Nosocomial infections (NI), also known as healthcare-associated infections (HAI) complicate 30% of intensive care unit (ICU) admissions and are associated with increased mortality and morbidity including both longer ICU and hospital stays [1, 2]. In the ICU, mechanically ventilated patients are at the highest risk for NIs, likely due to the presence of multiple risk factors [3]. An increasingly recognized potential risk factor for infection in the critically ill is immunological dysfunction, which has been described in both critically ill adults and children since the 1980s [4, 5]. Immune dysfunction can occur in patients who experience severe trauma, are post-operative or in patients with sepsis [4, 6, 7]. The exact mechanism of immune dysfunction is unknown, but is likely related to abnormal regulation of inflammation, as well as the development of tolerance to stimulation by the immune system. Further, there is probably a genetic predisposition to the development of immune dysfunction, but the exact genetic markers have yet to be identified [5].
Studies that have examined immune dysfunction and the relationship with patient outcomes have reported inconsistent findings. In patients with either sepsis or trauma, most studies show a significantly lower survival rate in patients with prolonged low human leukocyte antigen (HLA)-DR expression by monocytes, as well as higher rates of major infections compared to those patients who only had a transient or less severe drop in HLA-DR expression [8–13]. However, Perry and colleagues did not find a relationship between HLA-DR expression in septic patients and patient outcomes [14]. Further studies also found that HLA-DR expression had poor discriminating power in identifying septic patients at high risk of dying [15]. Oberholzer et al. found that selected baseline cytokines including interleukin (IL)-6 and soluble tumor necrosis factor (sTNF) were helpful in predicting patient outcomes, while other cytokines, including TNF-α, as well as the change in cytokine concentrations over time, were not predictive of patient outcomes [16].
The relationship between immune dysfunction and the acquisition of NIs remains uncertain. Studies looking at cytokine levels in patients admitted with sepsis and the correlation with the development of NIs, have yielded mixed results [17, 18]. One study found no difference in cytokine levels between patients admitted with sepsis who later developed NIs, and those admitted with sepsis who did not develop NIs [17]. However, Van Vught et. al found significant elevations of inflammatory cytokines in patients with sepsis who developed NIs as compared to patients who did not, and suggested that these patients have concomitant hyperinflammation and immune suppression to a greater degree than those patients who only had sepsis [18]. Immune dysfunction has also been studied in the development of NIs after trauma and elective surgery [5, 6, 19–21]. These studies suggested that distinct inflammatory marker patterns exist in patients who develop NIs. Currently, there are few studies that look at cytokine levels as they relate to NIs in all patients admitted to an ICU. Further, it is still unknown whether higher or lower levels of pro-inflammatory cytokines correlate to the development of NIs, and whether admission cytokine levels can help predict who develops these infections.
The best way to measure immune dysfunction is unknown, but some studies suggest that TNF-α levels in lipopolysaccharide (LPS)-stimulated whole blood are more accurate in predicting patient outcomes than using HLA-DR expression [22, 23]. LPS, also known as endotoxin, is a component of the outer membrane of gram-negative bacteria and is known to stimulate monocytes to release cytokines, including TNF-α. Studies of TNF-α levels post-LPS stimulation in healthy adults, show significant variation in both baseline levels of TNF-α and levels post-LPS stimulation [24–28]. Patients with middle range initial levels of TNF-α had a response to LPS stimulation, while people with high levels and some with low levels of TNF-α did not respond [28]. Bruunsgard et al and von Haehling et. al showed that there are differences in immune stimulation between age groups, but their results are conflicting [29]. There is little research on the use of an ex-vivo LPS assay in critically ill patients, but the data available suggests that there is less of a response to LPS in patients in the ICU versus healthy patients [22, 30]. Further, there is minimal data on whether TNF-α response to LPS is related to patient outcomes. Ploder et al. and Heagy et al. suggested that patients who had a lower TNF-α response to LPS at baseline had a worse prognosis than patients who had a higher TNF-α response [22, 31]. Few studies have looked at TNF-α response to LPS as it evolves over the course of a patient’s admission to the ICU.
To investigate the relationship between TNF-α and the acquisition of NIs, we conducted a secondary analysis of a randomized, multi-centre, double-blinded placebo controlled trial studying the effect of lactoferrin of the acquisition of NIs [32]. The objectives of this descriptive analysis were:
1. To describe the characteristics of an ex-vivo whole blood LPS stimulation assay in critically ill, mechanically ventilated patients in the ICU as measured by change in the level of TNF-α;
2. To explore how levels of TNF-α after stimulation by an LPS assay are associated with clinical outcomes including mortality and the development of NIs.
We hypothesized that patients with lower levels of stimulated TNF-α on ICU admission would develop more NIs, have longer ICU and hospital lengths of stays, and increased mortality.