Screening test principles in children — formative SAQs

SAQ 1 (10 marks)

A public health team proposes a new universal school-entry blood test for a rare treatable condition (prevalence about 1 in 2,000). A laboratory assay is available. Confirmatory testing would require tertiary travel. ^{[1] [2]}

1. List five programme criteria you would apply before supporting rollout. (5) ^{[1] [2] [3]}
2. Explain why positive predictive value may still be low despite high specificity. (3) ^[5]
3. State one equity risk of launching without a local confirmatory pathway. (2) ^{[2] [8]}

Model answer

Apply criteria in the Wilson–Jungner / modern lineage: important condition; acceptable accurate test; effective early treatment; agreed policy and quality programme; facilities for diagnosis and treatment; informed choice; equity; and explicit net benefit after harms (false positives, overdiagnosis, cost). ^{[1] [2] [3]} When prevalence is very low, most screened children are well, so even a highly specific test can generate many false positives relative to true positives among those who flag positive—PPV falls. ^[5] Launching without usable local confirmation shifts burden onto families least able to travel, creating inequitable incomplete pathways and harm without benefit. ^{[2] [8]}