2) A reasonable sensitivity and specificity to the test (meaning people with disease are more likely to have a positive test and those without the disease to have a negative test),
3) A test that is reasonably cheap and acceptable to patients (tests like mammography and colonoscopy, for example, are both more expensive and more uncomfortable than, say, a blood test),
4) A more definitive test available to say more definitely whether people who screen positive actually have the disease (for most screening, although those who screen positive are more likely to have the disease than those who do not, the majority of those who screen positive still may not have the disease),
5) An intervention that can be done in the asymptomatic stage that will prevent the disease from progressing (or else, why not wait until it is symptomatic?)
 Bleyer A, Welch HG, “Effect of three decades of screening mammography on breast-cancer incidence”, NEJM 22Nov2012;367(21):1998-2005.