Laboratory Examination and Imaging Examination

Routine Examination

In the early stage of the disease, the total number of leukocytes in the peripheral blood is normal or decreased, and the lymphocyte count is reduced. Some patients may have increased liver enzymes, lactate dehydrogenase (LDH), creatine kinase, and myoglobin; some critically severe patients may have increased troponin. C-reactive protein (CRP) and erythrocyte sedimentation rate are elevated in most patients, and procalcitonin is normal. In severe cases, D-dimer is increased, and peripheral blood lymphocytes are progressively decreased. Inflammatory factors are often increased in severe and critically severe patients. The ratio of neutro-phils/lymphocytes ratio is helpful to determine the severity of the disease.

Virological Testing

Common virological testing includes virus nucleic acid detection and serological tests.

Virus Nucleic Acid Testing Testing Methods

Using RT-PCR and/or Next-Generation Sequencing (NGS) methods, 2019-nCoV nucleic acid can be detected in nasopharyngeal swabs, sputum, and other lower respiratory secretions, blood, feces, and other specimens. It is more accurate to detect lower respiratory tract specimens (sputum or airway extracts). Specimens should be sent for examination as soon as possible after collection.

When collecting samples from the oral, nasopharyngeal swabs, and other parts of the upper respiratory tract, it is recommended to collect nasopharyngeal swabs for virus nucleic acid testing. In order to improve the detection accuracy, it is recommended to collect multiple samples (oropharyngeal swabs, nasopharyngeal swabs, nasal swabs, etc.) from the same patient for combined detection. For suspected patients with digestive tract symptoms, stool or anal swabs can be collected at the same time of testing. Reasons for False Negative Results and Countermeasures The advantage of nucleic acid testing is that it shortens the window period of infection detection and can detect infected persons early. The false negative of nucleic acid testing may be due to poor quality specimens. The possible influencing factors include improper collection, preservation, transportation, and handling of specimens, virus mutation, PCR inhibition, and so on. In addition, as 2019-nCoV is a single-stranded positive-stranded RNA virus with a large molecular weight, it is easy to mutate. Nucleic acid sequence mutations may occur in the process of transmission. If it is located in the primer binding area for nucleic acid amplification, false negative results will occur. It is suggested that multiple nucleic acid regions should be amplified to effectively avoid the influence of nucleic acid variation on the detection results. When the nucleic acid testing result is negative, only the negative result of this testing can be reported. The 2019-nCoV infection cannot be ruled out, and repeated confirmation is required.

Serological Test

After the virus infects the body, the immune system defends against the virus and produces the specific antibody. Among them, specific IgM antibody is an early antibody produced after infection, which can indicate acute infection or recent infection. IgG antibody is the main antibody produced by the reimmune response, indicating that the disease has entered the convalescent period or there was a previous infection. Therefore, the combined detection of immunoglobulin IgM and IgG antibodies can not only provide early diagnosis of infectious diseases but also contribute to the evaluation of the infection stage of the body.

The clinical sensitivity of 2019-nCoV-specific IgM antibody and IgG antibody detection is 70.24% and 96.10%, and the clinical specificity is 96.20% and 92.41%, respectively. The total coincidence rate of 2019-nCoV specific antibody detection and nucleic acid detection in diagnosing infection is 88.03%.

Serum specific antibody detection has shown that 2019-nCoV-specific IgM antibodies mostly start to be tested as positive 3-5 days after the onset of the disease, and the titer of IgG antibodies during the convalescent period is 4 times or even higher than that in the acute phase.

2019-nCoV-specific IgM antibody and IgG antibody detection cannot only make up for the inadequacy of nucleic acid testing and improve the diagnosis rate of 2019-nCoV but also avoid the risk of infection during the collection of nasopharyngeal swab specimens. At the same time, it is useful for assessing the immune status of patients, and it is also of great significance for the selection of some high-titer individuals as plasma donors for antibody therapy.

Because any single test has a certain false negative and false positive rate, a reasonable interpretation of the combined detection of nucleic acid and antibodies can better determine the patient’s current condition and outcome (see Table 1.1).

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