Hepatic Proteins

Hepatic proteins are a group of proteins synthesized in the liver that may be used in the assessment of nutritional status. Hepatic proteins include albumin, prealbumin (transthyretin), retinol-binding protein (RBP), and transferrin.

Diagnosis

Indications for Testing

Known disease that would cause compromised nutritional status (eg, cancer, malabsorption)
Failure to thrive in children

Laboratory Testing

Albumin – usual first-line test in evaluating nutritional status
- Elevated concentrations – dehydration
- Decreased concentrations are very common
  - Impaired synthesis
    - Primary (eg, liver disease)
    - Secondary (eg, low protein intake)
  - Increased catabolism – result of tissue damage and inflammation
  - Reduced absorption of amino acids – malabsorption or malnutrition
  - Excessive protein loss in urine, feces, or skin – glomerulonephritis, nephrotic syndrome, protein-losing enteropathy
  - Altered distribution that sequesters large amounts of albumin in extravascular compartment
    - Nephrotic syndrome
    - Congestive heart failure
- Albumin measurement in urine may aid in early detection of renal involvement in chronic diseases
Prealbumin – may be a better early screening test due to short half-life
- Used as marker of nutritional status in
  - Premature infants
  - Cancer patients
  - Surgical patients
- Negative acute phase reactant – concentrations fall in the following diseases due to decreased synthesis
  - Inflammation
  - Malignancy
  - Cirrhosis of the liver
  - Protein-wasting diseases of the gut or kidney
- Recommended test for protein measurement in evaluation of nutrition in hospitalized patients
Retinol-binding protein (RBP), transferrin, and fecal alpha-1-antitrypsin – less widely used as screening tests
- Decreased concentrations of RBP – cystic fibrosis, liver disease
- Elevated concentrations of transferrin
  - Malnutrition
  - Acute inflammation
  - Infection
  - Renal disorders
  - Red blood cell disorders (eg, iron deficiency)
  - High concentrations can occur in pregnancy and during estrogen administration
- Decreased concentrations of transferrin
  - Transferrin is a negative acute phase reactant
  - Low concentrations occur in
    - Inflammation
    - Malignancy
    - Chronic liver disease
    - Protein loss
- Elevated fecal clearance of alpha-1-antitrypsin in protein-losing enteropathy
Vitamin/mineral assay testing also recommended

Background

Etiologies

Starvation
Chronic disease
- HIV
- Malignancy
- Chronic liver disease
- Chronic kidney disease
- Chronic obstructive pulmonary disease (COPD)
Anorexia
Malabsorption
Critical illness or trauma

Pathophysiology

Albumin
- Function – carrier protein for minerals, fatty acids, vitamins, and hormones
- Most abundant protein in human plasma – 55-65% of total protein content
- Most commonly monitored protein – long half-life (20 days) makes it a relatively insensitive marker
Prealbumin (transthyretin)
- Function – carrier protein for thyroid hormone
- 2-day half-life
  - Short half-life makes it a good indicator for early monitoring
  - Unaffected by hydration status
Retinol-binding protein (RBP)
- Function – responsible for binding and transporting retinol (vitamin A)
- Short half-life (11 hours) makes it an excellent indicator of early malnutrition
Transferrin
- Function – carrier protein for iron
- Presence of transferrin in serum and other body fluids aids in differential diagnosis
Alpha-1-antitrypsin (fecal)
- Function – protease inhibitor

Clinical Presentation

Constitutional – weight loss, muscle wasting, fatigue, failure to thrive (children)
Skin changes from vitamin deficiencies may occur in chronic loss
Extremes – kwashiorkor manifesting with ascites, edema