Page 15 - DISINVESTMENT AND IMPLEMENTATION OF VISION SCREENING TESTS BASED ON THEIR EFFECTIVENESS

- The natural history of the condition, including development from latent to declared disease, should be adequately understood
- should be an agreed policy on whom to treat
- The cost of case-finding (including diagnosis and treatment of patients diagnosed)
should be economically balanced
- Case-finding should be a continuing process
The potential negative effects of screening include the risk of imposing anxiety on positively screened individuals, an erroneous conception of health for false negative cases, unnecessary examinations for false positive cases. Also limited economic resources and growing medical expenses have to be taken into account.
Sensitivity, specificity and positive predictive value
The sensitivity of a test is the ability to detect persons who truly have the disease. The specificity of a test is the ability to detect persons free of the disease. A test with low sensitivity fails to detect a substantial part of affected individuals (“underreferrals”). A test with low specificity wrongly suspects disease in a large number of healthy subjects (“overreferrals”). The positive predictive value is the proportion of subjects found positive upon testing who truly are affected with the target condition (Table). A low positive predictive value means that few of those found positive at screening actually are affected by the disease. This might lower the confidence of the screening result among the public and can lead to low compliance with referral for more specialized care.
Table 1. Sensitivity, specificity, positive and negative predictive value.
Positive screening test Negative screening test Total
Truly diseased
a
c
a + c
Truly healthy
b
d b+ d
Total
a + b c + d
General introduction
a = the number of subjects who have the disease and for whom the screening test is positive (true positive)
b = the number of subjects who are healthy, but for whom the screening test is positive (false positive)
c = the number of subjects who have the disease, but for whom the screening test is negative (false negative) d = the number of subjects who are healthy and for whom the screening test is negative (true negative)
Sensitivity: a/ a+c
Specificity: d / b+d
Positive predictive value: a / a+b Negative predictive value: d / c+d
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