Child and Adolescent Psychiatry

312 Chapter 37

Box 37.1 shows some of the terms used when using screening tests.

Box 37.1Screening test terminology

True diagnosis

Case Non-Case

Positive ab

Screening test result Negative cd

True positive a Correctly identified case

True negative d Correctly identified non-case

False positive b A non-case misleadingly identified as positive by

the screen

False negative c A case misleadingly identified as negative by

the screen

Sensitivity a/a+c the proportion of cases correctly identified

Specificity d/b+d the proportion of non-cases correctly identified

Positive predictive value a/a+b the proportion of screen positive predictions

that are cases

Negative predictive value d/c+d the proportion of screen negative predictions

that are non-cases

Conditions that make prevention feasible

1 For targeted prevention programmes, there needs to be an effective
screening or identification test or procedure. This needs to be sensitive,
that is, it should not miss many cases (low false negative rate), and
specific, so that cases that would not get the disorder are not identified
unnecessarily (low false positive rate). However, since most psychiatric
conditions are dimensional, some false positive cases may not be a
bad thing, since the children an adolescents in question may have
substantial problems that could befit from intervention even if they
don’t quite have a full set of symptoms warranting a diagnosis. There
are now good psychometric data across large and diverse populations
on easily administered screening instruments such as the Strengths
and Difficulties Questionnaire (SDQ) that show the power to predict
emotional and behavioural disorders with acceptable sensitivity and
specificity.
2 There needs to be an effective preventive intervention that is taken up
by a substantial proportion of the population to whom it is offered. For
example, as shown below, there are effective programmes for conduct
disorder.
3 There are serious, long-standing consequences if the full-blown con-
dition develops. The case for prevention is particularly strong if the