Using highly sensitive methods to identify a broad range of viruses, we have estimated the prevalence of respiratory viruses and viremia in hematological patients with neutropenia other than allogeneic HSCT recipients. Viral infections were commonly detected in patients with CLL and NHL. Among subpopulations of immunological cell types investigated, decline in the CD4+ T cell count was associated with these infections. Viral infections were associated with fever, and specifically respiratory viruses in NPA were associated with URTS. Although the majority of viral infections did not co-occur with bacterial infection, viral and bacterial co-infections were common.
The proportion of different virus species in NPA differed from an earlier large study by Chemaly et al on adult hematological patients 
. HRV was the commonest finding in our study which could be explained by the use of PCR instead of culture. Moreover, the study by Chemaly et al was retrospective and only included patients with documented positive viral culture. However, our findings in NPA were comparable to recent studies on children with febrile neutropenia 
where PCR was used. Moreover, with the exception of corona virus and PIV, the proportions of viral species corresponded well to a study on community acquired pneumonia in the same geographical area 
. The latter confirms an earlier finding that the temporal occurrence of viral infections in immunocompromised patients tends to mirror their occurrence in the community 
Approximately half of the viral findings were viremia with BKV and CMV as dominating pathogens. A comparable screening in young children with febrile neutropenia showed lower frequency 
, but this discrepancy is probably explained by the fact that the prevalence of latent viral infections increases with age. It has been shown that allogeneic transplanted patients with a BK viral load of more than 104
copies/mL had a significantly higher risk of having hemorrhagic cystitis than patients with a viral load of less than 104
. In a recent pediatric study on a post-HSCT population, the degree of BK viremia seemed to predict renal, urologic, and overall outcome 
. However, in our less immunosuppressed cohort with mostly very low titers, we had no reports on symptoms of disease associated to BKV.
Although we found an independent association between viral findings and fever, one should interpret the results with caution because of the discrepancy in grade of immunosuppression between the febrile and afebrile patients. Furthermore, multivariate analysis with this low number of patients carries the risk of small sample bias. Even though the levels of virus in blood were lower than those expected from symptomatic disease, the independent association prompts the question whether the viral load was sufficient to generate fever. Furthermore, not all viral respiratory tract infections cause fever, and different virus types are known to be variously pyrogenic 
. In our study, not all patients with fever were completely sampled for detection of viruses. We thus risk underestimating the association between fever and viral findings. Although bacterial culture is limited in sensitivity, it is of interest to note that virus existed more often as the sole pathogen identified here than as a cofactor with bacteria. The fact that PCR is a highly sensitive method always raises the question of the findings' relevance. We did, however, determine an association between respiratory viruses and URTS, indicating symptomatic infection.
Until recently, viral infections were regarded as a concern in hematological malignancies primarily in patients who had undergone allogeneic HSCT. However, in the last years, the use of monoclonal antibodies directed against B and/or T cells for treatment of hematological malignancies such as NHL and CLL has had potential to increase the incidence of viral infections in these groups as well. These two diagnoses as well as receiving monoclonal antibody therapy were associated to detection of virus in our material. After multivariate analysis, only CLL showed to be independently associated, but once again, with risk of small sample bias.
In allogeneic HSCT recipients, lymphocytopenia is a known risk factor for the development of lower respiratory tract infection 
, and reconstitution of the lymphocyte population is known to reduce the risk for viremia with CMV and AdV 
. In our study, all measured cell counts were lower in the virus-positive patients but only the difference in CD4+
cell count was statistically significant. The function of lymphocytes can be altered by both the treatment and the underlying disease 
and it would therefore be interesting to investigate an association between not only the cell count but also the cell function to risk of viral infection in this patient category.
In conclusion, both respiratory virus infections and low titers of viruses in blood from patients with neutropenia were common. Patients with CLL and patients with fever were independently associated to these infections, and viral findings in NPA were associated to URTS indicating active infection. Although this is a small study in a heterogeneous group of hematological patients, it is possible that a proportion of episodes of febrile neutropenia could originate from virus infections. Larger prospective longitudinal studies should be conducted to investigate the possible causality between neutropenic fever and viral infections.