Which protein has the longest half-life among the following plasma proteins?

• A. Albumin
• B. Transferrin
• C. Prealbumin
• D. Retinol-binding protein

Answer: (A)

Knowing how long a plasma protein stays in circulation depends on how quickly it’s cleared and how slowly it’s turned over. Albumin is a large, liver-produced protein that tends to stay in the bloodstream for a long time because minimal renal loss occurs and its turnover is relatively slow. Its half-life is about 19–21 days, making it the longest among the options. Transferrin, the iron-transport protein, is cleared more quickly through receptor-mediated uptake after delivering iron, giving it a half-life of around a week. Prealbumin (transthyretin) is a transport protein with a short half-life, roughly 2 days, reflecting rapid turnover. Retinol-binding protein is small and is cleared quickly, with a half-life of only hours to about a day. Therefore, albumin has the longest half-life.