In the natural history of a disease, the 'critical point' relevant to screening programmes is best described as:

• A The point at which symptoms first appear in the natural history
• B The point at which the screening test attains its highest sensitivity
• C The point at which the disease reaches maximum prevalence in the community
• D The point before which treatment is more effective than treatment started after it, determining whether early detection can improve outcome ✓

Explanation

The critical point is the dividing line in the natural history before which therapy is more effective than therapy begun afterwards. For screening to be worthwhile, the detectable preclinical phase must begin before the critical point, so that detection and treatment during the preclinical phase can alter outcome. If the critical point precedes the start of the detectable preclinical phase, as classically illustrated for lung cancer, earlier detection by screening cannot improve prognosis even with a sensitive test.

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

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