As respiratory viruses circulate, a common concern is whether an influenza infection can mistakenly trigger a positive COVID-19 test result. Since the symptoms of both illnesses are similar, people wonder if testing technology can truly differentiate them. Diagnostic tools are designed to provide clear and specific identification of the responsible pathogen. This article examines the technical answer, clarifies the confusion created by symptom overlap, and explains the real reasons why a test result might be inaccurate.
The Specificity of Viral Testing
The short answer is that influenza does not cause a positive COVID-19 test result. This high accuracy is due to the fundamental design of diagnostic tests, which rely on high specificity. Testing methods, including rapid antigen tests and laboratory-based polymerase chain reaction (PCR) tests, are engineered to look for unique biological markers.
PCR Tests
PCR tests search for specific genetic sequences, or nucleic acids, exclusive to the SARS-CoV-2 virus. Before approval, tests are rigorously validated against a panel of other common respiratory viruses, including influenza A and B. This validation confirms the assay’s probes only bind to the target sequence of SARS-CoV-2, preventing influenza from being confused with COVID-19.
Antigen Tests
Antigen tests operate on a similar principle but target specific surface proteins on the virus. These rapid tests contain antibodies designed to physically latch onto a protein, such as the nucleocapsid protein of SARS-CoV-2. The protein structures of influenza and SARS-CoV-2 are distinct enough that the SARS-CoV-2 antibody will not bind to the influenza protein.
Symptom Overlap and Co-Infection
The confusion surrounding test results is understandable because the clinical presentation of influenza and COVID-19 can be nearly identical. Both are respiratory illnesses that frequently cause fever, cough, fatigue, sore throat, and muscle aches. It is impossible to reliably distinguish between the two infections based on symptoms alone, requiring diagnostic testing to confirm the cause.
The necessity of differentiation is compounded by the possibility of co-infection, meaning a person can be sick with both influenza and COVID-19 simultaneously. Knowing the exact pathogen is important because the antiviral treatments for influenza and COVID-19 are different. Antiviral medications for both illnesses are most effective when started shortly after symptoms begin.
This diagnostic challenge led to the widespread use of combined or multiplex testing. Multiplex assays simultaneously check a single nasal sample for the presence of influenza A, influenza B, and SARS-CoV-2. These co-tests streamline the diagnostic process, allowing healthcare providers to quickly identify the specific virus or viruses present and initiate the correct treatment plan.
When COVID-19 Test Results Are Incorrect
While influenza does not cause a false positive, test results can occasionally be incorrect for other reasons. A false positive result means the test indicates infection when the person is not actually infected. Though rare with laboratory-based PCR tests, this can happen if a sample is cross-contaminated during processing.
For at-home rapid antigen tests, a false positive may occur due to user error, such as reading the result after the specified time window. In low-prevalence settings, a positive result has a higher chance of being a false positive. These causes relate to procedural or environmental factors, not biological cross-reactivity with the flu virus.
A more common issue is a false negative result, where the test misses an infection that is present. False negatives are more frequent with rapid antigen tests because they are less sensitive than molecular PCR tests. Typical causes include poor sample collection technique or testing too early or too late in the course of the infection, resulting in a viral load too low for detection.