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Overview :
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:
• The impact of
interventions in patients with false positive results needs to be taken into
account.
• 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.
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