When can a risk factor be used as a worthwhile screening test?

This publication has 2 references indexed in Scilit:

• Apolipoproteins and ischaemic heart disease: implications for screening
  The Lancet, 1994
• Maternal serum screening for Down's syndrome: the effect of routine ultrasound scan determination of gestational age and adjustment for maternal weight
  BJOG: An International Journal of Obstetrics and Gynaecology, 1992