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Health condition

A1AT phenotypes explained

Alpha-1 antitrypsin results are reported as phenotypes - MM, MZ, SZ, ZZ and others - and what each combination means for your lungs and liver differs substantially.

Why trust this guide

  • 01

    Editorial team

    Written by our editorial team from the UK sources listed on each page.

  • 02

    Sourced from guidance

    Checked against current NHS and UK specialist society guidance at the time of review.

  • 03

    Current for 2026

    Reviewed on 2026-09-27 with the next review scheduled for 2027-09-27.

Key facts

The essentials at a glance.

Which gene and test confirm it, who should be tested in the family, and which specialist to see.

  • M is the normal variant; S produces moderately reduced levels; Z produces low levels and can injure the liver

  • ZZ carries the major risk of emphysema and liver disease; SZ carries an intermediate lung risk, particularly in smokers

  • MZ carriers have a mildly increased risk, mainly relevant if they smoke or have another liver condition

  • Phenotype (or genotype) testing distinguishes these combinations and guides family testing

Understanding the condition

What it is and what happens next.

What causes it, how it is confirmed, and the testing and specialist routes available in the UK.

What it is and how it presents

Because everyone inherits one SERPINA1 allele from each parent, results come as pairs. MM is normal. MZ and MS carriers usually have enough protective protein for ordinary life, though MZ status modestly raises liver and lung risk, mostly when combined with smoking or alcohol. SZ sits in between, and ZZ - about 1 in 2,000 Northern Europeans - is the classic severe deficiency associated with early emphysema and liver fibrosis.

Testing, surveillance and specialist care

Interpreting a phenotype properly means pairing it with the measured level, lung function and liver checks, which is where a respiratory physician with alpha-1 experience earns their keep; carriers also often want advice about testing partners and children, best addressed through genetic counselling. NHS clinical genetics services carry some of the longest waits in the health service, often many months for a first appointment, whereas private genetic testing and a specialist review can typically be arranged within around two weeks.

Seeing a specialist

Who to see, and how quickly you can be seen.

The specialist for this condition is usually a respiratory physician. Your GP can refer you on the NHS, or you can arrange a private consultation directly.

A quiet reminder

This guide is for information, not medical advice.

Your GP knows your history and can tell you which parts apply to you. If in doubt, see them - and seek urgent care for any emergency symptoms.

Frequently asked

Common questions about A1AT phenotypes explained.

Quick, plain-English answers to what people ask us most.

  • What are A1AT phenotypes?

    A1AT phenotypes describe the different genetic variants of the alpha-1 antitrypsin gene that a person carries, such as MM (normal), MZ, SZ, or ZZ, which determine how much functional protein is produced and the resulting health risk.

  • What causes different A1AT phenotypes?

    Phenotypes arise from inherited variations in the SERPINA1 gene, passed down from both parents. Combinations of normal (M) and deficient (S or Z) alleles determine whether a person has normal, intermediate, or significantly reduced A1AT levels.

  • What are the symptoms linked to abnormal A1AT phenotypes?

    Low-risk phenotypes usually cause no symptoms, while high-risk combinations (like ZZ) increase the risk of early emphysema, chronic liver disease, and occasionally skin or vascular problems, particularly in smokers.

  • Are abnormal A1AT phenotypes dangerous?

    Some phenotypes, particularly ZZ and certain rarer null variants, carry a substantial risk of lung and liver disease, especially with smoking, so identifying phenotype matters for risk assessment and prevention.

  • Are A1AT phenotypes genetic?

    Yes, they are entirely inherited, following a pattern where each parent contributes one allele. Knowing your phenotype can help relatives understand their own risk and consider testing.

  • Can A1AT phenotype-related disease be prevented?

    You cannot change your phenotype, but you can substantially reduce disease risk by never smoking, avoiding occupational dust and fume exposure, and having regular liver and lung monitoring if you carry a high-risk phenotype.

  • How is A1AT phenotype tested and interpreted?

    Testing combines a blood A1AT level with genetic phenotyping or genotyping to identify the specific allele combination, which a specialist then uses to estimate lifetime risk and guide monitoring.

  • How often should A1AT phenotype carriers be monitored?

    Monitoring frequency depends on phenotype risk level; high-risk carriers are typically reviewed annually with lung function tests and periodic liver assessment, while low-risk carriers may need little or no routine follow-up.

  • ZZ phenotype vs MZ phenotype — what's the difference?

    ZZ phenotype means both copies of the gene are the deficient Z variant, causing markedly low A1AT levels and high disease risk, while MZ means one normal and one Z allele, giving intermediate levels and a much smaller, though not zero, increased risk.

  • When should I see a doctor about A1AT phenotype testing, and how quickly can I be seen privately?

    Consider testing if you have unexplained early emphysema, liver disease, or a family history of A1AT deficiency. NHS genetic testing can take several weeks, while private testing and specialist interpretation is often available within about two weeks.