Proteins

Diagnosis

Indications for Testing

Laboratory Testing

  • Albumin – usual first-line test in evaluating nutritional status
    • Elevated concentrations – dehydration
    • Decreased concentrations are very common
      • Impaired synthesis
      • 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
    • 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
    • Prealbumin used as marker of nutritional status in the following
      • Premature infants
      • Cancer patients
      • Surgical patients
    • Prealbumin is a negative acute phase reactant
      • Due to decreased synthesis, prealbumin serum concentrations fall in these diseases
        • 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
  • 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, including 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 the following
        • Inflammation
        • Malignancy
        • Chronic liver disease
        • Protein loss
    • Elevated fecal clearance of alpha-1-antitrypsin in protein-losing enteropathy
  • Also recommend vitamin/mineral assay testing

Clinical Background

Hepatic proteins refer to a group of proteins synthesized in the liver that may be used in the assessment of nutritional status. Hepatic proteins include the following

  • Albumin
  • Prealbumin (transthyretin)
  • Retinol-binding protein (RBP)
  • Transferrin

Etiologies

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
    • Prealbumin has 2-day half-life
      • Short half-life makes it a good indicator for early monitoring
      • Unaffected by hydration status
  • 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

Indications for Laboratory Testing

  • Tests generally appear in the order most useful for common clinical situations
  • Click on number for test-specific information in the ARUP Laboratory Test Directory
Test Name and Number Recommended Use Limitations Follow Up
Albumin, Serum or Plasma by Spectrophotometry 0020030
Method: Quantitative Spectrophotometry

Evaluate production of albumin by liver; assess nutritional status

Assess nephrotic syndrome and protein-losing enteropathy

Albumin may be acutely decreased in sepsis or trauma

 
Prealbumin, Serum 0050435
Method: Immunoturbidimetry

Assess nutritional status in premature infants, in cancer patients and surgical patients

Recommended protein measurement to evaluate nutritional status in hospitalized patients

Assess nephrotic syndrome and protein-losing enteropathy

Prealbumin may be acutely decreased in sepsis or trauma  
Retinol Binding Protein 0050467
Method: Quantitative Nephelometry

Indicate early malnutrition, acute and chronic hepatitic disease, advanced chronic renal insufficiency and cystic fibrosis

Assess nephrotic syndrome and protein-losing enteropathy

   
Transferrin, Serum 0050570
Method: Quantitative Immunoturbidimetry

Aid in differential diagnosis of malnutrition

Monitor iron deficiency anemia

   
Alpha-1-Antitrypsin Clearance, Quantitative by ELISA, Timed Stool 2011043
Method: Enzyme-Linked Immunosorbent Assay/Quantitative Immunoturbidimetry

Order as a follow-up test when protein-losing enteropathy is suspected

If serum or plasma specimen is not submitted in conjunction with timed stool collection, order alpha-1-antitrypsin random stool test

   
Alpha-1-Antitrypsin, Quantitative by ELISA, Random Stool 2011041
Method: Quantitative Enzyme-Linked Immunosorbent Assay

Order as a follow-up test when protein-losing enteropathy is suspected

   
Alpha-1-Antitrypsin 0050001
Method: Quantitative Immunoturbidimetry

Calculations for the alpha-1-antitrypsin fecal test require that this test also be ordered