in a previous issue of critical care , prowle and colleagues describe the results of a cohort study in which incidence and attributable mortality of bloodstream infection ( bsi ) were investigated in university - affiliated hospitals .
the cohort included 6,339 patients with an icu stay > 72 hours , of which 330 developed an icu - acquired bsi .
mortality associated with bsi was 41.2% and thereby substantially higher compared to patients without bsi ( 22.5% ) .
after adjustment for confounding covariates , bsi remained an independent risk factor for death ( hazard ratio 2.9 , 95% confidence interval 2.4 to 3.5 ) .
the study by prowle and colleagues , however , goes beyond impact estimates for patients who are affected by this infectious complication .
the investigators linked the attributable risk of death to the incidence of bsi , which was approximately 5% in this cohort .
the authors state that , taking into account the low incidence , the overall impact of nosocomial bsi on the total study population was small as it only caused a decrease in overall survival of approximately 1% .
as such , the study by prowle and colleagues is remarkable as it evaluates the deleterious impact of bsi in bacteremic patients and in the overall icu cohort . for what concerns
the impact for patients with bsi , the study results concur with those of other investigators who found dramatic excess mortality rates , albeit that some researchers reported attributable mortality rates of non - significant proportions [ 2 - 4 ] . within certain confidence limits ,
all of these estimates might match reality as clinical outcomes are multifactorial and variable depending on the specific characteristics of the study cohort .
more unusual and controversial is to express the excess mortality of bsi for the overall population .
even in a situation where substantial excess mortality was noted in the bacteremic cohort , the deleterious impact appears to be of minor importance when using the overall cohort as denominator .
as the authors themselves point out , survival benefit of untargeted interventions to prevent bsi is hardly feasible , and any claims of improved survival from interventions to reduce bsi must therefore be interpreted cautiously .
as bsis hardly affect overall mortality , huge trials in prevention are necessary to demonstrate benefits in mortality .
as already mentioned , mortality always is a product of many components . also , plenty of studies , including the present one by prowle and colleagues , show that bsis do result in a significant excess risk of death .
it seems appropriate , therefore , that infection rate in itself , and not mortality , is the best suited outcome variable .
the data provided by prowle and colleagues show that , for interventional studies in infection prevention , study power should be calculated as a function of a predefined reduction in infection rate , rather than decreased mortality .
vice versa , study results of such trials indicating a reduced infection risk but without difference in mortality should not be taken as a justification of non - adherence , on the condition that associated costs are not excessive . although mathematically correct , the approach of considering the impact of bsi on population level calls for some vigilance . by diluting the effect of infection
even though prowle and colleagues clearly do not enlighten their study results with this purpose , the present data could be used as an argument to minimize the problem and a reason not to invest in infection prevention . in this way
, the consequences of many other severe complications can be downgraded to apparently negligible proportions .
for example , acute kidney injury with need for renal replacement therapy is well - known to be one of the strongest risk factors for death in patients with severe burn injury .
acute kidney injury occurs in approximately 3% of severely burned patients and is associated with a mortality rate of about 30% .
based on these figures , it can be calculated that acute kidney injury increases mortality by only 0.9% ( from 4.85% to 5.75% ) .
this example illustrates how the impact of complications can be diminished by alterations in denominator .
the study by prowle and colleagues demonstrates that assessing the impact of bsi should preferably be performed on the patient group affected as many other icu patients have only a very low risk profile for the acquisition of nosocomial bsi .
diluting the impact of infection to the overall population , however , illustrates that infection prevention studies should use infection rates as endpoints , as proving substantial benefits in survival does not seem to be achievable .
 mortality associated with nosocomial bloodstream infection is multifactorial . 
 source of infection , etiology , age , underlying disease , acute illness , and appropriateness of antimicrobial therapy all contribute to the final outcome . 
 as such , estimates of mortality attributable to bloodstream infection may differ largely according to the presence or absence of risk factors in distinct patient populations . 
 the adverse effect of nosocomial bloodstream infection for the individual patient is substantial , with about a doubling of the risk of death . yet , in settings with a high standard of care in terms of infection prevention and control , the occurrence rate of bloodstream infection is relatively low and therefore its impact on overall icu mortality rather limited . as a consequence , 
 untargeted interventional studies focused on infection prevention should use occurrence rate of infection rather than mortality as outcome variable . 
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