Prealbumin is a lab test used to evaluate nutritional status.  Prealbumin is a protein. 

Tests are used to evaluate protein status at time of surgery. Serum albumin has the longest half-life at 18 to 20 days and is the most extensively used parameter. Low serum albumin (<2.2 g/dL) is a marker of a negative catabolic state and a predictor of poor outcome. Surgical stress, burns, infection, other acute stresses, hepatic disease, and renal disease decrease serum albumin levels. Neither starvation nor supplementation has any impact on levels.[1]

Serum transferrin has an intermediate half-life of eight to nine days and is reflective of recent systemic inflammatory response. Transferrin also reflects iron status, and low transferrin should be considered an indicator of protein status only in the setting of normal serum iron. [1]

Serum prealbumin (transthyretin) has the shortest half-life at two to three days. Prealbumin responds quickly to the onset of catabolism or inflammation and rises rapidly when it resolves. In general, inflammatory cytokines reduce the level of prealbumin synthesis by the liver, and it can also be reduced with renal and hepatic disease but, like all "negative acute phase reactants," is likely lowered via capillary leak. [1]

Use of prealbumin in the ICU might not be useful.Although nutritional chemistries are often measured and are prognostic indicators of poor outcome, most experts are not proponents of this approach. Such surrogates, particularly reduced serum proteins (eg, albumin, prealbumin/transthyretin), are nonspecific and should be assumed to be due to systemic inflammatory critical illness. In addition, they are not responsive to nutrient intake and therefore, cannot be used as therapeutic biomarkers. [2]