Alpha-1 Antitrypsin

Alpha-1 antitrypsin (AAT) is a 52 kDa serine protease inhibitor made mainly in hepatocytes and encoded by SERPINA1 on chromosome 14. Its main role is to inhibit neutrophil elastase in the lung and so protect the alveolar wall. The normal allele is M. The common deficiency alleles are Z (Glu342Lys), which polymerises in the endoplasmic reticulum of hepatocytes and causes both low circulating levels and liver injury, and S, which is milder.

The protein is measured in serum by immunonephelometry or immunoturbidimetry in g/L (or micromol/L). Phenotyping by isoelectric focusing or genotyping then identifies the variants.

The test identifies alpha-1 antitrypsin deficiency in patients with early or atypical emphysema, unexplained liver disease at any age, bronchiectasis and some vasculitis and skin disorders. It is also a family screen. Quantification gives the level, and phenotype or genotype gives the cause.

Severe deficiency (PiZZ) affects about 1 in 2000 to 5000 people of European descent, with the highest frequency in northern and western Europe and a low frequency in East Asia and sub-Saharan Africa. It is under recognised: only a small fraction of affected people are diagnosed, and the average delay to diagnosis is several years. It accounts for 1 to 3% of chronic obstructive pulmonary disease.

Indications. Chronic obstructive pulmonary disease or emphysema before 45 years, in a non-smoker or with basal predominant emphysema, bronchiectasis of unclear cause, unexplained chronic liver disease or cirrhosis at any age, neonatal cholestasis, necrotising panniculitis, ANCA-associated vasculitis (especially anti-proteinase 3), poorly controlled asthma with fixed obstruction, family history of deficiency, and a missing alpha-1 band on serum protein electrophoresis.

Specimen. Serum or heparin plasma. Fasting is not needed. Stable for days at 2 to 8 °C. A level is best measured when the patient is well, since AAT is an acute phase protein.

Ranges are method specific, and local ranges take precedence.

Serum AAT level. Range: 0.9 to 2.0 g/L (about 20 to 53 micromol/L). Raised: acute and chronic inflammation, infection, pregnancy, oestrogen therapy, malignancy, tissue injury, and stress, with rises up to 2 to 3 times the baseline. Lowered: genetic deficiency, advanced liver disease, nephrotic syndrome, protein losing enteropathy, malnutrition, and severe burns.

Levels by genotype are approximate:

• MM (normal): 0.9 to 2.0 g/L.
• MZ (carrier): 0.6 to 1.2 g/L.
• SZ: about 0.4 to 0.8 g/L.
• ZZ: 0.1 to 0.7 g/L, about 10 to 15% of normal.
• Null variants (no protein produced): under 0.1 g/L.

A serum level under 0.5 g/L (about 11 micromol/L) is the protective threshold under which the risk of emphysema rises steeply.

Phenotype (isoelectric focusing). Identifies M, S, Z and rarer variants (for example I, F, P and Mmalton). Used when the level and genotype do not agree and to confirm the severe forms.

Genotype. Targeted testing for the S and Z alleles identifies more than 95% of cases. Full sequencing of SERPINA1 is needed for rare and null variants, and when the level is low but S and Z are not found.

Lung disease. Panacinar emphysema, basal predominant on imaging, with onset in the third to fifth decade, airflow obstruction, chronic bronchitis and bronchiectasis. Smoking accelerates decline: ZZ smokers develop symptoms 10 to 15 years earlier than non-smokers. Annual loss of FEV1 is 50 to 100 mL in smokers with ZZ. Replacement therapy with pooled human plasma AAT is considered in non-smokers or ex smokers with ZZ or null genotype, serum level under 11 micromol/L, and FEV1 between 30 and 65% of predicted.

Liver disease.

• Neonatal cholestasis occurs in 10 to 15% of ZZ infants, and most recover, though 2 to 3% need transplant in childhood.
• In adults, ZZ is linked to cirrhosis and hepatocellular carcinoma, with a risk that rises after age 50 and with obesity, alcohol and metabolic liver disease.
• MZ and SZ heterozygotes have a modest increase in risk of liver fibrosis, mostly with a second insult.
• Biopsy shows periodic acid-Schiff positive, diastase resistant globules in periportal hepatocytes.

Other disorders. Necrotising panniculitis (tender nodules on thighs and buttocks that ulcerate), ANCA-associated vasculitis, and granulomatosis with polyangiitis. Fibromuscular dysplasia and aneurysm are reported.

Absent alpha-1 band. A flat alpha-1 region on serum protein electrophoresis is a clue to deficiency in a patient with lung or liver disease.

Interpreting the level.

• Normal level (over 0.9 g/L) with an inflammatory state: a level in the low normal range may reflect an acute phase rise from a deficient baseline, so a repeat in the well state or a genotype is justified when suspicion is high.
• Level 0.5 to 0.9 g/L: heterozygous deficiency (MZ, MS) or intermediate (SZ), with mild or no risk.
• Level under 0.5 g/L: severe deficiency (ZZ, null) until proven otherwise.

Critical values and findings needing immediate action.

• Level under 0.5 g/L (11 micromol/L), which needs specialist referral and smoking cessation.
• Neonatal jaundice with pale stools and a low level.
• Rapid fall in lung function, haemoptysis or respiratory failure in a patient known to have deficiency.
• Cirrhosis with ascites, variceal bleeding or an abnormal liver mass in a patient with ZZ.

Interferences and pitfalls.

• A normal or near normal level during an acute phase response can hide heterozygous or even homozygous deficiency.
• Oestrogen, pregnancy and corticosteroid effects raise levels.
• Recent plasma transfusion, or replacement therapy, alters the level, so the test is done before treatment starts or at a trough.
• Nephelometry may overestimate the level in the presence of dysfunctional or polymerised protein, and phenotype corrects this.
• Null alleles give very low levels with no band and need sequencing.
• Genotyping for S and Z only misses rare variants.
• A level alone does not predict who will develop disease, since smoking and other factors strongly modify risk.
• Family members of any person with deficiency are offered testing.

Special groups.

• Neonates and infants: neonatal hepatitis with cholestasis, prolonged jaundice and failure to thrive. Screening of siblings is advisable.
• Children: liver disease more often than lung disease, with a variable course.
• Pregnancy: levels rise by 30 to 50%, and counselling concerns the risk to the child.
• Older adults: lung and liver disease appear in non-smokers late in life.
• Regions: the Z allele is rare in East Asia and sub-Saharan Africa, so deficiency is infrequent there, and the S allele is common in southern Europe.

Follow up tests. Phenotyping, SERPINA1 genotyping and sequencing, spirometry with bronchodilator response, transfer factor, CT chest, liver function tests, liver elastography, ultrasound, liver biopsy, alpha fetoprotein, serum protein electrophoresis, ANCA, family screening, and smoking cessation support.