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]

SAQ 2 (10 marks)

Parents receive a positive newborn screening flag. Confirmatory testing one week later is negative. They remain anxious. [7]

1. Distinguish false positive from overdiagnosis in one sentence each. (2) [4]
2. Outline your counselling after the confirmatory negative result. (4) [7] [5]
3. List four programme-level actions that reduce false-positive harm. (4) [2] [8]

Model answer

False positive: screen positive but target disease absent on the reference pathway. Overdiagnosis: a real finding that would not have caused harm yet may trigger labelling or treatment. [4] Counselling: the baby does not have the target disease; explain why screens are set sensitive; acknowledge that worry is common and valid; avoid further cascade tests without indication; safety-net for unrelated future symptoms without blaming the cleared screen. [7] [5] Programme actions: optimise cut-offs and two-step design; rapid confirmatory access; clear parent information at offer and result; active psychosocial support and exit counselling; audit false-positive rates and time-to-clearance. [2] [8]