## 2 Screening  
A high prevalence of malnutrition is observed at ICU admission [5], with a further energy and protein deficit (50% and 60%, respectively) occurring during ICU stay [51; 52]. Indeed, an international multicenter observational study revealed an association between the increase of energy and protein intake and the decrease of mortality in patients with a body mass index (BMI) <25 or >35 kg/m\({}^{2}\)[6]. The same research group failed to validate this concept in the TOP-UP trial [53]. More generally, no nutritional intervention reduced mortality in critically ill patients, except withholding early glutamine in patients with multiorgan failure [14].  
On the other hand, some recent RCTs investigating the timing of initiation as well as the quantity and the route of MNT showed contradictory results [7; 54; 55; 56; 57]. Waiting for more conclusive studies, actual guidelines suggest that all critically ill patients should be routinely screened with the aim of preventing malnutrition [17; 27].  
Many different screening tools to identify the malnutrition risk are available. Some scores are widely used, such as (1) the Nutrition Risk Screening (NRS) 2002 [58], focusing on nutritional state,illness severity, and age; (2) the Nutrition Risk in the Critically Ill (NUTRIC) score [59], including a severity score, comorbidities, and the number of hospital days; (3) the Subjective Global Assessment (SGA), combining historical data and a clinical examination [60]; and (4) the Mini-Nutrition Assessment (MNA) [61], which is specific for the elderly population. Despite the large number of existing scores, none has been validated for the critically ill thus far [62] and none have been able to identify patients who improve with enhanced feeding.  
Laboratory data, such as albumin and pre-albumin, are frequently used as markers of nutritional status, but cannot be clearly interpreted during critical illness, as low albumin and prealbumin levels occur as a response to increased inflammation [63].  
Weight changes are also difficult to interpret considering the relevant amounts of fluid administration and depletion during the ICU and IMC-U stay. Thus, weight loss does not reflect loss of lean body mass. Nevertheless, this aspect is an important issue for the detection of malnutrition, as the loss of lean body mass has been associated with longer hospitalization and reduced quality of life and functional abilities [60]. Some methods, such as ultrasound [64] or computerized tomography (CT) scan [65], are promising tools that can help to determine the extent of lean body mass loss. However, these methods still need to be validated in clinical practice. Moreover, the way in which they influence therapy is not yet clear. Bioelectrical impedance to assess body composition and lean body mass may be useful, but its interpretation is complicated by important fluid changes that occur in the critically ill [66].  
Concerning these screening tools, ASPEN [27] and ESPEN [17] have taken different positions. While the current ASPEN guidelines recommend using either the NRS 2002 or the NUTRIC-Score for nutritional risk determination [27], the new ESPEN guidelines propose a more general clinical assessment including anamnesis, evaluation of muscle mass and strength, unintentional weight loss, and body composition (GPP). Furthermore, all critically ill patients with a length of stay of more than 48 h should be considered at risk for malnutrition (GPP) [17].