Alpha-1 Antitrypsin (AAT) Level
A protein made by the liver that helps protect the lungs. A low level can be a first clue to alpha-1 antitrypsin deficiency, an inherited condition.
AI-verified against cited sources. Awaiting expert reviewAlso called AAT, A1ATLOINC 1825-9
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What it is
Alpha-1 antitrypsin (AAT) is a protein your liver makes and releases into the blood. It helps protect your lungs from damage caused by inflammation and irritants such as smoke. This test measures how much AAT is in your blood. It is mostly used when someone has unexplained lung or liver problems, or has a family member with AAT deficiency.
Why it matters
AAT deficiency is an inherited condition. It can lead to lung disease such as COPD and, in some people, to liver damage. A low AAT level is a reason to look further, usually with a genetic test or a phenotype test that shows which AAT variants a person has. Knowing early helps people and their care teams focus on protecting the lungs and liver, for example by avoiding smoke.
If it’s low
A level below your lab's range can be a sign of AAT deficiency, but a blood level alone cannot confirm it. The usual next step is a genetic (genotype) or phenotype test.
Common causes
- AAT deficiency, caused by inherited changes in both copies of the SERPINA1 gene
- Being a carrier (one changed copy), which can give a mildly low level or sometimes a level in the usual range
- Conditions that lower many blood proteins at once, such as severe liver disease or large protein loss through the kidneys or gut
What can help
- Ask about a genotype or phenotype test to confirm what a low level means
- Ask whether close family members should be offered testing
- MedlinePlus lists not smoking and avoiding alcohol among the steps for people with AAT deficiency. Ask your clinician about support if you need it.
If it’s high
A level above your lab's range is often a sign of inflammation or another temporary cause, rather than a problem with AAT itself. AAT is an 'acute-phase' protein, which means the body makes more of it during infection, inflammation and some other stresses.
Common causes
- Infection, injury or other inflammation
- Pregnancy
- Estrogen-containing medicines, such as birth control pills or hormone therapy
- Some long-lasting conditions that cause ongoing inflammation
What can help
- Ask your clinician whether an illness, pregnancy or a hormone medicine could explain the result
- If AAT deficiency is still a question, ask whether a genetic test makes sense, because inflammation can hide a low level
Usual ranges
What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.
The usual range depends on sex and age. Choose women or men to see the one that applies.
- 18+90–200 mg/dL
Sources: Labcorp, ARUP Laboratories, Orphanet Journal of Rare Diseases
See Alpha-1 antitrypsin by age and sex, with the U.S. survey numbers where they exist.
What can affect your result
- Infection or inflammation at the time of the draw, since AAT is an acute-phase protein
- Pregnancy and estrogen-containing medicines
- Age: levels change in children, so children have their own ranges
- Lab method: ARUP prints 90–200 mg/dL, Labcorp's current page lists 101–187 mg/dL above age 40, and older reference lists give 150–350 mg/dL
- Sex: Labcorp lists slightly different ranges for males and females aged 13 to 40
- AAT is an acute-phase protein, so infection, injury and other inflammation raise it. This can hide a low level in a person who carries a changed gene.
- Labcorp advises retesting in 10 to 14 days if a CRP (inflammation) test is positive.
- Pregnancy and estrogen-containing medicines can also raise the level.
- ARUP rejects hemolyzed samples.
Rises with inflammation can appear within days and settle as the illness passes.
When to talk to a clinician
Talk with your clinician soon if a low level comes with a cough that won't go away, wheezing or shortness of breath, especially if you smoke. Contact your clinician promptly if you notice yellow skin or eyes, dark urine or swelling in the belly. A baby with jaundice or unexpected liver test results should be seen by the baby's clinician. Call 911 or your local emergency number for severe trouble breathing, chest pain or fainting.
Questions you could ask
- Is the AAT level enough for my situation, or do we also need a genetic test?
- Could an infection, pregnancy or a medicine explain a higher level?
- Should we recheck when I'm well?
- Should I have a genotype or phenotype test to find out what a low level means?
- Should my close family members be offered testing?
- Which lung or liver checks, if any, make sense for me?
Questions people ask
My AAT level is in the usual range. Can I still be a carrier?
Yes. A carrier has one changed copy of the gene and can have a level in the usual range, and inflammation can push any level up. If AAT deficiency is a real question, such as when a family member has it, a genetic test is the way to settle it.
My AAT level is low. Does that mean I have AAT deficiency?
It means more testing is worth considering. MedlinePlus says a low level may mean AAT deficiency, and that a genotype or phenotype test is needed to confirm it. Your clinician can explain what a low result means for you.
Why does my report show AAT in g/L instead of mg/dL?
Labs outside the US often report in g/L. To convert, multiply g/L by 100 to get mg/dL, so 1.5 g/L is 150 mg/dL.
Show the science and sources
Units
- 1 mg/dL = 1 mg/dL
- 1 mg/dL = 0.01 g/L
Open the Alpha-1 antitrypsin unit converter
Is a change real?
Alpha-1 antitrypsin naturally varies about 3.8% from test to test in the same healthy person. Between two results from the same lab, a change of roughly 11% or more is unlikely to be day-to-day noise or lab variation (95% reference change value). Source: The European Biological Variation Study (EuBIVAS): a summary report.
For clinicians and researchers
AAT is a serine protease inhibitor encoded by SERPINA1, made mainly by hepatocytes, whose main job is to inhibit neutrophil elastase in the lung. The common deficiency alleles are Z and S, and most severe deficiency is PiZZ. Liver disease is seen mainly with PiZZ (Fregonese 2008; Nelson 2012), and the COPD Foundation guidance also lists SZ and a few rare variants as at risk (Sandhaus 2016). Lung disease is linked to PiZZ and Null types, and less strongly to PiSZ (Fregonese 2008). AAT is an acute-phase reactant, so levels rise with inflammation, pregnancy and estrogen use, and can be misleadingly normal in heterozygotes. Concentration is measured by nephelometry or immunoturbidimetry (older methods include radial immunodiffusion and rocket immunoelectrophoresis). Levels overlap between genotypes, so a level cannot identify the genotype. The 2016 COPD Foundation guidance recommends genotyping for at least the S and Z alleles in symptomatic people, with Pi typing or expanded genotyping as confirmatory options, because a level alone cannot identify people at risk (Sandhaus 2016). A level below 50% of the lower limit of normal is regarded as clear evidence of deficiency (Nelson 2012). Calibration differs by assay and era, which is why older lists show 150–350 mg/dL, ARUP shows 90–200 mg/dL and Labcorp's current page shows 101–187 mg/dL above age 40.
LOINC 1825-9 (primary), 6771-0, 76481-1. Specimen: serum-or-plasma.
Sources
- Alpha-1-Antitrypsin, Serum (test 001982), sample report, Labcorp
- Alpha-1-Antitrypsin, Serum (test 001982), Labcorp
- Pediatric Testing Reference Ranges, Labcorp
- Alpha-1-Antitrypsin (test 0050001), ARUP Laboratories
- Alpha-1 Antitrypsin Testing, MedlinePlus, National Library of Medicine
- The Diagnosis and Management of Alpha-1 Antitrypsin Deficiency in the Adult, Chronic Obstructive Pulmonary Diseases: Journal of the COPD Foundation (2016)
- Diagnosis and Management of Patients with α1-Antitrypsin (A1AT) Deficiency, Clinical Gastroenterology and Hepatology (2012)
- Hereditary alpha-1-antitrypsin deficiency and its clinical consequences, Orphanet Journal of Rare Diseases (2008)
AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.