Alpha-1-Antitrypsin Deficiency - AAT

Diagnosis

Indications for Testing

  • American Thoracic Society/European Respiratory Society (2003)
    • Adult with early-onset COPD or emphysema without recognized risk factors
    • Necrotizing panniculitis (unexplained)
    • Family history of emphysema, alpha-1-antitrypsin (AAT), bronchiectasis, liver disease, panniculitis
    • Bronchiectasis with no known etiology
    • Unexplained liver disease
    • ANCA vasculitis (anti-PR3 type)
  • Other
    • Newborns with bleeding disorder or prolonged jaundice

Laboratory Testing

  • Initial testing
    • Serum AAT – <60 mg/dL signifies substantial risk for AAT deficiency-related disorders
      • AAT is also an acute phase reactant and may be elevated by other disease processes
        • Up to fourfold increase observed in inflammatory conditions, cancer, and liver disease
        • Plasma concentration can be elevated into the normal range in PI MZ heterozygotes
  • Phenotyping and genotyping
    • Phenotype testing to identify AAT protein variants if AAT concentration is <100 mg/dL
    • Molecular testing of SERPINA1 gene for S and Z alleles if phenotype is abnormal

Histology

  • Immunohistochemistry – alpha-1-antitrypsin (AAT)
    • Homozygosity for Z allele confirms AAT severe phenotype
    • Homozygosity for S allele confirms AAT intermediate phenotype
    • SZ compound heterozygosity confirms AAT intermediate phenotype

Differential Diagnosis

Screening

  • Screening tests available prior to testing for alpha-1-antitrypsin (AAT) levels
    • Pulmonary function studies (irreversible airflow obstruction)
    • Chest x-ray/CT scan
  • Population-based screening not recommended even though disease recognized as being underdiagnosed

Clinical Background

Alpha-1-antitrypsin (AAT, alpha-1-protease inhibitor) is the chief protease inhibitor in human serum. The loss of this protease inhibitor results in the degradation of the connective protein elastin in lung alveoli and increases the risk for developing severe lung disease during early adulthood.

Epidemiology 

  • Prevalence
    • AAT deficiency affects ~2-3% of the 2-3 million patients with chronic obstructive pulmonary disease (COPD) in the U.S.
    • Severe deficiency – 1/6,000 in Caucasian populations; less frequent in other ethnicities
  • Incidence – 1/3,000-5,000
  • Age 
    • Smokers develop disease in 40s
    • Nonsmokers develop disease in 50s

Risk Factors

  • Genetics – SERPINA1
    • Autosomal recessive
    • Isoelectric focusing (PI typing) has identified >100 AAT allelic variants classified according to electrophoretic mobility; most variants have no clinical significance
      • Mutations in the AAT glycoprotein gene on chromosome 14q31-32.3; homozygosity for the Z allele is the most common cause
    • Most common normal phenotype is PI MM (100% AAT activity)
      • PI MM is found in 95% of Caucasians
    • Other common phenotypes and percentage of AAT activity include PI MS (80%), PI SS (60%), PI MZ (57.5%) and PI ZZ (15%)
      • S and Z alleles represent 95% of deficiency phenotypes in the general population
        • PI ZZ is associated with severe liver and lung disease
        • PI SS is associated with milder lung disease
      • Heterozygotes (PI MS and PI MZ) are at slightly increased risk for AAT deficiency-related disorders
  • Tobacco use
    • Tobacco smoke contains oxidants capable of inactivating AAT protein, further impairing reduced AAT function
    • Increases risk of developing severe lung disease; symptoms begin ≥10 years earlier than nonsmokers

Pathophysiology

  • AAT is an acute phase reactant synthesized by the liver
    • Most important role is inhibition of protease neutrophil elastase
  • Inherited deficiency is associated with liver and lung disease
    • Decreased quantities of AAT allow elastase to degrade lung parenchyma
    • Hepatic disease is secondary to accumulation of unsecreted AAT in hepatocytes

Clinical Presentation

Treatment

  • Early diagnosis is crucial in order to begin enzyme-replacement therapy

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
Alpha-1-Antitrypsin (SERPINA1) Enzyme Concentration and 2 Mutations with Reflex to Alpha-1-Antitrypsin Phenotype 0051256
Method: Immunoturbidimetry/Polymerase Chain Reaction/Fluorescence Monitoring/Isoelectric Focusing

Primary test to identify AAT deficiency; includes reflex to phenotyping if AAT concentration is inconsistent with genotype

Mutations other than those targeted in S allele (c.791A>T) and Z allele (c.1024G>A) are not detected

95% clinical sensitivity

AAT-deficient patients may have falsely normal AAT concentrations

Analytic sensitivity may be compromised by rare primer site mutations

 
Alpha-1-Antitrypsin Phenotype (Includes Alpha-1-Antitrypsin) 0080500
Method: Qualitative Isoelectric Focusing/Immunoturbidimetry

Order when AAT level <100 mg/dL to predict particular AAT variant

Acutely ill, AAT-deficient patients may have falsely normal AAT concentrations

 
Alpha-1-Antitrypsin 0050001
Method: Quantitative Immunoturbidimetry

May use to identify AAT deficiency

Acutely ill, AAT-deficient patients may have falsely normal AAT concentrations

Does not determine phenotype or genotype

Alpha-1-Antitrypsin (AAT) by Immunohistochemistry 2003424
Method: Immunohistochemistry

Aid in histologic diagnosis of AAT

Stained and returned to client pathologist; if consultation required, contact anatomic pathology, surgical consult or hematopathology