A common concern regarding COVID-19 tests is whether a bloody nose can affect their accuracy. Understanding how these tests work and how blood might interfere can help ensure reliable results.
How COVID Tests Work
COVID-19 tests, particularly those involving nasal swabs, rely on collecting an adequate sample from the nasal passages. Rapid antigen tests detect specific proteins from the SARS-CoV-2 virus in nasal secretions. These tests involve inserting a soft-tipped swab just inside the nostrils, then rotating it to collect mucus, which is applied to a testing device containing chemical reagents. This technology, known as lateral flow immunoassay, involves the sample flowing along a strip where viral antigens, if present, bind to antibodies, triggering a visible line.
PCR tests, a more sensitive method, detect the virus’s genetic material (RNA). A swab collects mucus or secretions, often from deeper in the nose, and the sample is then sent to a lab for analysis. The RNA is extracted, converted to DNA, and then amplified through a process called polymerase chain reaction to detect even small amounts of viral genetic material. Proper sample collection is crucial for accurate results, as insufficient viral material can lead to inaccuracies.
Does Blood Affect Test Results?
The presence of blood on a nasal swab can affect the accuracy of COVID-19 test results. For rapid antigen tests, blood may physically interfere with the flow of the sample along the test strip, potentially leading to an invalid or inaccurate result. Some manufacturers advise against using blood-contaminated specimens, as it can interfere with sample flow. While some studies found blood did not cause false positives in rapid antigen tests, other sources indicate it can lead to inaccurate results, including false positives.
For PCR tests, blood contamination can also cause issues, though the impact might differ. Blood can interfere with the chemical reagents used in the PCR process or dilute the sample, making it harder to detect viral RNA if present. This interference can result in false negative results, where the test incorrectly indicates the virus is not present even if an infection exists.
Steps to Take When Testing with a Bloody Nose
If you experience a bloody nose before or during a COVID-19 test, several steps can help ensure accurate results. The most important action is to wait for the bleeding to stop before proceeding with the test. Gently blowing your nose to clear out any clotted blood can be helpful, but avoid aggressive blowing that might restart bleeding. If you are performing a self-test and the swab becomes bloody, discard that swab and use a new one, swabbing the non-bleeding nostril if possible.
For significant or persistent bleeding, delay testing until the nosebleed has fully resolved. Informing a healthcare provider about the nosebleed, especially if they are administering the test, allows them to account for potential interference or suggest alternative testing methods, such as a saliva test if available. If you suspect the bleeding significantly compromised the sample, consider retesting after the bleeding has completely stopped to ensure a reliable result.