Tell me the difference between genetic screen and diagnostic test.?

Pap smear vs. biopsy: Screening tests are devised to label a small proportion of the screened population as high-risk for a certain condition (cervical cancer in the case of pap smear, fetal anomalies in the case of fetal ultrasound). Diagnostic tests clinch the pathological diagnosis with virtual certainty (cervical biopsy for cancer, genetic testing or conclusive demonstration of anomaly by u/s in prenatal diagnosis).