Test Positivity Rate Calculator
What is Test Positivity Rate?
The Test Positivity Rate (TPR), often referred to simply as the "positivity rate," is a critical epidemiological metric used to assess the current level of disease transmission within a community and the adequacy of testing efforts. It represents the percentage of all diagnostic tests conducted that produce a positive result.
Public health organizations, such as the World Health Organization (WHO) and the CDC, utilize this percentage to make decisions regarding lockdowns, reopening strategies, and resource allocation during infectious disease outbreaks.
How to Calculate Test Positivity Rate
The calculation for the test positivity rate is straightforward. It requires two specific data points within a set timeframe (usually daily or a 7-day rolling average):
Example: If a laboratory processes 1,000 tests in a single day and 50 of them come back positive:
- Total Tests: 1,000
- Positive Tests: 50
- Calculation: (50 / 1000) × 100 = 5%
Interpreting the Results
Understanding what the number means is crucial for public health response. While thresholds can vary by pathogen and jurisdiction, general guidelines are as follows:
Low Positivity (< 5%)
According to the WHO, a positivity rate of less than 5% generally indicates that the spread of the infection is under control. It also suggests that testing capacity is sufficient to detect most active cases in the community.
Moderate Positivity (5% – 10%)
Rates in this range suggest moderate community spread. It acts as a warning sign that transmission is increasing or that testing may not be keeping up with the volume of infections.
High Positivity (> 10%)
A high positivity rate indicates widespread transmission. Crucially, it often implies that testing is insufficient; only the most symptomatic people are getting tested, meaning many mild or asymptomatic cases are likely going undetected.
Why Not Just Count Total Cases?
Looking at the raw number of positive cases alone can be misleading. If you conduct more tests, you will naturally find more cases. The positivity rate corrects for this testing volume. If the number of positive cases rises but the positivity rate remains stable or drops because testing increased significantly, the situation might actually be improving. Conversely, if testing numbers drop but the positivity rate spikes, it indicates a worsening outbreak.