ratio of the combined test results. Sensitivity of the test is 94.0%, and specificity is 99.0%. The positive likelihood ratio [likelihood ratio test (LR+)] is 94.00; a negative likelihood ratio [likelihood ratio test (LR-)] is 0.06. The pretest probability that pregnant women carry a fetus with chromosomal abnormality is 1:250. Posttest odds after the combined test to discover this abnormality are 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.