Calculating SARS-CoV-2 Laboratory Test Percent Positivity: CDC Methods and Considerations for Comparisons and Interpretation
SARS-CoV-2 test result data are reported to state, local, and territorial health departments by public health laboratories, clinical laboratories, some large chain drug stores, and other testing entities as required by state and local laws. Data are then transmitted from states to CDC as required by the Coronavirus Aid, Relief, and Economic Security (CARES) Act. (See CARES Act Section 18115pdf iconexternal icon.) All information is de-identified to protect individual privacy. CDC and HHS process and analyze the data, and results are made available in HHS Protect for use by the federal response. CDC makes SARS-CoV-2 test result data available to the public on CDC’s COVID Data Tracker website. Data can be downloaded for analysis from healthdata.govexternal icon.
Using Percent Positivity Calculation for Public Health Surveillance
CDC has a formula for calculating percent positivity of laboratory test results.
- Obtain the numerator — the number of positive SARS-CoV-2 Nucleic Acid Amplification Test (NAAT) results.
- Obtain the denominator — the total number of SARS-CoV-2 NAAT test results, both positive and negative.
- Divide the numerator by the denominator.
- Multiply the result by 100 to obtain the percentage.
Another way to look at this formula is positive tests/total tests x 100. When calculating percent positivity, CDC excludes antigen and antibody tests from both the numerator and denominator.
In general, laboratory test percent positivity as reported by CDC has represented the percentage of all SARS-CoV-2 NAATs conducted that are positive. While calculation methods among jurisdictions can differ, percent positivity has provided insights into transmission of SARS-CoV-2 within a geographic area (e.g., national, regional, state, county).
Percent positivity can vary depending on the volume of testing and the population tested. A high NAAT percent positivity occurs when many of the test results among those being tested and reported in a community are positive. This can mean the following:
- There are widespread infections in the community tested.
- Only those at greatest risk of infection within a community are being tested.
- There are reporting processes or delays that skew the results. An example would include prioritizing reporting of positive test results over negative results.
Varying Methods to Calculate Percent Positivity
Methods used by jurisdictions to calculate percent positivity can differ, including the following:
- Differences in the numerators or denominators used — such as tests/tests, people/tests, people/people. State, local, and territorial health departments have access to personal identifiers in their datasets. This allows them to identify and de-duplicate people with multiple positive tests. CDC, however, is unable to perform this function and can only calculate tests/tests.
- Differences in the timeframe for including data (a seven-day versus a 14-day rolling average, for example). Other differences can include the date used — specimen collection date, test date, result date — to assign tests to specific timeframes.
- Differences in the inclusion or exclusion of antigen test results, which are inconsistently reported. CDC recommends reporting positive results for SARS-CoV-2 antigen tests as a separate metric.
- Differences in inclusion of screening test results. Screening has been increasing. Sometimes the criteria for testing — routine screening versus diagnostic testing of symptomatic people — is not known. This affects how the meaning of percent positivity results are interpreted. Learn more about screening tests at Testing Strategies for SARS-CoV-2.
- Differences in how test results are assigned to jurisdictions, including by the person’s place of residence, the provider’s clinic location, the location the test specimen was collected, or the location of the laboratory performing the test.
Assigning Test Results
CDC assigns test result data provided by state, local, and territorial health departments by date. Assignment is determined by the first date available, in this order:
- Test date (day the laboratory had a test result)
- Result date (day the laboratory sent the result to the requestor)
- Specimen received date (day accessioned by laboratory)
- Specimen collection date (day specimen was collected from the patient)
Geographic assignment is based on the first location available, in this order:
- Patient residence location
- Provider facility location
- Ordering facility location
- Performing organization location
Reporting Percent Positivity for SARS-CoV-2
CDC provides data on NAAT percent positivity at the county and national level on the CDC COVID Data Tracker website.
The Centers for Medicare and Medicaid Services publishes NAAT percent positivityexternal icon by county. This helps guide the frequency of COVID-19 screeningpdf iconexternal icon for long-term care facility residents and staff.
The White House Coronavirus Task Force reports NAAT percent positivity at the national, state, and county levels in the State Profile Report.
- When consistently applied across a jurisdiction, different methodologic choices for calculating percent positivity are useful for monitoring trends and magnitude of disease for surveillance purposes and public health decision-making.
- State, local, and territorial public health officials may consider adopting some or all of the methods outlined above if these methods suit their surveillance needs.