A malaria rapid diagnostic kit shows sensitivity 92% and specificity 96% both in a rural primary health centre and in a medical college hospital. However, its positive predictive value is 60% in the PHC but 93% in the referral hospital. The single factor that best explains this difference in PPV is:
- A Higher technical skill of laboratory staff in the hospital
- B Different parasite species prevalent in the two areas
- C Higher pretest probability (prevalence) of malaria among patients referred to the hospital ✓
- D Storage-related degradation of test kits in the rural centre
Correct answer: C. Higher pretest probability (prevalence) of malaria among patients referred to the hospital
Explanation
Predictive values depend directly on disease prevalence: as prevalence rises, PPV rises and NPV falls, even though sensitivity and specificity remain unchanged because they are intrinsic test properties. Referred hospital patients have a higher pretest probability of malaria than unselected PHC attendees, so a positive result is more likely to be true. Species mix and staff skill affect measured accuracy, not the gap between intrinsic properties and PPV.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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