Risk factors for false negatives were discussed. Compared with the nucleic acid positive group, the false-negative group had less epidemiological exposure (52.6% vs 83.9%; P = .025), less chest discomfort (5.3% vs 32.3%; P = .035), and faster recovery (10 [8, 13] vs 15 [11, 18.5] days; P = .005). The number of involved lung lobes was (2 [1, 2.5] vs 3 [2, 4] days; P = .004), and the lung damage severity score was (3 [2.5, 4.5] vs 5 [4, 9] days; P = .007), which was lighter in the nucleic acid falsenegative group. Thus, the absence of epidemiological exposure may be a potential risk factor for false-negative nucleic acids. The false-negative cases of COVID-19 are worth noting because they have a risk of viral transmission without positive test results, lighter clinical manifestations, and less history of epidemiological exposure. severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) has become a major public health incident of worldwide concern. As of 20 April 2020, there have been nearly 2.6 million cases of infection in 200 countries and regions around the world. The SARS-CoV-2 infection has a latent period and is highly contagious. Its spread is much higher than the Middle East respiratory syndrome-Coronavirus and SARS-CoV. 1 Therefore, timely identification of COVID-19 cases is critical for controlling the pandemic. Currently, the SARS-CoV-2 realtime polymerase chain reaction (RT-PCR) test is the gold standard for diagnosing COVID-19. However, clinical studies have shown that many cases present typical clinical symptoms and lung imaging changes, but their RT-PCR test results for SARS-CoV-2 are negative. [2] [3] [4] These cases can easily become loopholes in the prevention and control of the COVID-19 pandemic. Recent studies have shown that SARS-CoV-2 nucleic acid detection combined with specific antibody detection significantly improves the diagnostic accuracy of COVID-19. [5] [6] [7] [8] However, due to the outbreak of the COVID-19 pandemic, the number of patients has grown rapidly. In this setting, there is an insufficient supply of SARS-CoV-2 nucleic acid detection kits or antibody detection kits. Some countries only give priority to testing high-risk or critically ill patients, that is, patients with a clear history of epidemiological exposure, or clinical and imaging characteristics of suspected COVID-19. Therefore, understanding the clinical and imaging characteristics of nucleic acid false-negative COVID-19 patients is of great significance for their timely identification and reducing the spread of epidemics due to missed diagnosis.
Some studies have reported the clinical and imaging characteristics of COVID-19 patients positive for SARS-CoV-2 nucleic acid, [9] [10] [11] [12] but there are limited reports on those negative for SARS-CoV-2 nucleic acid. In addition, the differences in epidemiology, clinical manifestations, and imaging characteristics between COVID-19 patients with false-negative nucleic acid results and those with positive nucleic acid results are not clear. In this study, we retrospectively analyzed and compared the detailed clinical data of mild-to-moderate COVID-19 patients with positive or negative results for SARS-CoV-2 nucleic acid, who were hospitalized in the Hezheng Ward of Shenzhen Hospital of Southern Medical University. All the patients were diagnosed according to the latest diagnosis and treatment plan. 13 Meanwhile, the possible risk factors of a nucleic acid false negative in COVID-19 patients were analyzed.
This is a retrospective analysis. We included COVID-19 patients who were hospitalized in Hezheng Ward of Shenzhen Hospital of Southern Medical University from 3 February 2020 to 7 March 2020.
All included patients who underwent SARS-CoV-2 nucleic acid detection, specific IgM and IgG antibody detection, and lung CT examination. There were no exclusion criteria for the patients. in which the symptoms included fever and cough and other respiratory symptoms, and lung imaging showed viral pneumonia. However, the vital signs were stable, and the blood oxygen saturation was more than 93% without oxygen support. In the end, 50 eligible cases of mild or moderate COVID-19 were included in the study. According to RT-PCR results, we divided the cases into two groups of the nucleic acid positive group and nucleic acid false-negative group. In the nucleic acid positive group, all cases had positive RT-PCR results for SARS-CoV-2 of nasal or pharyngeal swabs. In the nucleic acid false-negative group, all cases had clinical features of suspected cases of COVID-19 and positive specific IgM/IgG antibody, but their nucleic acid detection of nasal, pharyngeal and anal swabs was continuously negative from the initial screening to hospitalization. This study was approved by the Ethics Committee of Shenzhen Hospital, Southern Medical University (NYSZYYEC20200013). The data were anonymous and the informed consent was therefore waived. 
All data were statistically analyzed using SPSS 25.0. Categorical variables, presented as n (%), were analyzed with χ 2 or Fisher exact tests. The continuous variables of non-normal distribution were expressed as median (25%, 75%) and compared with the Mann-Whitney U test. Univariate and multivariate logistic binary regression analysis was used to analyze risk factors. A P < .05 is considered statistically significant. The nucleic acid false-negative group had fewer epidemiological exposure history (52.6% vs 83.9%; P = .025). There was no significant difference in chronic comorbidities between the two groups (19.4% vs 31.6%, P = .496). The symptoms of chest tightness and chest discomfort in the nucleic acid false-negative group were less than those in the nucleic acid positive group (5.3% vs 32.3%; P = .035) ( Table 1 ).
The other symptoms were similar between the two groups. The hospitalization days of the nucleic acid false-negative group were 15 (13, 18) The results of the blood routine test of both groups were within the normal range (Figure 2A-E) . The median absolute value of lymphocytes in the nucleic acid positive group was lower than the nucleic acid false-negative group (0.99 × 10 9 vs 1.34 × 0 9 /L, P = .037) ( Figure 2C ). The median of hypersensitive C-reactive protein increased slightly ( Figure 2E ). The overall condition of the nucleic acid false-negative group was lighter than that of the nucleic acid positive group. The severity of lung lesions was not significantly related to the viral load of the upper respiratory tract. 12 To et al 16 The reason for the false-negative of SARS-CoV-2 nucleic acid test is not only related to the disease itself, but also with some other factors. The viral load of the nasal and pharyngeal parts of the upper respiratory tract is different. 10, 16, 21 Thus, different sampling sites may also affect nucleic acid detection results. In addition, nucleic acid detection kits and standard operating procedures are also factors that may affect nucleic acid detection results. 6 ,21 Therefore, attention should be paid to the combination of multi-site sampling 22 and the standardization of the operation process during nucleic acid detection. The epidemiological, clinical, imaging and nucleic acid and antibody test results should be combined to make the diagnosis of However, this study still has some limitations. For example, the sample size was relatively small. In addition, the retrospective design itself was limited. Further studies with larger sample size are needed.
In conclusion, the clinical and imaging characteristics of mild-to- 