Pathophysiology
Clinical meaning
Advanced nutritional assessment requires understanding the kinetics of visceral protein markers and nitrogen metabolism. Albumin (molecular weight 66 kDa, half-life 14-20 days, pool size 4-5 g/kg) is synthesized exclusively by hepatocytes at a rate of 150-250 mg/kg/day. As a negative acute-phase reactant, albumin synthesis decreases during inflammation when hepatic protein synthesis shifts to positive acute-phase reactants (CRP, fibrinogen, ferritin). Albumin levels are also influenced by hydration status, capillary permeability, and hepatic synthetic function, making it a poor marker of acute nutritional changes. Prealbumin (transthyretin, molecular weight 55 kDa, half-life 2-3 days, pool size 10 mg/kg) reflects recent protein intake more accurately because of its short half-life and small body pool. However, prealbumin is also a negative acute-phase reactant and decreases with inflammation, renal failure (filtered and catabolized by the proximal tubule), and hepatic dysfunction. Retinol-binding protein (half-life 12 hours) is the most rapidly responsive visceral protein marker but is rarely used clinically due to cost and availability. CRP should be measured alongside prealbumin to differentiate inflammation-driven decreases from true malnutrition. Nitrogen balance quantifies the relationship between protein intake and protein...