Calculate Annual Parasite Index, total parasite incidences, or total population from two known values with unit options per 100 to per 10,000.
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Annual Parasite Index Formula
The annual parasite index, or API, is usually calculated as the number of confirmed parasite incidences per population over one year. It is most often reported per 1,000 people, but the calculator also supports per 100 and per 10,000.
- APIN = annual parasite index for the selected population basis
- TPI = total parasite incidences during the year
- TP = total population at risk or total population being measured
- N = population basis, such as 100, 1,000, or 10,000
If you enter total parasite incidences and total population, the calculator finds the annual parasite index. If you enter the annual parasite index and total population, it finds the number of parasite incidences. If you enter total parasite incidences and the annual parasite index, it estimates the total population represented by those values.
API Units and Result Interpretation
Use the same population basis when comparing API values between places or years. Per 1,000 is the common format for malaria annual parasite index reporting.
| Selected API unit | Population basis | Formula used |
|---|---|---|
| Per 100 | 100 people | API = (TPI / TP) × 100 |
| Per 1,000 | 1,000 people | API = (TPI / TP) × 1,000 |
| Per 10,000 | 10,000 people | API = (TPI / TP) × 10,000 |
| API per 1,000 | Simple interpretation |
|---|---|
| Less than 1 | Very low recorded parasite incidence |
| 1 to 10 | Low to moderate recorded incidence |
| More than 10 | Higher recorded incidence that may need closer review |
Example Calculations
Example 1: Calculate annual parasite index
You have 250 parasite incidences in a population of 50,000. Calculate API per 1,000.
The annual parasite index is 5 per 1,000 people.
Example 2: Calculate total parasite incidences
You know the API is 2.5 per 1,000 and the population is 120,000.
The total parasite incidences are 300.
FAQ
What does annual parasite index mean?
Annual parasite index measures the number of confirmed parasite incidences in a population during one year. For malaria reporting, it is commonly shown as cases per 1,000 people per year.
What population should you use?
Use the population covered by the parasite incidence count. If the incidence count is for a district, use that district’s population. If it is for a village, health area, or study group, use the matching population for that same area and time period.
Can API be compared between two areas?
Yes, but only when the values are calculated with the same population basis and similar case definitions. For example, compare per 1,000 with per 1,000. If one value is per 100 and another is per 1,000, convert them to the same basis before comparing.
