The results of this prospective observational study show that the determination of IFNγ production by CMV-specific CD8+ T lymphocytes using the QuantiFERON-CMV test at admission in ICU is a good marker for identifying the risk of CMV reactivation in critically ill non-immunosuppressed patients.
The main utility of QuantiFERON-CMV® in these patients is its high negative predictive value as patients with QuantiFERON-CMV® reactive show protection against CMV reactivation. The administration of prophylaxis is therefore not recommended in these patients, thus preventing the effects of exposure to antiviral drugs (adverse reactions, resistance) [18].
In our study, only 10 of the 27 (37 %) QuantiFERON-CMV non-reactive patients presented CMV reactivation. Given this low positive predictive value, the QuantiFERON-CMV non-reactive result would not be adequate for identifying patients who may develop subsequent reactivation as 63 % of the QuantiFERON-CMV non-reactive patients would receive prophylaxis unnecessarily. Therefore, the QuantiFERON-CMV non-reactive result would be useful to select a population in which monitoring for early detection and treatment of CMV reactivation (pre-emptive therapy) during their stay in the ICU would be a better strategy than universal prophylaxis.
The potential usefulness of evaluating the function of CMV-specific T cells using the QuantiFERON-CMV test has been demonstrated in both hematopoietic progenitor and solid organ transplant recipients [13, 15, 19]. Cantisán et al. reported that the decreased production of IFNγ by CMV-specific CD8+ T lymphocytes has been shown to predict CMV reactivation in both the pre-transplant and post-transplant periods [13].
At present, data on the functionality of CMV-specific T lymphocytes in critically ill patients are scarce and present contradictory results. Two previous studies reported no differences in terms of the functionality of CMV-specific T cells in patients with and without CMV reactivation [20, 21]. Subsequently, the results of one study of cases and controls, which included 31 patients subjected to mechanical ventilation, showed how decreased levels of IFNγ-producing CMV-specific CD4+ and CD8+ T lymphocytes were associated with the presence of active CMV infection or increased risk of subsequent viral reactivation, although the absence of multivariate analysis does not rule out the possible existence of confounding factors [22].
Studies conducted so far show that although it may be variable, the mean incidence of CMV reactivation in critically ill patients is 25 % [9]. This variability in the incidence of reactivation may be due to differences in the type of population and the technique and frequency of viral monitoring employed. In our series, an incidence of CMV reactivation of 24.5 % was found, which is similar to the level reported previously [6, 11, 23]. Similarly, the median time of onset of viremia in our study was 14 days, which is similar to that reported in previous studies using PCR in viral monitoring [6]. Our results show that although reactivation is more frequent in QuantiFERON-CMV non-reactive, no differences were found in the timing of onset, duration, and magnitude of viremia. A study with a larger sample would probably reveal differences in these aspects.
As a result of the limited number of patients in the study, the multivariate analysis included only three risk factors (age, diabetes mellitus, and SAPS II score) in addition to the QuantiFERON-CMV. Numerous studies have shown the influence of age on immune response to CMV [24, 25], although our study did not find age to be associated with increased risk of CMV reactivation. In a similar manner, although diabetes mellitus has been associated with increased risk of infections and sepsis [26, 27], it was not associated with increased risk of CMV reactivation in our study. Moreover, in line with previously published data [6], this study found no association between severity of illness (as assessed by the SAPS II score) and CMV reactivation, thus decreasing the likelihood of CMV reactivation being an intermediate marker of disease severity.
In our study, the mortality rate was significantly higher in patients who developed CMV reactivation compared to those who did not. However, because of the small sample size (only 6 of the 13 patients with replication died), we did not perform a multivariate analysis to verify the joint influence of other potential risk factors. Studies evaluating the influence of CMV reactivation on the mortality of critically ill patients have reported contradictory results [5, 7]. This may be because these studies include different populations with different degrees of severity and different strategies for monitoring CMV reactivation. A placebo-controlled clinical trial of antiviral prophylaxis would have to be conducted in order to determine the real influence of CMV reactivation on mortality in this population.
Our study has some strengths, among them the prospective design, the use of a robust PCR-based method to detect CMV reactivation, the evaluation of immune function using a standardized method, and the inclusion of representative patients in a general ICU setting.
Our study also has certain limitations. The main limitation is the small size of the sample studied, which does not allow definitive conclusions to be drawn regarding the existence of differences in the kinetics of viral reactivation based on the results of the QuantiFERON-CMV test. Another limitation is that the sample size does not allow one to perform an adequate assessment of the possible role of clinical variables on CMV reactivation. On the other hand, we do not know the value of QuantiFERON-CMV immediately before CMV reactivation because the QuantiFERON-CMV test was performed at admission. However, our objective is to identify patients according to their risk of reactivation at the time of admission to the ICU, and on the other hand, we cannot know the moment at which viral reactivation will occur.