The Number Needed to Screen (NNS) is the number of people in a population who need to be screened for a genetic disorder in order to prevent 1 person from being affected. This is an extension of the Number Needed to Treat (NNT, see chapter on Decision Analysis).
Parameters:
(1) prevalence of carriers in the target population in percent
(2) percent risk of disease for untreated carriers
(3) percent risk of disease in treated carriers
number of people developing disease per thousand untreated carriers =
= 1000 * (percent risk of disease in untreated carrier as a decimal fraction)
reduction in risk if treated =
= (percent risk in untreated carriers) – (percent risk in treated carriers)
number of carriers per 1000 who benefit from treatment =
= (reduction in risk if treated) * (number of people developing disease per thousand untreated carriers)
number of carriers needed to treat to prevent 1 case =
= 1000 / (number of carriers per 1000 who benefit from treatment)
number of people to screen =
= (number of carriers needed to treat to prevent 1 case) / (prevalence of carriers in target population as a decimal fraction)
Interpretation:
• A reasonable Number Needed to Screen is usually found with a highly-penetrant mutation (a large number of carriers develop disease) in a high-risk family.
• The Number Needed to Screen tends to be too high when (a) screening the general population for a highly penetrant mutation or (b) screening for a low penetrant polymorphism (only a small number of carriers develop disease).
Other issues:
(1) The impact of interventions in patients with false positive results needs to be taken into account.
(2) Ideally there should be an effective treatment for people at risk if screening is to have a significant benefit. If there is no effective treatment, genetic counseling and early screening may have some benefit.
