# Situation: The Rural Health Unit (RHU) is planning community screening programs, and the nurse helps evaluate the screening tests. For a separate program, the RHU must choose a test to screen 1,000 pregnant women for an infection that is curable, can be passed to the newborn, and is serious if missed. The expected prevalence is 5%. Every positive screen needs a confirmatory test, and the laboratory can confirm no more than 150 positive screens. Which test is BEST?

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> language: ko  
> subject: Nursing Practice I — Community Health Nursing

## 문제

Situation: The Rural Health Unit (RHU) is planning community screening programs, and the nurse helps evaluate the screening tests.

For a separate program, the RHU must choose a test to screen 1,000 pregnant women for an infection that is curable, can be passed to the newborn, and is serious if missed. The expected prevalence is 5%. Every positive screen needs a confirmatory test, and the laboratory can confirm no more than 150 positive screens. Which test is BEST?

## 보기

1. Sensitivity 98%, specificity 80%
2. Sensitivity 86%, specificity 98%
3. Sensitivity 80%, specificity 96%
4. Sensitivity 90%, specificity 90% **✔ 정답**

**정답: 4**

## 해설

Two rules apply. Because missing a case is dangerous and treatable, the test should be as sensitive as possible, but its positives must fit within the 150 confirmations. With 5% prevalence, 50 women are infected and 950 are not. Positives per test: 98%/80% = 49 + 190 = 239 (over capacity); 86%/98% = 43 + 19 = 62; 80%/96% = 40 + 38 = 78; 90%/90% = 45 + 95 = 140. Of the tests within capacity, the 90%/90% test misses the fewest infections (5 of 50).

## 심화 해설

Two rules for choosing the test
The infection to be screened is curable, can be passed to the newborn, and is serious if missed. These features call for the most sensitive test available, since every missed case is harmful and treatment is effective. But the program has a hard limit: every positive screen must be confirmed, and the laboratory can confirm no more than 150. The best test is the most sensitive one whose number of positive screens fits within the confirmation capacity.

Calculating positives for each test
With 1,000 women and 5% prevalence, 50 are infected and 950 are not. True positives = sensitivity × 50; false positives = (1 − specificity) × 950. The test at 98%/80% gives 49 + 190 = 239 positives, above capacity. The test at 86%/98% gives 43 + 19 = 62 positives and misses 7. The test at 80%/96% gives 40 + 38 = 78 positives and misses 10. The test at 90%/90% gives 45 + 95 = 140 positives, within the limit, and misses only 5.

| Sensitivity / specificity | True positives | False positives | Total positives | Missed infections |
| --- | --- | --- | --- | --- |
| 98% / 80% | 49 | 190 | 239 (over 150) | 1 |
| 86% / 98% | 43 | 19 | 62 | 7 |
| 80% / 96% | 40 | 38 | 78 | 10 |
| 90% / 90% | 45 | 95 | 140 | 5 |

Why the other tests are not best
The 98%/80% test misses only one infection, which makes it attractive, but its 239 positives exceed what the laboratory can confirm, so many women would be left without confirmation. The 86%/98% and 80%/96% tests fit easily within capacity, but they miss 7 and 10 infections, more than the 90%/90% test. Watch out! With low prevalence, false positives come mostly from the large uninfected group, so even a modest drop in specificity adds many positives; always calculate them rather than judging by the percentages.

Exam takeaway
First eliminate tests that exceed the practical limit, then choose the most sensitive of those that remain. This reasoning reflects real program planning, where test accuracy must be balanced against laboratory capacity, cost, and follow-up. Women with a positive screen are confirmed and treated promptly, and the newborn is protected through treatment during pregnancy.

## 임상 시나리오

Choosing a Screening TestSensitivity within confirmation capacity
The infection is curable and serious if missed, so choose the most sensitive test whose positives fit within 150 confirmations.

With 50 infected and 950 uninfected: positives = sensitivity × 50 + (1 − specificity) × 950.

The 90%/90% test gives 140 positives and misses 5; the 98%/80% test gives 239, over capacity.

CautionAt low prevalence, small drops in specificity add many false positives; calculate before choosing.

## 핵심 개념

- **Sensitivity** — The proportion of people with the disease whom the test correctly identifies as positive.
- **Specificity** — The proportion of people without the disease whom the test correctly identifies as negative.
- **False positive** — A positive screen in a person who does not have the disease.
- **Confirmatory test** — A more accurate test used to verify a positive screening result.

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