In order to effectively prevent and control the development of the new crown epidemic, comprehensive nucleic acid or serological testing has been carried out for high-risk groups. With the expansion of the detection range, various nucleic acid and serological results have appeared, and it is necessary to scientifically judge the infectiousness and infection stage of patients in combination with clinical manifestations.

The new coronavirus is infectious from the start of the incubation period after infection: about one week after infection, the patient develops respiratory tract-based symptoms, the viral nucleic acid load reaches a peak, and the infectivity also peaks; at the same time, a large number of viral antigens are secreted to induce antibody expression. About 2 weeks after infection, serum IgM antibodies increased rapidly, IgG antibodies also began to appear, and viral nucleic acids and antigens began to decline. About 3 weeks after infection, the level of IgM antibody reached its peak, and the IgG antibody increased steadily. At the same time, the viral nucleic acid and antigen rapidly dropped below the detection level, and the infectivity gradually disappeared. Four weeks after the infection, the patient was in convalescence, the IgM antibody began to decline, the IgG antibody steadily increased, the nucleic acid test was negative, and the infectivity disappeared. Current research has found that the duration of IgM antibodies of the new crown is generally about 1 to 2 weeks, and the IgG antibodies can last for a longer time.

The current diagnosis standard is still based on the nucleic acid results. In the early stage of the disease, the nucleic acid is positive and the infectivity is strong; after 3 weeks of infection, the viral nucleic acid load gradually decreases and the infectivity disappears. However, a negative nucleic acid test in the early stage of infection cannot completely rule out infection, and the method of sample collection may affect the nucleic acid test results. A small number of patients have repeatedly tested negative for throat swabs in the early stage of infection, but when lung imaging and symptoms progress, changing test samples such as bronchoalveolar lavage fluid can increase the positive rate. Nucleic acids detected late in infection may only be fragments and not infectious. Antibody detection also has limitations, and there may be false positive antibodies, which still need to be comprehensively judged based on clinical manifestations and nucleic acid test results.

Therefore, the possible nucleic acid and serological results are summarized as follows:

IgG negative, IgM negative, nucleic acid positive - patients in the early stage of infection, highly infectious

IgG-negative, IgM-positive, nucleic acid-positive - those diagnosed at the early stage of the disease are highly contagious

IgG-positive, IgM-positive, nucleic acid-positive - those diagnosed in the middle and late stages of the disease are contagious

IgG positive, IgM negative, nucleic acid positive - patients with late infection (virus carriers), low infectivity

IgG positive, IgM negative, nucleic acid negative - recovered patients, non-infectious

IgG positive, IgM positive, nucleic acid negative - patients at the onset stage (recheck for nucleic acid required), may be infectious

IgG negative, IgM positive, nucleic acid negative - patients in the early stage of infection (recheck for nucleic acid), may be infectious

IgG negative, IgM negative, nucleic acid negative - healthy, uninfected, non-infectious

(The author of this article is an infectious disease expert, an expert of the National New Coronary Pneumonia Medical Treatment Expert Group, and an expert of the rear support team of the Overseas Anti-epidemic Medical Expert Group)

(Organized by our reporter Sun Guogen)