Retinol-binding Protein (RBP) is a transport protein that carries vitamin A (retinol) in the blood and reflects the liver's protein synthesis capacity. This test measures RBP levels to evaluate nutritional status, liver function, and kidney function. RBP has a short half-life of approximately 12 hours, making it a sensitive marker for recent changes in protein metabolism and nutritional status. Low RBP levels can indicate protein malnutrition, liver disease, or kidney dysfunction, while elevated levels may suggest vitamin A toxicity or certain inflammatory conditions. This test is particularly valuable for monitoring patients on total parenteral nutrition, evaluating early malnutrition, and assessing liver synthetic function in various disease states. RBP measurements provide more immediate feedback on nutritional interventions than other longer-lived proteins.