What is Prealbumin?

Prealbumin AKA transthyretin is a transport protein synthesized by the liver that binds thyroxine. It is used in hospital as a marker of a patients nutritional status, more specifically a measure of 'protein' nutritional status. (other markers for protein nurtional status include: albumin, retinol-binding protein, transferrin, creatinine, blood urea nitrogen). Serum prealbumin is often requested when patients are on parenteral nutrition or  other forms of nutritional support. Normal serum prealbumin concentrations range from 16 to 40 mg/dL; values of <16 mg/dL are associated with malnutrition. It has a half life of around two days, much shorter than albumin's which is around 20 days, making it a good marker to understand acute changes in nutritional status. There will also be changes in prealbumin with liver disease and chronic kidney disease. The problem is that prealbumin is related to acute phase reactants meaning that it is affected in inflammatory states, such as with injury or infections. It can help to look at C-reactive protein (CRP) when assessing prealbumin, CRP is also an acute phase protein (in fact many studies look at the prealbumin/CRP ratio). Another drawback is that the prealbumin test is more expensive than a simple serum albumin test and so in some institutions the test is not available. Some studies have been published recently that suggest that there is little relation between nutritional markers such as prealbumin and actual nutritional status probably because of too much interference by inflammatory states (source). 

Summary:

• synthesized by liver and bind thyroid hormone
• short half life, normal value between 16-40mg/dL
• marker of acute changes in protein-nutritional status
• best assessed with a complete nutritional panel and with CRP index
• nutritional markers are not a replacement for a good physical exam and history

sources: http://www.medscape.com/viewarticle/474066_6, http://www.mayomedicallaboratories.com/test-catalog/Clinical+and+Interpretive/9005, http://emedicine.medscape.com/article/985140-overview, https://www.urmc.rochester.edu/encyclopedia/content.aspx?contentTypeID=167&ContentID=prealbumin, https://labtestsonline.org/understanding/analytes/prealbumin/tab/test/, http://pen.sagepub.com/content/39/7/870.long, http://www.ncbi.nlm.nih.gov/pubmed/25912205