Evaluate False Positive Negative Risk
Use molecular-test evidence to evaluate false positive/negative risk accurately.
- Distinguish Sensitivity vs Specificity: Rung 8 of the clinical ladder: false positive and false negative rates are the flip side of sensitivity and specificity, so learn those names first.
- Signal interpretation: Bands, colors, curves, and E-values must be compared to a rule.
Re-learn the skill with worked practice and clear examples.
Use a 2x2 table to weigh a false positive against a false negative and pick the error to avoid most.
| Test result | Truly infected | Truly healthy |
|---|---|---|
| Test says POSITIVE | 90 (true positive) | 8 (false positive) |
| Test says NEGATIVE | 10 (false negative) | 92 (true negative) |
A clinic uses a first SCREENING test for a serious, treatable infection. Why do they design the screen to avoid false negatives even if it raises false positives?
Reviewed| Test result | Truly infected | Truly healthy |
|---|---|---|
| Test says POSITIVE | 90 (true positive) | 8 (false positive) |
| Test says NEGATIVE | 10 (false negative) | 92 (true negative) |
- A.A false negative tells an infected person they are clear, so the disease goes untreated and spreads
- B.Screening tests are not allowed to make any errors, so both kinds are ruled out
- C.A false positive is always more dangerous than a false negative, no matter which disease is screened
- D.Avoiding false negatives makes the test cheaper to run for the clinic
Show the worked solution ▾
Answer: A. A false negative tells an infected person they are clear, so the disease goes untreated and spreads
- Step 1: Spot the goal of a screen: A screen should not miss real cases.
- Step 2: Trace the harm: Missing a case = an infected person believes they are clear and gets no treatment.
Why it's right: Missing a real case (false negative) leaves an infectious, treatable disease untreated, so a screen is built to avoid that error first.
- B: Every real test makes some errors; the goal is to choose which error to minimize.
- C: Which error is worse depends on the situation, not a fixed rule.
- D: Cost is not the reason; the reason is patient and public-health harm.
Aligned to Clinical reasoning · reading level ~grade 9
- In Unit 1.1 ELISA Lab & Controls, this skill turns class evidence into a result another person can check.
Fill these in as you work through the lesson.
- False positive (the test says ____ when the person is really healthy):
- False negative (the test says ____ when the person is really sick):
- Screening test (the first, wide test used to ____ as many real cases as possible):
- Confirmatory test (the second test that must be ____ before treatment starts):
A screening test should avoid negatives so it does not miss a case, and a confirmatory test should avoid positives so it does not treat a healthy person.
- Which box in the 2x2 table is a false positive, and which is a false negative?
- For a first screen of a treatable infection, which error is worse and why?
- Why does a confirmatory test flip which error matters most?
A screen flags 8 healthy people and misses 0 sick people. Decide whether that trade-off is acceptable for a first screen, then say what those 8 people need next.