A false positive COVID-19 test occurs when an individual receives a positive result, indicating the presence of the SARS-CoV-2 virus, even though they are not truly infected. Understanding how these tests work, the rarity of false positives, and contributing factors helps in navigating a positive test outcome.
How COVID Tests Work
Two primary types of COVID-19 tests are widely used: PCR tests and rapid antigen tests. Each operates on a distinct principle to detect the virus.
PCR (Polymerase Chain Reaction) tests are highly sensitive, designed to detect the genetic material (RNA) of the SARS-CoV-2 virus. A sample, typically collected via a nasal swab, undergoes a laboratory process where the viral RNA is converted into DNA. This DNA is then repeatedly copied, or amplified, millions of times until even minute traces of the virus become detectable.
Rapid antigen tests, often used for at-home testing, work by identifying specific proteins, known as antigens, found on the surface of the virus. These tests utilize a lateral flow technology, where a sample is applied to a strip containing antibodies. If viral antigens are present in the sample, they bind to these antibodies, triggering a visual line on the test strip, usually within 15 to 30 minutes.
Understanding False Positives
False positive COVID-19 test results are uncommon for both PCR and rapid antigen tests. Their frequency is low compared to true positive results. For rapid antigen tests, reported false positive rates can be less than one percent. One study from 2022 estimated that 0.05% of positive tests were false positives, while another found that 1.7% of individuals in a large cohort experienced at least one false positive rapid antigen test.
PCR tests also have a low rate of false positives, though this possibility increases in settings where the virus prevalence is low, or when testing asymptomatic individuals. False negatives, where a person is infected but tests negative, are more common with rapid antigen tests than false positives. Receiving a false positive result can lead to unnecessary isolation, delays in other medical care, or transfer to a COVID-19 treatment area. Such outcomes can also contribute to an overestimation of the virus’s incidence within a community.
Factors Contributing to False Positives
Several factors can contribute to a false positive COVID-19 test result, from issues in the testing process to specific characteristics of the individual being tested. One significant cause, particularly for laboratory-based PCR tests, is contamination, which can occur if viral genetic material from highly positive samples, reagents, or laboratory equipment inadvertently contaminates other samples. Rigorous laboratory procedures minimize this risk.
Improper sample collection or handling can also lead to inaccurate results, especially with at-home rapid antigen tests. Not following manufacturer’s instructions precisely, such as reading the result outside the recommended time window, can produce an erroneous positive. Issues with the test kit itself, like manufacturing errors, have caused false positives. Some research indicates specific test strip lots may exhibit higher false positive rates.
Certain individual factors can also play a role; for instance, some studies have noted a higher prevalence of persistent false positive rapid antigen tests among women and individuals with autoimmune disorders. Cross-reactivity, where antibodies in the test interact with components in the person’s system (like rheumatoid factor), explains cases. Incorrect reagents or buffers, or even intentional contamination of samples with substances like fruit juice, can interfere with test chemistry, causing false positive outcomes. In situations where community COVID-19 prevalence is very low, the likelihood of a positive result being a false positive increases proportionally.
What to Do After a Positive Test
Upon receiving a positive COVID-19 test result, the first step is to follow public health guidelines, involving isolating yourself from others for at least five days. This helps prevent transmission of the virus, regardless of whether you suspect it’s a false positive. Isolation is recommended even if vaccinated or previously had COVID-19.
Contact a healthcare provider to discuss your positive test result and any symptoms. They can offer guidance on managing symptoms and determine if treatments are appropriate. If symptoms worsen, such as difficulty breathing, seek immediate medical attention.
Retesting is generally not required to end isolation if symptomatic and symptoms are improving. However, if asymptomatic and test positive, or if you strongly suspect a false positive, consulting your healthcare provider about a confirmatory test, potentially a PCR test after a rapid antigen test. If you retest after day five and still receive a positive result, continue isolation and retest every 24 to 48 hours until a negative result is obtained, or complete a full ten days of isolation. The Centers for Disease Control and Prevention (CDC) does not recommend retesting for returning to work or other activities if symptoms have cleared or are improving.