The usefulness of a 'screening test' in a community depends on its -
Screening refer to 'the search for unrecognized disease or defect by means of rapidly applied tests, examinations or other procedures in apparently healthy individuals'
For screening a disease the most important parameter is sensitivity

Sensitivity is defined as the ability of a test to identify correctly all those who have the disease i.e., true positives as it reflects the ability to detect those who have the disease amongst the hidden cases in the community.

For confirming a patient as diseased most important parameter is specificity

Specificity is defined as the ability of a test to identify correctly all these who do not have the disease i.e., true negative.

Memo: ELISA is a sensitive test for HIV: ELISA is used as screening test for HIV
Western blot is a specific test for HIV : Western blot is used as a confirmatory test for HIV.

Usefulness of a screening test is given by sensitivity
Statistical index of diagnostic accuracy → specificity
Sensitivity
Specificity
Reliability
Predictive value