Proteins

Dx
Background
Lab Tests

Diagnosis

Indications for Testing

Patient with known disease that would cause compromised nutritional status (eg, cancer, malabsorption), children with failure to thrive

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
- 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

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
- 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
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, Feces 0099991 Method: Quantitative Radial Immunodiffusion	Order as a follow-up test when protein-losing enteropathy is suspected Serves as a clearance measure; calculations require that serum alpha-1-antitrypsin test also be ordered	Blood in stool or diarrhea may increase clearance
Alpha-1-Antitrypsin 0050001 Method: Quantitative Immunoturbidimetry	Calculations for the alpha-1-antitrypsin fecal test require that this test also be ordered