# Situation: The nurse at a Rural Health Unit (RHU) joins a team evaluating a point-of-care hemoglobin test for anemia in pregnant women. The laboratory hemoglobin test is the reference standard. The RHU is considering a second device with a sensitivity of 90% and a specificity of 80% to screen 1,000 adolescent girls, among whom the prevalence of anemia is 10%. Assuming sensitivity and specificity stay the same in this group, what positive predictive value (PPV) should the nurse expect? Round off to one decimal place.

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> subject: Nursing Practice I — Community Health Nursing

## 문제

Situation: The nurse at a Rural Health Unit (RHU) joins a team evaluating a point-of-care hemoglobin test for anemia in pregnant women. The laboratory hemoglobin test is the reference standard.

The RHU is considering a second device with a sensitivity of 90% and a specificity of 80% to screen 1,000 adolescent girls, among whom the prevalence of anemia is 10%. Assuming sensitivity and specificity stay the same in this group, what positive predictive value (PPV) should the nurse expect? Round off to one decimal place.

## 보기

1. 9.0%
2. 31.0%
3. 33.3% **✔ 정답**
4. 90.0%

**정답: 3**

## 해설

With 10% prevalence, 100 girls have anemia and 900 do not. True positives = 0.90 × 100 = 90; true negatives = 0.80 × 900 = 720, so false positives = 900 − 720 = 180. PPV = 90 ÷ (90 + 180) × 100 = 90 ÷ 270 × 100 = 33.3%. Because PPV depends on prevalence, a good test gives many false positives when the condition is uncommon.

## 심화 해설

Core concept: Predictive value depends on prevalence

This question asks you to calculate the positive predictive value (PPV) of a screening test when the prevalence of the target condition is known. The PPV is the probability that a person with a positive test result actually has the disease. Unlike sensitivity and specificity, which describe the test itself, PPV changes with the population being screened.

Here, the prevalence of anemia is 10%. In a group of 1,000 adolescent girls, that means 100 girls truly have anemia and 900 do not. The device has a sensitivity of 90%, so it correctly identifies 90 of the 100 anemic girls as positive (true positives). The specificity is 80%, meaning it correctly identifies 720 of the 900 non-anemic girls as negative (true negatives). The remaining 180 non-anemic girls are incorrectly labeled positive (false positives).

PPV is calculated as true positives divided by all positive results. Therefore, PPV = 90 ÷ (90 + 180) × 100 = 33.3%. Even with a sensitivity of 90%, only about one-third of positive results represent true anemia when the prevalence is low.

Key point! The large number of false positives arises because the specificity of 80% is applied to a very large group of healthy girls. When a condition is uncommon, even a small false-positive rate produces many false-positive results, diluting the PPV.

Why this matters for point-of-care hemoglobin screening

Point-of-care (POC) hemoglobin devices are attractive in rural or resource-limited settings because they provide rapid results without a full laboratory. The study by Sobhy et al. [1] reviewed the accuracy of on-site tests for anemia during prenatal care and noted that access to laboratory facilities is limited in low- and middle-income countries, making on-site testing a practical option. However, the clinical value of such screening depends not only on how well the device measures hemoglobin but also on how the test result is interpreted in a specific population.

Ssuuna et al.  evaluated four POC hemoglobin devices used in routine HIV and maternity care in Uganda against a laboratory gold standard. Their work highlights that multiple factors can affect POC device performance in real-world settings. This reinforces the idea that a device with fixed sensitivity and specificity will produce different predictive values depending on the prevalence of anemia in the group being screened. In a high-prevalence antenatal clinic, a positive POC result is more likely to be a true positive than in a low-prevalence adolescent screening program.

Jegadeesan et al.  compared clinical pallor at different anatomical sites for detecting anemia in pregnant women, using a portable hemoglobin device as the reference standard. Their study illustrates a broader point: in primary care, clinicians often rely on simple, low-cost screening methods, but the interpretation of any screening result must account for the underlying prevalence of anemia in that setting.

| Term | Definition | Depends on prevalence? |
| --- | --- | --- |
| Sensitivity | Proportion of truly anemic individuals who test positive | No |
| Specificity | Proportion of truly non-anemic individuals who test negative | No |
| PPV | Proportion of positive results that are true positives | Yes |
| NPV | Proportion of negative results that are true negatives | Yes |

Watch out! A common error is to assume that a test with high sensitivity will have a high PPV. That is only true when the prevalence is high. In this scenario, the prevalence of 10% is low enough that the PPV falls to 33.3% despite the 90% sensitivity.

When interpreting a positive POC hemoglobin result in a low-prevalence population, the nurse should recognize that many positive results will be false positives. This has practical implications: a positive screening result in an adolescent girl with a low pretest probability of anemia should be confirmed with a laboratory hemoglobin measurement before initiating treatment or referral.

The calculation also demonstrates why screening programs must consider the target population. Screening 1,000 adolescent girls with a 10% anemia prevalence will generate 270 positive results, but only 90 of those girls truly have anemia. The remaining 180 would undergo unnecessary follow-up testing or potentially receive iron supplementation they do not need.
References (research sources)

- [1]Accuracy of on-site tests to detect anemia during prenatal care.Research articleSobhy S, Rogozinska E, Khan KS (2017) · DOI: 10.1002/ijgo.12289

## 임상 시나리오

PPV in Low-Prevalence ScreeningWhy a good test can still yield many false positives
When screening for anemia in a population with 10% prevalence, a device with 90% sensitivity and 80% specificity produces a positive predictive value of only 33.3%. This means only about one in three positive results is a true case.

The low PPV occurs because the 20% false-positive rate is applied to the large group of 900 non-anemic girls, generating 180 false positives. These dilute the 90 true positives.

CautionDo not interpret a positive point-of-care hemoglobin result as definitive in low-prevalence settings. Confirm with a laboratory standard before initiating treatment.

## 핵심 개념

- **Positive Predictive Value (PPV)** — Probability that a person with a positive test result truly has the disease; varies with disease prevalence.
- **Sensitivity** — Ability of a test to correctly identify those with the disease (true positive rate).
- **Specificity** — Ability of a test to correctly identify those without the disease (true negative rate).
- **Prevalence** — Proportion of a population that has the condition at a given time.
- **False Positive** — A person without the disease who receives a positive test result.

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