A rapid diagnostic test for COVID-19 (antigen test) has a sensitivity of 80% and specificity of 95%. In a low-prevalence setting (0.5% prevalence), a 35-year-old woman tests positive. Her symptoms are mild and non-specific. What is the most appropriate next step?

## Understanding Post-Test Probability in Low-Prevalence Settings **Key Point:** In low-prevalence diseases, even a highly specific test can have a low PPV, meaning a positive result is more likely to be a false positive than a true positive. ## PPV Calculation for COVID-19 Antigen Test $$PPV = \frac{\text{Sensitivity} \times \text{Prevalence}}{\text{Sensitivity} \times \text{Prevalence} + (1 - \text{Specificity}) \times (1 - \text{Prevalence})}$$ $$PPV = \frac{0.80 \times 0.005}{(0.80 \times 0.005) + (0.05 \times 0.995)}$$ $$PPV = \frac{0.004}{0.004 + 0.04975} = \frac{0.004}{0.05375} \approx 7.4\%$$ ## Interpretation | Metric | Value | Clinical Implication | |--------|-------|----------------------| | PPV | ~7.4% | Only 7.4% of positive tests are true positives | | False Positive Rate | ~92.6% | 92.6% of positive tests are false positives | | Likelihood Ratio Positive (LR+) | 16 | Increases odds of disease 16-fold | | Post-test odds | 1:13.5 | Still more likely to be negative than positive | **High-Yield:** LR+ = Sensitivity / (1 − Specificity) = 0.80 / 0.05 = 16. Despite an LR+ of 16, the post-test probability remains low because the pre-test probability (prevalence) is very low. ## Why Confirmatory Testing Is Needed **Mnemonic: CONFIRM** — **C**ontext (low prevalence), **O**ne test alone insufficient, **N**eed gold standard, **F**alse positives common, **I**nvestigate further, **R**T-PCR is gold standard, **M**anagement depends on confirmation. 1. **Low prevalence setting:** When disease prevalence is low, positive screening tests are enriched for false positives 2. **Mild symptoms:** Non-specific symptoms do not increase pre-test probability 3. **Gold standard available:** RT-PCR is more sensitive and specific than antigen testing 4. **Clinical consequences:** Treatment decisions (antivirals, isolation) should not be based on a test with 92.6% false positive rate **Clinical Pearl:** Antigen tests are useful for rapid screening in high-prevalence settings or symptomatic patients. In low-prevalence asymptomatic or mildly symptomatic individuals, a positive antigen test should always be confirmed with RT-PCR before clinical decisions are made.