Usefulness of real-time RT-PCR to understand the kinetics of SARS-CoV-2 in blood: A prospective study

Abstract

Background: SARS-CoV-2 viral load and kinetics assessed in serial blood samples from hospitalised COVID-19 patients by RT-PCR are poorly understood.

Methods: We conducted an observational, prospective case series study in hospitalised COVID-19 patients. Clinical outcome data (Intensive Care Unit admission and mortality) were collected from all patients until discharge. Viremia was determined longitudinally during hospitalisation, in plasma and serum samples collected sequentially, using two commercial and standardised RT-PCR techniques approved for use in diagnosis of SARS-CoV-2. Viral load (copies/mL and log10) was determined with quantitative TaqPath™COVID-19 test. Persistent viremia (PV) was defined as two or more consecutive quantifiable viral loads detected in blood samples (plasma/serum) during hospitalisation.

Results: SARS-CoV-2 viremia was studied in 57 hospitalised COVID-19 patients. PV was detected in 16 (28%) patients. All of them, except for one who rapidly progressed to death, cleared viremia during hospitalisation. Poor clinical outcome occurred in 62.5% of patients with PV, while none of the negative patients or those with sporadic viremia presented this outcome (p < 0.0001). Viral load was significantly higher in patients with PV than in those with Sporadic Viremia (p < 0.05). Patients presented PV for a short period of time: median time from admission was 5 days (Range = 2-12) and 4.5 days (Range = 2-8) for plasma and serum samples, respectively. Similar results were obtained with all RT-PCR assays for both types of samples.

Conclusions: Detection of persistent SARS-CoV-2 viremia, by real time RT-PCR, expressed as viral load over time, could allow identifying hospitalised COVID-19 patients at risk of poor clinical outcome.

Keywords: Intensive care unit; Mortality; Plasma viral-load; RT-PCR; SARS-CoV-2; Viremia-kinetics.

Fig. 1

STROBE flow chart of the study including participation, sample collection and tests.

Fig. 2

Scatter plot of mean Ct obtained by Taqpath™ and Cobas® RT-PCR tests in plasma (a) and serum (b). Correlation between results was analysed by Pearson correlation (r = 0.86 and 0.76, respectively. P < 0.0001 for both techniques).

Fig. 3

Comparison of viral load in plasma and serum samples. Viral loads were determined by qTaqPath-test. Differences between plasma and serum were analysed by student's t-test (p = 0.65) (a). Correlation between data was evaluated by Pearson correlation coefficient (r = 0.89; p < 0.0001) (b).

Fig. 4

Distribution of genes detection by TaqPath™ kit in patients with positive viremia.

Fig. 5

Viral load kinetics curves throughout hospitalisation in plasma from patients with persistent viremia. Clinical characteristics defining poor outcome are included within the graphics for each patient. ICU: intensive care unit.

Fig. 6

Analysis of SARS-CoV-2 viremia kinetics assessed by different RT-PCR methods in plasma and serum samples. The headings of the graphs represent the code of the anonymised patients, followed by their clinical outcome (ICU: Intensive Care Unit admission; M: Death during hospital admission). The curves show the kinetics of viremia determined by the TaqPath™-test and Cobas®-test, expressed as Ct values (A), and the q-TaqPath test, expressed in copies/mL (B).

Fig. 7

Relationship of persistent viremia with clinical outcome. Distribution of patients according to the presence or absence of persistent viremia is shown. The comparison between both groups was analysed with the χ2 test.

Fig. 8

Viral load is significantly higher in patients with persistent viremia than those with sporadic viremia in serum and plasma samples. The viral load expressed as log10 showed a normal distribution, so the difference of viral loads between both groups was analysed by student's t-test.