Background

The latest pneumonia epidemic caused by a novel coronavirus (Severe Acute Respiratory Syndrome Related Coronavirus 2; SARS-CoV-2) termed coronavirus disease 2019 (COVID-19) has raised a major risk for global public health. It has had a significant impact on clinical microbiology laboratories over the last few months. Compared to the tests developed for other SARS-CoV diagnosis, the optimal test method to detect COVID-19 is reverse transcription quantitative polymerase chain reaction (RT-qPCR) in respiratory samples [1]. Although respiratory samples have the greatest yield, other samples including stool and blood, could also be used to detect the COVID-19 virus [2, 3], as for other coronaviruses such as SARS and Middle East Respiratory Syndrome (MERS) [4,5,6]. Numerous studies have used blood samples to diagnose COVID-19 disease, although variable results have been reported (Table 1).

Table 1. Studies performed on blood samples for COVID-19 viral RNA detection
Full size table

The review by Wang, distinctly showed that the nucleic acids were presented in the serum or plasma samples for all novel coronaviruses, but the timeout of viremia remained unclear [7]. In another study, Chang et al. [8] suggested that the SARS-CoV-2 RNA was relatively stable in plasma, although it might not indicate any infection.

On February 2, 2020, the Hubei Province Center for Disease Control and Prevention conducted follow-up tests on plasma samples by reporting poor positive results close to the limit of detection. After a week, donor-collected throat swab specimens became rather positive, suggesting the presence of an extremely low viral load in plasma samples [9]. Peng et al. [10] analyzed different patient samples including blood, urine, anal and throat swabs, tested with RT-qPCR. Virus screening in all samples revealed that coronavirus could affect respiratory, digestive, urinary and hematological systems. Thus, clinical symptoms and signs in COVID-19 disease could result from the involvement of different organs. So far, only a few studies have shown that the viral RNA in COVID-19 patients might be detectable in plasma or serum samples (Table 1).

Viral RNA in blood has been detected in COVID-19 patients on the first 2 to 3 days after the onset of symptoms; however, there is no evidence on the plasma and serum viral load during the incubation period [7, 11]. Zheng et al. [12] reported that, from the first week, the load of viral RNA in serum samples gradually increased, followed by a decline in the third week of the disease. In addition, they reported that the rate of viral RNA detection in serum samples was greater in patients with advanced disease than mild cases (45% vs 27%), although the difference was not significant. Since there are limited number of studies indicating the presence of COVID-19 RNA in blood samples, more investigations are needed to determine the time period when the viral RNA is present in blood. Also, it is not clearly known which particular type of patients are linked with the higher occurrence of RNAaemia [13]. According to the experiments carried out to diagnose SARS-CoV-2 RNA in blood samples, on average 10–15% of the critically ill patients have RNA in plasma or serum [7, 10, 14,15,16]. However, a zero detection rate of viral RNA in serum samples has also been reported [17, 18].

Chen et al. reported that the combination of the substantial level of IL-6 with a meaningful Ct (cycle threshold) value of viral RNA in serum samples can be considered as an effective and accurate biomarker unveiling adverse outcomes [15]. Since the increased IL-6 could be part of a broader cytokine storm that may intensify the outcome, IL-6 could be a possible treatment option for critically ill patients with an uncontrolled inflammatory response [15, 19].

In a recent study, Zhang et al. [20] reported that the current strategy for the detection of viral RNA in oral swabs is not ideal. They indicated that the virus could be present in patients' anal swabs or blood while the oral swab test was negative. Similar studies have also confirmed that the virus can exist in blood or rectal swabs and not to be presented in the throat swab. These patients could act as carriers and spread the disease to other individuals. Thus, it is important to examine samples from different sources to validate the infection [20, 21]. While SARS-CoV-2 RNA has been identified in serum or plasma of infected patients, there is no evidence demonstrating the possibility of SARS-CoV-2 transmission through blood transfusion [22].

Conclusion

In conclusion, while the detection of SARS-CoV-2 viral load in respiratory specimens is commonly used to diagnose COVID-19, detecting SARS-CoV-2 nucleic acid in plasma or serum should not lose sight. In some situations, such as patients in coma condition, the oropharyngeal or nasopharyngeal sampling is seldom feasible, and thus blood sampling could be an alternative. Moreover, the blood test could have less harm for the hospital staffs who conduct the sampling with the respiratory swaps. It should be noted that the detection of viral RNAaemia might not reveal active infection, but rather at times of post-viral infection. Furthermore, RNA detection by itself does not necessarily indicate the presence of live virus and only patients having positive blood tests with clinical symptoms should receive medical treatment.

However, it remains unclear that whether the RNAaemia is associated with the frequency of cytokine storms or it is related to a particular type of patients. Another challenge for the identification of viral RNA in blood samples may be due to the small amounts of RNA in plasma or serum samples. In addition, large amounts of background RNA can mask the low-abundance of viral RNA fragments leading false negative outcomes. Further investigations in future studies are needed to lower the false negative results. We should keep in mind that although the sampling of oropharyngeal/nasopharyngeal compartments may be challenging, it may not be impossible. This may be justified further as blood specimens may not help in better yields. Once more, further studies are needed.