These are the methods of discriminating the patients who are malnourished or who are at risk of malnutrition during their ICU stay. In short, one makes a "subjective global assessment of nutritional status" on the basis of history and examination, and then collects supporting anthropometric and laboratory data, which can be used to monitor response to therapy. Previous SAQs on this topic have included:
Biochemistry and physiology:
This technique is a wholistic history-and-examination guided method of classifying a patient as well nourished, moderately malnourished, or severely malnourished. It can be combined with supportive anthropometric indices, and with biochemical analysis of endocrine and metabolic function.
In short, it has none. Even the patholgists agree. Albumin is what one might call a "negative acute phase protein", and one's production of albumin is among the first frivolities sacrificed by the fascist austerity measures of the hypercatabolic stress response. In critical illness, the serum albumin is not a reliable measure of nutritional status, because (almost by definition) everybody in ICU is sufficiently unwell to have a vigorous stress response, and markedly diminished albumin production. It is probably more related to the severity of the illness (by magnitude of its decrease, rather like a magnitude of increase in the CRP). However, it has seemed attractive in the past, because its production is also sacrificed in chronic malnutrition, and it has a long halflife.
Prealbumin is perhaps slighly more useful than albumin as a means of assessing recent nutrition. Its halflife is only 2 days, which means that its levels will fall rapidly if your stressed-out body has recently prioritised the synthesis of CRP and fibrinogen. In fact, it seems prealbumin levels are inversely proportional to CRP. So in ICU a single prealbumin level is probably meaningless. However, sequential measurements can be used to assess the adequacy of nutritional support. There has been at least one study which has demonstrated that for an ICU patient, an increase in prealbumin indicates that at least 65% of protein-energy needs have been met.
The equation for calculating nitrogen balance is as follows:
Obviously, the inconvenience of 24-hour urine collection and the wild inaccuracy of this tecnique have limited its clinical use. The errors tend to overestimate intake and underestimate output, thereby leading to falsely positive balances. Intake, after all, is dependent on a working gut (and who in the ICU can confidently report one of those?). Urinary output can be measured, but what of sweat and faecal losses? And what if the patient produces no urine? Don't get me started on nitrogen balance in dialysis.
The repsonse to skin-prick hypersensitivity testing is a very indirect method of assessing ntritional status. It rests on the premise that malnutrition results in decreased lymphocyte number and function, which in turn will result in a delayed response to skin-prick testing. This has been observed in 1973 among malnourished children, an it is possible that somebody somewhere did this routinely to adults, but interest in this practice has been lost since perhaps the early 1980s, and now it is only found in CICM college answers.