The 'number needed to screen' (NNS) to prevent one adverse outcome is mathematically:

• A 1 / Relative risk reduction
• B 1 / (Absolute risk reduction in the screened vs. unscreened group) ✓
• C Sensitivity × Specificity
• D Prevalence / Incidence rate

Explanation

NNS = 1 / Absolute Risk Reduction (ARR), where ARR is the difference in event rates between the unscreened (control) and screened groups. It quantifies how many people must be screened to prevent one additional adverse outcome, integrating screening uptake, test performance, treatment efficacy, and baseline risk. A lower NNS implies greater screening efficiency.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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