Immature granulocytes can help the diagnosis of pulmonary bacterial infections in patients with severe COVID-19 pneumonia
Journal of Intensive Care volume 9, Article number: 58 (2021)
During COVID-19, immature granulocyte (IG) concentration is heterogeneous with higher concentrations than those found in bacterial sepsis. We investigated the relationship between IG levels at ICU admission and on days 7 (± 2) and 15 (± 2) and associated pulmonary bacterial infections in intensive care unit (ICU) patients hospitalized for an acute respiratory distress syndrome (ARDS) related to SARS-CoV-2. Patients with associated pulmonary bacterial infection had a peak of IGs. IG thresholds of 18% or 2 G/L allowed discriminating patients with ventilator associated pneumonia with 100% sensitivity and specificity. Our study supports that IGs could help identifying pulmonary bacterial infections in this population.
Patients hospitalized in the intensive care unit (ICU) with an acute respiratory distress syndrome (ARDS) related to SARS-CoV-2 frequently develop associated pulmonary bacterial infections, including ventilator-associated pneumonia (VAP) which diagnosis is challenging in this clinical setting [1, 2]. In bacterial sepsis and severe COVID-19, the myeloid cell compartment is dysregulated and circulating levels of immature granulocytes (IG) may increase [3, 4]. The range of IG increase appears highly variable in COVID-19 [4, 5]. We previously showed that septic patients exhibit higher IG levels than patients with severe SARS-CoV-2 infection [3, 5]. We hypothesized that IG levels heterogeneity could be related to the development of pulmonary bacterial infections in patients mechanically ventilated for a SARS-CoV2-induced ARDS.
Between December 2020 and March 2021, consecutive patients without known immunosuppression who required invasive mechanical ventilation for severe COVID-19 pneumonia were prospectively enrolled. The evolution of peripheral blood leukocyte populations were studied, from ICU admission to day 7 (± 2) and day 15 (± 2). Using flow cytometry, leukocyte populations were discriminated with CD3 for the T cells, CD19 for the B cells, CD14 for the monocytes, and CD16 for the granulocytes. CD45 was used to identify the hematopoietic cells and CD64 was used as a marker of neutrophils activation. Immature granulocytes or “band cells” were characterized as CD45+CD3−CD19−CD14−CD16dim/neg (Fig. 1). Monocyte expression of HLA-DR, CD4+ and CD8+ T lymphocytes counts were also analyzed.
An independent committee blindly adjudicated the diagnosis of pulmonary bacterial infections during the ICU stay based on clinical findings (fever, new onset of purulent endotracheal sputum or modification of sputum characteristics, auscultation abnormalities, increasing need of oxygen therapy), biological abnormalities (hyperleukocytosis, decreased PaO2/FIO2), radiological data (new onset or worsening of pulmonary infiltrate), and microbiological documentation. Pulmonary bacterial infections diagnosed within the first 2 days of ICU stay were considered as co-infections, while those diagnosed later were reported as VAP.
Nineteen patients ventilated for severe COVID-19 were studied (72 [63.0–74.5] y.o; SAPS II: 29 [26.0–35.5]; mortality rate: 32%). Severity scores, comorbidities and steroids use were similar, irrespective of the presence of a pulmonary bacterial infection (Table 1). Two patients were admitted to ICU with a pulmonary bacterial co-infection, whereas 10 patients developed VAP (median diagnosis: 6.5 [4.3–7.8] days). On ICU admission, patients without pulmonary co-infection (n = 17) exhibited markedly lower circulating IG levels in absolute count (0.40 ± 0.75 G/L) and in percentage (3.22 ± 3.78%) than those with bacterial co-infection (2.30 G/L and 9.37 G/L in absolute count, or 75% and 84% in percentage) (Fig. 2A). In the two patients with bacterial co-infection at admission, IG absolute numbers and frequencies decreased with time (Additional file 1: Figure S1). On day 7, patients with confirmed VAP had a major peak of IG, both in percentage and absolute numbers, when compared to patients without VAP (55.6 ± 26.6% vs 9.0 ± 5.9%, p = 0.0001 and 6.9 ± 4.72 G/L vs 0.95 ± 0.75 G/L, p = 0.0002; Fig. 2B). IG thresholds of either 18% or 2 G/L allowed discriminating patients with or without VAP with a 100% sensitivity and specificity (Fig. 2B). On day 15, IG levels decreased in patients who developed VAP on day 7, and were close to those observed on ICU admission (8.7 ± 5.6% and 1.75 ± 1.13 G/L; Fig. 2C). IG levels were moderately correlated to the SOFA score (Sequential Organ Failure Assessment) (Fig. 3; Spearman test r = 0.62). Total neutrophil count tended to be increased in patients with VAP but without reaching statistical significance. No significant difference was noticed on day 7 in the neutrophil to lymphocyte ratio (NLR) (Fig. 2D). CD4 and CD8 T lymphocyte counts as well as HLA-DR monocyte expression were similar between patients (Fig. 2E, F).
Our results, even if obtained on a small cohort, suggest a strong association between level of IGs and pulmonary bacterial infection. Provided consolidation, they indicate that the peak of IG observed in case of bacterial associated infection could be a characteristic reaction of COVID-19.
In contrast to previous studies [4, 6, 7], IG levels were not associated with clinical severity in our patients as reflected by the absence of correlation between IG level and SOFA score. Nevertheless, the occurrence of secondary bacterial infection was not documented in these studies [4, 6, 7]. Associated pulmonary bacterial infections affect approximatively 50% of ventilated patients with severe COVID-19 . This incidence appears superior to that observed in influenza or non-viral pneumonia . In the clinical setting of acute viral ARDS, bacterial infection is associated with an increased risk of death . In addition, radiological and clinical criteria are often inconclusive and, the diagnosis as well as the start of antibiotics mainly rely on microbiological documentation [2, 11]. Due to the severity of SARS-CoV-2 pneumonia in ICU patients, the occurrence of an associated bacterial pulmonary infection requires early antibiotic therapy. Nevertheless, inappropriate antibiotic prescription could favor the emergence of multidrug resistant bacteria which could also jeopardize outcome. Therefore, a reliable biomarker would be highly beneficial in the context of the challenging diagnosis of associated bacterial infection in severe COVID-19.
Even if our results need to be validated by further larger scale studies, our proof of concept study supports that IGs could be an interesting biomarker of bacterial over-infections in ICU patients with severe COVID-19, especially since more and more hospitals have access to flow cytometry.
Availability of data and materials
Acute respiratory distress syndrome
Coronavirus disease 2019
Intensive care unit
Neutrophil to lymphocyte ratio
Simplified Acute Physiology Score
Sequential Organ Failure Assessment
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Daix, T., Jeannet, R., Hernandez Padilla, A.C. et al. Immature granulocytes can help the diagnosis of pulmonary bacterial infections in patients with severe COVID-19 pneumonia. j intensive care 9, 58 (2021). https://doi.org/10.1186/s40560-021-00575-3