1  Introduction
Since December 2019, the rapid propagation of a novel coronavirus (SARS-CoV-2) has broken out in China, and SARS-CoV-2 causes a novel pneumonia named COVID-19 [1]. SARS-CoV-2 is a β-coronavirus with a genome highly homologous to bats, which probably originated from wild animals [2]. Interpersonal transmission is the main cause of infection [3]. The World Health Organization (WHO) has declared it as a public health emergency of international concern [4]. As of May 3, 2020, a total of 3,405,914 cases were confirmed and 240,573 cases died globally [5]. The clinical features of severe COVID-19 are similar to those of severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS). It can cause acute respiratory distress syndrome (ARDS), acute heart injury, and even death. Its main clinical features are fever, cough and sore throat. According to the clinical classification method, the patients were divided into four types: ordinary type, mild type, severe type and critically ill type according to the severity of the disease [6], [7]. In addition, the correlation between specific laboratory diagnosis and disease severity deserves attention. Several studies have reported different laboratory findings at the beginning of the outbreak of COVID-19 [8], [9], [10]. The purpose of this survey is to reveal the characteristics of laboratory findings of COVID-19 through the included articles, especially the changes of severe and critically ill patients, so as to provide more information for COVID-19 's diagnosis.