Protein Nutritional Status: Serum Albumin and Prealbumin

Serum albumin and prealbumin (transthyretin) are circulating transport proteins synthesised by the liver that have long been used as laboratory indicators of protein nutritional status. Both are now understood to be strongly influenced by inflammation and disease, so contemporary consensus treats them less as direct measures of nutrition and more as markers of illness severity and prognosis.

Definition

Albumin and prealbumin (transthyretin) are hepatically synthesised plasma transport proteins whose serum concentrations have been used as biochemical markers of protein nutritional status, but which also fall as negative acute-phase reactants during inflammation.

Scope

This topic covers the two classic visceral protein markers - albumin and prealbumin - what they reflect, why their plasma concentrations fall during acute illness, and how their role in defining malnutrition has been reframed by ASPEN/Academy and GLIM consensus statements. It is a reference treatment of measurement meaning and limitations, not a guide to interpreting any individual's values.

Core questions

• What do albumin and prealbumin concentrations actually reflect in a sick patient - nutrition or inflammation?
• Why does prealbumin's shorter half-life make it more responsive to short-term change than albumin?
• Why have consensus frameworks removed serum proteins from the core criteria for diagnosing malnutrition?

Key concepts

• Visceral (transport) proteins
• Negative acute-phase reactants
• Protein half-life (albumin ~20 days, prealbumin ~2 days)
• Inflammation as a confounder
• Distribution volume and hydration effects
• Prognostic versus nutritional interpretation

Mechanisms

Albumin and prealbumin are made by the liver and circulate in plasma; their concentrations depend on synthesis rate, distribution between compartments, catabolism, and losses. During the acute-phase response, hepatic priorities shift toward positive acute-phase proteins and away from albumin and prealbumin, so their levels fall in inflammation and critical illness regardless of nutrient intake - they are negative acute-phase reactants (Shenkin, 2006). Albumin has a long half-life of roughly three weeks, making it slow to change, whereas prealbumin's half-life of about two days makes it more responsive to short-term shifts but equally confounded by inflammation (Ikizler, 2014). Because of this, a low level signals illness and risk rather than isolated protein deficiency.

Clinical relevance

These proteins are widely measured and frequently discussed in nutrition appraisal, so understanding what they do and do not indicate is important for reading the literature. The entry describes their physiological meaning and confounders at a conceptual level and does not provide thresholds or recommendations for evaluating an individual's nutritional status.

Epidemiology

Low albumin and prealbumin levels are common in hospitalised, critically ill, and chronically ill populations and correlate with adverse outcomes, but largely because they track inflammation and disease severity. This recognition drove the ASPEN/Academy consensus (White et al., 2012) and the GLIM criteria (Cederholm et al., 2019) to exclude serum proteins from the defining criteria for malnutrition.

History

For much of the twentieth century, hypoalbuminaemia was treated as a hallmark of protein-energy malnutrition, and prealbumin was later promoted as a more sensitive marker because of its shorter half-life. Growing evidence that both are dominated by the acute-phase response prompted a reappraisal, and by the 2010s consensus statements had reclassified them as indicators of inflammation and prognosis rather than direct nutritional markers (Shenkin, 2006; White et al., 2012; Cederholm et al., 2019).

Debates

Should albumin and prealbumin still be called nutritional markers?

Because both proteins fall during inflammation independently of intake, the prevailing view is that they reflect disease severity and prognosis more than nutritional status, and they have been removed from the core diagnostic criteria for malnutrition.

Key figures

• Alan Shenkin
• T. Alp Ikizler
• Jeanette White
• Gordon Jensen
• Rosalind Gibson

Seminal works

• shenkin-2006
• white-2012
• cederholm-2019

Frequently asked questions

Why is prealbumin sometimes preferred over albumin?

Prealbumin has a much shorter half-life (about two days versus three weeks for albumin), so it responds faster to change; however, like albumin it falls during inflammation, so it is not a clean nutritional marker.

Does a low albumin mean a person is malnourished?

Not on its own. Low albumin most often reflects inflammation, illness, or fluid shifts, which is why current consensus frameworks do not use it as a defining criterion for malnutrition.