Discussion
Our study revealed that WBC, LYM% and PLT decreased, while CRP and Hb increased significantly in nucleic acid positive group compared to those in nucleic acid negative pneumonia group. LYM%, Hb and WBC had a good predictive value to distinguish the nucleic acid positive patients from negative patients. The dynamics of WBC, LYM%, Hb and CRP in nucleic acid positive patients were more sensitive to clinical treatment and gradually returned to normal level. Only LYM% had a significant correlation with C t value.
COVID-19 caused by SARS-CoV-2 with high transmissibility[10] is an ongoing global pandemic[11]. Given the global spread of COVID-19, strict discharge standard is of great significance for the prevention and control of the epidemic.
At present, the criteria for discharge is 1) temperature returns to normal for more than three days, 2) respiratory symptoms improve sig­nificantly, 3) pulmonary imaging shows significant improve­ment in acute exudative lesions, and 4) nucleic acid test is negative for two consecutive times for samples of sputum, nasopharyngeal swabs, and other respiratory tract specimens, tested at intervals of at least 24 h.
Therefore, negative nucleic acid test is indispensible for patients’ discharge[12]. However, there are no available parameters to accurately determine when patients will test negative for SARS-CoV-2 during clinical treatment.
The nucleic acid testing time is based on the clinical improvement of patients which is partially subjective and lacks objective indicators. The clinical and radiographic manifestations of many patients improved significantly with time, but the actual viral load was still high. Therefore, patients often need to take nucleic acid test again and again which brings discomfort to patients during the sampling process, increases the infection risk of medical staff and consumes limited medical resources, resulting in their wastage[13].
Lymphocytes% commonly decreases in the early stage of disease[14, 15] and could be an indicator of reflecting COVID-19 severity and prognosis[16, 17]. LYM% can be used as a reliable indi­cator to classify the moderate, severe, and critical ill patients independent of any other auxiliary indicators[18]. The COVID-19 is much more serious with lower Lymphocytes [19]. SARS-CoV-2 RNA load is also closely related to COVID-19 severity [20]. The relative RNA load is higher, the lymphocyte count was lower[21].
LYM% can be easily acquired from Blood routine test which requires low cost and the whole process is easier and convenient [22]. There is no standard guideline to accurately determine the timing of SARS-CoV-2 nucleic acid detection until now. Our study confirmed that: 1) LYM% of COVID-19 pneumonia patients was decreased compared to those of common pneumonia patients, 2) LYM% had a good predictive ability to distinguish the nucleic acid positive from negative pneumonia, 3) LYM% dynamics of COVID-19 were sensitive to clinical treatment and 4) LYM% had a significant correlation with C t value(reverse to viral RNA load). So, we suggested dynamically observing the changes of LYM% may be used to estimate the time for SARS-CoV-2 nucleic acid test results turning negative in COVID-19 Patients.
However, more laboratory test results need to be comprehensively analyzed to explore a quantifiable standard of LYM% to accurately guide the timing of SARS-CoV-2 nucleic acid turning negative detection during the treatment course of COVID-19.
This study has three limitations. First, this is a retrospective study. The cases were only from the Xiangyang central hospital. Second, only a few relevant studies are available, and this study only analyzed the correlation between LYM% and C t value, but the quantifiable LYM% is not clear. Third, for greater convenience in statistical analysis, we used a range of lymphocyte count that did not account for the effects of age and gender on the lymphocyte count.