The most common methods for determining a COVID-19 infection are the at-home rapid antigen test and the laboratory-based PCR test. These tests differ significantly in how a positive result is displayed or communicated, which can lead to confusion. Understanding what to look for is important for taking appropriate next steps to protect your health and the health of those around you. This guide clarifies the visual appearance of a positive result from an at-home test and explains how a lab test result is delivered.
Interpreting Rapid Antigen Test Lines
A positive rapid antigen test relies on the appearance of two distinct lines within the test window. These tests use a lateral flow immunoassay to detect specific viral proteins (antigens) from a nasal swab sample. A positive result requires the presence of both a Control line, marked ‘C’, and a Test line, marked ‘T’.
The Control (C) line must always appear; its presence confirms the test functioned correctly and the fluid moved through the strip as intended. The Test (T) line contains specialized antibodies that bind to SARS-CoV-2 viral proteins, causing a colored line to appear if the virus is present. If both the C line and the T line are visible, the result is positive, confirming viral proteins were detected.
If the Test line appears faint or blurry, it must still be interpreted as a positive result. Even the slightest visible colored line in the ‘T’ region indicates a positive result. The faintness may reflect a lower concentration of viral proteins in the sample. Regardless of the line’s darkness, a confirmed positive result means you should take precautions to avoid spreading the virus.
Recognizing an Invalid Test
An invalid test is a result that cannot be trusted, and it must be distinguished from a true negative result. An invalid test occurs when the Control (C) line fails to appear in the designated window, regardless of whether the Test (T) line is present. The absence of the C line signals a procedural malfunction, indicating the test did not run correctly.
Invalid results can happen due to various factors, such as not following instructions precisely, using incorrect amounts of solution, or a faulty test strip. Reading the result outside of the designated time window, often 15 to 30 minutes, is another common cause of failure. If your test returns an invalid result, you must repeat the testing process with a brand-new kit.
How PCR Test Results Are Communicated
The laboratory-based Polymerase Chain Reaction (PCR) test does not produce a visual line on a strip, unlike rapid antigen tests. PCR tests are processed in a clinical lab and detect the virus’s genetic material (RNA) using a highly sensitive method. Results are typically communicated to the patient through secure electronic means, such as email, text message, or a patient portal.
A positive PCR result is delivered as a text report using specific lab terminology. Look for language such as “Detected,” “Positive,” or “SARS-CoV-2 RNA Detected” in the result report. This confirms the test successfully identified the genetic sequences specific to the virus in your sample. While a report may include a Cycle Threshold (Ct) value, the overall interpretation will be summarized as positive.
Immediate Steps After a Positive Result
Upon receiving a positive result from either a rapid antigen test or a PCR test, the immediate next step is to begin isolating at home. Public health guidance recommends isolating for at least five full days, starting from the onset of symptoms or the day the test was taken if asymptomatic. This isolation period reduces the risk of transmission to others.
During isolation, monitor your symptoms and stay away from other household members as much as possible. Ideally, use a separate bathroom and avoid sharing personal items. Inform recent close contacts so they can monitor for symptoms and consider testing. Seek medical attention immediately if you experience emergency warning signs, such as difficulty breathing or persistent chest pain.