# Situation: A public health nurse in a rice-farming municipality reviews a published prospective cohort study of farmers before planning leptospirosis prevention with the local farmers' association. Using the same study (24 cases among 400 farmers who waded without boots; 6 cases among 600 farmers who did not), what percentage of leptospirosis among the farmers who waded without boots can be attributed to that exposure (attributable risk percent)? Round off to one decimal place.

> source: MyMerci (mymerci.kr)  
> url: https://mymerci.kr/pages/nclex_q.php?qn_id=627208  
> language: ko  
> subject: Nursing Practice I — Community Health Nursing

## 문제

Situation: A public health nurse in a rice-farming municipality reviews a published prospective cohort study of farmers before planning leptospirosis prevention with the local farmers' association.

Using the same study (24 cases among 400 farmers who waded without boots; 6 cases among 600 farmers who did not), what percentage of leptospirosis among the farmers who waded without boots can be attributed to that exposure (attributable risk percent)? Round off to one decimal place.

## 보기

1. 5.0%
2. 16.7%
3. 75.0%
4. 83.3% **✔ 정답**

**정답: 4**

## 해설

Attributable risk percent = (incidence in exposed − incidence in unexposed) ÷ incidence in exposed × 100. Incidence in exposed = 24 ÷ 400 = 0.06; in unexposed = 6 ÷ 600 = 0.01. AR% = (0.06 − 0.01) ÷ 0.06 × 100 = 0.05 ÷ 0.06 × 100 = 83.3%. About five of every six cases among farmers who waded without boots could be prevented by removing that exposure.

## 심화 해설

Attributable risk percent (AR%) is the proportion of disease cases among exposed individuals that would be eliminated if the exposure were removed. It answers a practical prevention question: Of all leptospirosis cases that occurred in farmers who waded without boots, how many were actually caused by that specific exposure?

Step 1 — Calculate incidence in each group

The cohort data provide two incidence rates that must be computed before any comparison.

| Group | Cases | Total farmers | Incidence |
| --- | --- | --- | --- |
| Exposed (waded without boots) | 24 | 400 | 0.06 (6.0%) |
| Unexposed (did not wade without boots) | 6 | 600 | 0.01 (1.0%) |

Step 2 — Apply the attributable risk percent formula

The formula subtracts the baseline risk seen in the unexposed group from the risk in the exposed group, then expresses that difference as a proportion of the exposed group’s total risk.

AR% = (incidence in exposed − incidence in unexposed) ÷ incidence in exposed × 100

AR% = (0.06 − 0.01) ÷ 0.06 × 100

AR% = 0.05 ÷ 0.06 × 100 = 83.3%

Step 3 — Interpret the result

An AR% of 83.3% means that about 83 out of every 100 leptospirosis cases among farmers who waded without boots can be attributed to that exposure. Equivalently, roughly five of every six cases in the exposed group would be expected to disappear if boot use during wading were fully implemented and effective.

Watch out! Attributable risk percent is not the same as relative risk or the overall proportion of cases in the population. AR% applies only to the exposed group and reflects the preventable fraction within that specific subgroup.

Key point! The unexposed group still had a 1.0% incidence, which represents the background risk from other routes of Leptospira exposure in a rice-farming environment — contaminated soil, water contact during other activities, or peridomestic animal reservoirs. The AR% calculation removes that background risk before estimating the effect of wading without boots.

Why this matters for leptospirosis prevention planning

Leptospirosis is a zoonosis transmitted through contact between broken skin or mucous membranes and water or soil contaminated by urine from infected reservoir animals, particularly rodents. Occupational exposure in agricultural settings is a well-documented risk factor because farmers routinely enter flooded fields, irrigation canals, and muddy terrain where Leptospira can survive for weeks to months in warm, moist conditions . A case-control study in northern Iran similarly identified agricultural work and contact with surface water as significant risk factors for infection . Population-based surveillance data from the same region confirm that occupational and environmental exposures drive a substantial portion of the disease burden . Longitudinal surveillance in Fujian, China further links leptospirosis incidence to agricultural practices and reservoir animal activity, reinforcing the importance of targeting modifiable occupational exposures .

For the public health nurse planning prevention with the farmers’ association, the AR% value provides a concrete, evidence-based message: the majority of leptospirosis cases in farmers who wade without protective footwear are attributable to that single modifiable behavior. This supports prioritizing boot distribution, subsidized protective equipment, and education on avoiding bare-skin water contact during field work. The calculation also demonstrates why interventions aimed at the exposed group — rather than the entire farming population — yield the greatest reduction in cases per unit of effort.

Common calculation errors to avoid

One frequent mistake is dividing by the total population instead of the exposed group’s incidence. Another is subtracting incidence rates incorrectly or forgetting to multiply by 100. The answer 16.7% arises from dividing the difference by the unexposed incidence, while 75.0% results from using the relative risk reduction formula incorrectly. Only 83.3% follows the standard AR% equation and matches the interpretation that five of six exposed cases are preventable by removing the exposure.

## 임상 시나리오

Attributable Risk Percent in Field PreventionEstimating preventable cases among exposed farmers
For leptospirosis prevention planning, attributable risk percent answers how many exposed cases would disappear if boot use during wading were fully implemented. It is calculated as (Ie − Iu) ÷ Ie × 100.

Using the cohort data, exposed incidence is 24/400 = 0.06 and unexposed incidence is 6/600 = 0.01. The AR% is (0.06 − 0.01) ÷ 0.06 × 100 = 83.3%.

Interpretation: about 83 of every 100 leptospirosis cases among farmers who waded without boots are attributable to that exposure, meaning roughly five of six cases could be prevented by removing it.

CautionAR% applies only to the exposed group. Do not confuse it with relative risk or the population attributable fraction, which answers different prevention questions.

## 핵심 개념

- **Attributable risk percent** — Proportion of disease among exposed individuals that would be eliminated if the exposure were removed; calculated as (Ie−Iu)/Ie×100.
- **Incidence in exposed** — New cases divided by the exposed population at risk; here 24/400 = 0.06.
- **Incidence in unexposed** — New cases divided by the unexposed population at risk; here 6/600 = 0.01.
- **Absolute risk difference** — The simple arithmetic difference between exposed and unexposed incidence; here 0.05.
- **Relative risk** — Ratio of exposed incidence to unexposed incidence; here 0.06/0.01 = 6.0.

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