Rapid turnover proteins in critically ill ICU patients. Negative acute phase proteins or nutritional indicators?

Rapid turnover proteins are often used as metabolic indicators in patients receiving nutritional therapy. However, their plasmatic concentration can be influenced by activation of acute phase reaction due to stress. The aim of this prospective, observational study was to evaluate changes of positive and negative acute phase proteins in mechanically ventilated ICU patients with acute stress.

Plasmatic concentrations of prealbumin (PRA), retinol-binding protein (RBP), c-reactive protein (CRP) as well as resting energy expenditure (REE) and nitrogen balance were measured in thirty ICU patients before starting nutritional support (Baseline) and then after 3 and 8 days of parenteral nutrition (TPN).

Plasmatic concentrations of CRP were high at baseline and did not change (p = 0.47), while RBP and PRA were low and progressively increased during the study (p = 0.0001 and p = 0.004). Percentage changes from baseline of both PRA and RBP were significantly correlated with nitrogen balance (p = 0.01 and p = 0.009); while no significant correlation was observed between changes of rapid turnover proteins and CRP (p = 0.72 and p = 0.10).

All the variables involved in the study are known to be influenced by both metabolic state and resolution of inflammation. However, the observed changes of rapid turnover proteins significantly correlate with nitrogen balance in the face of a persistent inflammation, as documented by CRP plasmatic concentrations. This suggests that RBP and PRA monitoring may be used as complement clinical evaluation of nutritional therapy also in ICU patients with ongoing inflammation.