Understanding Sensitivity and Specificity of a Malaria Rapid Test
The correct answer is (a): A sensitivity of 95% means the test correctly identifies 95% of diseased individuals (those who actually have malaria), while a specificity of 85% means the test correctly identifies 85% of non-diseased individuals (those who do not have malaria).
Definitions in Practical Terms
Sensitivity (95%)
- Sensitivity measures the test's ability to detect disease when it is present 1
- With 95% sensitivity, if 100 people truly have malaria, this test will correctly identify 95 of them as positive 2
- The remaining 5 people with malaria will receive false negative results (the test misses them) 1
- High sensitivity is crucial for ruling out disease - a negative result on a highly sensitive test makes disease very unlikely 3
Specificity (85%)
- Specificity measures the test's ability to correctly identify people without disease 1
- With 85% specificity, if 100 people do not have malaria, this test will correctly identify 85 of them as negative 2
- The remaining 15 people without malaria will receive false positive results 1
- High specificity is important for ruling in disease - a positive result on a highly specific test strongly suggests disease is present 3
Why the Other Options Are Incorrect
- Option B is incorrect: This describes specificity, not sensitivity. The 95% figure refers to diseased individuals, not non-diseased 2
- Option C is incorrect: This reverses the numbers - 85% is the specificity (non-diseased identification), not sensitivity 2
- Option D is incorrect: This describes positive predictive value (PPV), which depends on disease prevalence in the population, not just the test's sensitivity and specificity 2, 3
Clinical Context for Malaria Testing
This test's performance (95% sensitivity, 85% specificity) falls within the range reported for malaria RDTs, which show sensitivity for P. falciparum ranging from 67.9% to 100% and specificity between 93.1% and 100% 1
Important Clinical Implications
- The 95% sensitivity is excellent - this test will catch nearly all malaria cases, making it valuable for screening 3, 4
- The 85% specificity is moderate - approximately 15% of people without malaria will test positive, leading to potential overtreatment 1, 5
- In low prevalence settings (such as returned travelers), the relatively lower specificity means many positive results will be false positives, requiring confirmatory microscopy 1, 3
- In high prevalence settings, the high sensitivity ensures few cases are missed, but false positives remain a concern 5
Common Pitfalls
- False negatives can occur with non-falciparum species, low parasitemia, or P. falciparum strains with gene deletions despite high sensitivity 1, 3
- False positives may result from rheumatoid factor, anti-nuclear antibodies, or persistent antigens from previous infections 1, 3
- Always confirm positive RDT results with microscopy for species identification and parasitemia quantification 2, 3