SARS-CoV-2 N-antigenemia in critically ill adult COVID-19 patients: Frequency and association with inflammatory and tissue-damage biomarkers

Abstract

The current study aimed at characterizing the dynamics of SARS-CoV-2 nucleocapsid (N) antigenemia in a cohort of critically ill adult COVID-19 patients and assessing its potential association with plasma levels of biomarkers of clinical severity and mortality. Seventy-three consecutive critically ill COVID-19 patients (median age, 65 years) were recruited. Serial plasma (n = 340) specimens were collected. A lateral flow immunochromatography assay and reverse-transcription polymerase chain reaction (RT-PCR) were used for SARS-CoV-2 N protein detection and RNA quantitation and in plasma, respectively. Serum levels of inflammatory and tissue-damage biomarkers in paired specimens were measured. SARS-CoV-RNA N-antigenemia and viral RNAemia were documented in 40.1% and 35.6% of patients, respectively at a median of 9 days since symptoms onset. The level of agreement between the qualitative results returned by the N-antigenemia assay and plasma RT-PCR was moderate (k = 0.57; p < 0.0001). A trend towards higher SARS-CoV-2 RNA loads was seen in plasma specimens testing positive for N-antigenemia assay than in those yielding negative results (p = 0.083). SARS-CoV-2 RNA load in tracheal aspirates was significantly higher (p < 0.001) in the presence of concomitant N-antigenemia than in its absence. Significantly higher serum levels of ferritin, lactose dehydrogenase, C-reactive protein, and D-dimer were quantified in paired plasma SARS-CoV-2 N-positive specimens than in those testing negative. Occurrence of SARS-CoV-2 N-antigenemia was not associated with increased mortality in univariate logistic regression analysis (odds ratio, 1.29; 95% confidence interval, 0.49-3.34; p = 0.59). In conclusion, SARS-CoV-2 N-antigenemia detection is relatively common in ICU patients and appears to associate with increased serum levels of inflammation and tissue-damage markers. Whether this virological parameter may behave as a biomarker of poor clinical outcome awaits further investigations.

Keywords: COVID-19; SARS-CoV-2 N-antigenemia; SARS-CoV-2 RNAemia; inflammation biomarkers; mortality.

Figure 1

(A) Evaluation of the limit of detection of the CLINITEST Rapid COVID‐19 Antigen Test (Siemens, Healthineersy) for detection of the SARS‐CoV‐2 N protein in plasma specimens. A prepandemic plasma pool testing negative by RT‐PCR was spiked with 10, 25, 50, 100, and 150 pg/ml of a recombinant N protein (MT‐25C19NC, Certest Biotec S.L.). (B) Representative example of a depleting experiment demonstrating the true nature of SARS‐CoV‐2 N protein detected in plasma specimens with N‐antigenenia and testing negative for SARS‐CoV‐2 RNA by RT‐PCR. A a rabbit anti‐N protein antibody (40143‐R019; SinoBiological) was used for N protein depletion. An isotype‐matched rabbit antibody was employed as a control. RT‐PCR, reverse‐transcription polymerase chain reaction

Figure 2

Box‐plot depicting SARS‐CoV‐2 RNA load (in copies/ml) in plasma (A) and tracheal aspirates (B) from critically ill patients in the presence or absence of SARS‐CoV‐2 N protein. p‐value for comparison is shown

Figure 3

Box‐plot depicting SARS‐CoV‐2 RNA load (in copies/ml) in plasma from critically ill patients according to the intensity of the N‐antigen line (1+ vs. 2+) on the CLINITEST Rapid COVID‐19 Antigen test. p value for comparison is shown

Figure 4

Box‐plot depicting serum levels of (A) ferritin, (B) D‐Dimer, (C) LDH, and (D) C‐reactive protein (CRP) from critically ill patients according to the intensity of the N‐antigen line (1+ vs. 2+) on the CLINITEST Rapid COVID‐19 Antigen test. P‐value for comparison is shown. LDH, lactate dehydrogenase