Antigen Testing: Finding the "Fingerprint" of Pathogens
The core of antigen testing is to determine whether a specific pathogen is currently present in the human body. Here, "antigen" refers to a part of the protein structure on the surface of a virus or bacterium, which serves as a "target" for the immune system to recognize and attack. By detecting these antigens, it can be determined whether the human body is infected with a certain pathogen.
Taking the novel coronavirus as an example, antigen testing typically detects its surface "spike protein" or "nucleocapsid protein." If these proteins are detected in a person's nasopharyngeal swab sample, it indicates that the virus is replicating in the body, indicating an active infection.
Antigen testing has the following characteristics:
Fast detection speed: Results can be obtained within 15-30 minutes, suitable for large-scale initial screening;
Simple operation: Can use test paper cards, and some products can be operated at home;
Relatively low sensitivity: May produce "false negatives" during the early stages of infection or when the viral load is low;
High specificity: Once positive, it can generally confirm the presence of viral antigens.
Antigen testing cannot provide complete information about the course of the disease. For example, it cannot determine whether the infected person has just been infected, is in the recovery phase, or has been infected before. Therefore, it is more of a quick snapshot tool for "whether there is a virus at this moment," commonly used in entry and exit screening, school checks, community epidemiological investigations, and other scenarios.
Antibody Testing: Finding the "Battle Traces" of the Immune System
Antibody testing belongs to another line of thinking: it does not directly look for the virus but checks whether the body has responded to a certain virus. In other words, it assesses not "whether there is a virus," but "whether there is a response to the virus."
Antibodies are proteins produced by B cells in the immune system after recognizing a pathogen, used to neutralize the virus and establish immune memory. Common antibody tests include two types:
IgM antibodies: Appear early in infection, indicating recent exposure to the virus;
IgG antibodies: Produced later in infection or after vaccination, representing long-term immune memory.
For example, during the COVID-19 pandemic, if a person has a positive IgG antibody and a negative IgM antibody, it usually indicates that the person has been infected in the past or vaccinated and is now recovered or in an immune state. If both are positive, it may indicate that the infection is still active, requiring further clinical symptom assessment.
The advantages and limitations of antibody testing include:
An effective tool for assessing past infections or vaccination history;
Cannot be used to diagnose current infections, as antibody production has a certain lag;
Possible cross-reactivity, for example, antibodies between different coronaviruses may interfere with each other;
Cannot determine whether immunity is sufficient for protection, i.e., positive antibodies do not equal sufficient immunity.
In large-scale epidemiological surveys, antibody testing is often used to assess the immune level of the population, such as how many people in a certain area have been infected and the effectiveness of vaccination.
Applicable Scenarios and Combined Use of Two Tests
Antigen testing and antibody testing often play different roles in medical practice; the former focuses on discovering whether the virus is currently present, while the latter is used to review whether there has been contact or an immune response. In different stages and populations, doctors will choose different strategies based on the purpose of the testing.
Here are several typical scenarios:
Initial diagnosis of patients with fever and cough: Prioritize antigen testing to determine if there is a current infection;
Screening of travelers returning from epidemic areas: Antigen testing is used for quick assessment of whether they are "carriers," while antibody testing is used to understand past infections or immune status;
Evaluation of vaccine effectiveness: Focus on IgG antibodies to determine whether protective immunity has been generated;
Plasma donation by recovered patients: Need to confirm the presence of neutralizing antibodies in their body;
Epidemiological investigation: Antibody testing is widely used to understand the proportion of asymptomatic infections and the coverage of immunity.
In addition, in certain complex situations, doctors may recommend the combined use of both tests to obtain more comprehensive information. For example, an asymptomatic person may test negative for antigens but positive for IgM antibodies, suggesting they may be in the late stage of infection or have a very low viral load. In this case, further nucleic acid testing may be recommended to confirm the status.
How to Correctly Interpret Test Results?
What do positive antigen and antibody results really mean? The public may easily misunderstand the interpretation of test results. Here are some common meanings of various results:
It is important to emphasize that any test result should not be judged independently of symptoms and epidemiological context. For example, a positive antibody result does not mean "permanent immunity," and a negative antigen result does not indicate "absolute health."
Therefore, after test results are released, it is recommended to communicate promptly with a doctor and analyze them in conjunction with personal symptoms, contact history, and vaccination history to avoid over-interpretation or incorrect decisions.
Real Cases: The Stories Behind Test Results
Case 1: Positive Antigen, Mistaken for Diagnosis
Mr. Zhang is an employee of a foreign company. During routine antigen screening, he tested positive and immediately self-isolated. However, subsequent PCR nucleic acid testing was negative, and both IgM and IgG antibodies were also negative. It was ultimately confirmed as a false positive result, possibly due to reagent contamination or operational error.
This case reminds the public that while antigen testing is convenient, its accuracy may be affected by multiple factors, and results need to be further confirmed by nucleic acid testing.
Case 2: Positive Antibody, Mistaken for Immunity
Ms. Li had received the COVID-19 vaccine and found her IgG positive during testing, believing she was "already immune," and thus relaxed her protective measures in social settings. Two months later, she still contracted the virus, but due to vaccination, her symptoms were mild, and she did not require hospitalization.
This indicates that a positive antibody result represents a certain level of protection but does not completely prevent infection, especially in the face of variants.
Case 3: Risks After Double Negative Results
During an overseas testing, a traveler tested negative for both antigen and antibody and boarded the plane smoothly. However, five days later, he developed fever symptoms and was diagnosed with COVID-19. Retrospective analysis revealed that his infection occurred very early, and the viral load was too low to be captured during testing.
This incident highlights that any testing method may have a "window period" issue, and dynamic judgment should be made in conjunction with the timing and symptoms to avoid blindly relying on a single test.
Changes in Testing Awareness in the Post-Pandemic Era
As the COVID-19 pandemic enters a chronic coexistence phase, testing methods will no longer be used solely for lockdowns and screening but will also become part of daily health management. The public's understanding of antigens and antibodies also needs to shift from "fear of results" to "scientific understanding."
First, it is essential to clarify that each testing method has its applicable boundaries and should not be generalized. Antigen testing does not equal diagnosis, and antibody testing does not equal immunity.
Second, it should be advocated to replace single indicators with combined judgments, conducting cross-analysis of multiple tests when conditions allow, which helps to accurately assess individual status.
Third, the value of testing lies not in "scaring" but in identifying risks, optimizing decisions, and ensuring reasonable protection. Timely testing and accurate interpretation can help individuals make appropriate social arrangements, treatment choices, and health strategies.
Finally, testing is not the ultimate goal but a tool. In the context of comprehensive use of vaccines, treatments, and daily protective measures, the role of testing will focus more on refined management rather than a simple "positive = risk."