Testing for IgA deficiency is straightforward: a simple blood draw measures the level of immunoglobulin A in your serum. If your IgA falls below 7 mg/dL while your other immunoglobulins (IgG and IgM) remain normal, and you’re older than four, that meets the international diagnostic threshold for selective IgA deficiency. Here’s what the full testing process looks like, from the initial blood work to follow-up evaluations.
The Core Blood Test
The primary test is a quantitative serum immunoglobulin panel. Your blood is drawn and sent to a lab, which measures levels of IgG, IgA, IgM, and sometimes IgE. The technique most commonly used is called nephelometry, which detects how much light scatters through a sample to determine the concentration of each antibody type. Results typically come back within a few days.
What matters isn’t just your IgA number in isolation. For a diagnosis of selective IgA deficiency, your IgA must be very low (below 7 mg/dL, or essentially undetectable) while IgG and IgM stay within their normal ranges. If all three antibody types are low, that points toward a different condition, such as common variable immunodeficiency (CVID), which involves more widespread immune dysfunction and carries a higher risk of serious complications like bronchiectasis, autoimmune disease, and certain cancers.
European guidelines from ESID require that undetectable IgA levels be confirmed on at least two separate blood draws before a formal diagnosis is made. This repeat testing helps rule out a temporary lab anomaly or a transient dip caused by illness.
Why Age Matters for Diagnosis
A definitive diagnosis of IgA deficiency cannot be made in children under four years old. The reason is developmental: the IgA system matures slowly after birth, and many young children with low IgA levels will see those levels rise naturally as their immune system catches up. Testing a toddler and finding low IgA might simply reflect a system that hasn’t fully come online yet, not a permanent deficiency. If your child has low IgA before age four, expect the doctor to recheck levels after that birthday before making a lasting diagnosis.
When Testing Is Typically Ordered
Most people with IgA deficiency have no symptoms at all, so testing usually happens because a pattern of recurring infections raises a red flag. The kinds of infections that prompt a workup include repeated ear infections (especially in young children), chronic sinus infections, recurrent bouts of bronchitis or pneumonia, and persistent digestive problems like giardiasis, a parasitic infection that causes diarrhea. Children who get sick frequently and struggle to gain weight may also be evaluated.
IgA testing is also commonly triggered during a celiac disease workup. IgA deficiency is 10 to 15 times more common in people with celiac disease than in the general population, affecting roughly 2 to 3% of celiac patients. This matters because the standard celiac blood tests, including tissue transglutaminase and endomysial antibody screens, detect the IgA form of those antibodies. If you’re IgA deficient, those tests can come back falsely negative even when celiac disease is present. A doctor who suspects celiac should check your total IgA level alongside the celiac panel. If IgA is low, IgG-based celiac antibody tests are used instead to get an accurate result.
Additional Tests After a Low IgA Result
Once low IgA is confirmed, the diagnostic process doesn’t always stop there. Your doctor may order several follow-up tests to understand the full picture of your immune function and rule out more serious immunodeficiencies.
- Vaccine response testing: You may be given a vaccine (or your existing titers checked) to see whether your immune system produces a normal antibody response. Poor responses to multiple vaccines suggest CVID rather than isolated IgA deficiency.
- IgG subclass testing: Some people with selective IgA deficiency also have low levels of certain IgG subtypes, which can increase their susceptibility to infections. Subclass testing helps identify this overlap.
- B-cell analysis by flow cytometry: A blood test that counts and categorizes your B cells (the immune cells that produce antibodies). In CVID, the percentage of memory B cells is dramatically reduced, often below 3% compared to a normal range above 20%. This test helps distinguish IgA deficiency from more serious conditions.
The distinction between selective IgA deficiency and CVID is clinically important. In CVID, IgG levels average around 255 mg/dL, far below the normal range of 767 to 1,590 mg/dL. IgA and IgM are also typically depressed, and T-cell counts trend lower. CVID patients face significantly higher rates of autoimmune complications, lung damage, and lymphoid cancers, so getting the right diagnosis shapes your long-term monitoring plan.
Partial vs. Complete IgA Deficiency
Not everyone falls neatly into the “undetectable” category. Some people have IgA levels that are low but still measurable, somewhere between 7 mg/dL and the lower end of the normal range (which starts around 61 mg/dL in most labs). This is sometimes called partial IgA deficiency. It’s a less clearly defined category, and guidelines for managing it are less standardized. If your levels are in this gray zone, periodic retesting over time helps determine whether your IgA is stable, trending downward, or normalizing.
Anti-IgA Antibody Testing
There’s one additional test that becomes relevant in a specific scenario: if you have confirmed IgA deficiency and ever need a blood transfusion or intravenous immunoglobulin therapy. Some people with very low IgA develop antibodies against IgA itself. When these individuals receive standard blood products (which contain IgA from the donor), they can have severe allergic reactions, including anaphylaxis with symptoms like swelling, dangerously low blood pressure, and difficulty breathing.
Not everyone with IgA deficiency develops these antibodies, so universal screening isn’t recommended. Testing for anti-IgA antibodies is typically reserved for people with severe (undetectable) IgA deficiency or anyone who has previously reacted to a blood transfusion. If you test positive, your medical team can plan around it by using washed blood products, sourcing donations from IgA-deficient donors, or using other specialized approaches.
How Common IgA Deficiency Is
Selective IgA deficiency is the most common primary immunodeficiency, but prevalence varies dramatically by ethnicity. It’s most frequent in people of European descent, where roughly 1 in 142 people are affected. In North America overall, estimates range from about 1 in 328 to 1 in 531. Among Japanese populations, the rate drops to roughly 1 in 18,500. If you’re of European ancestry and have a pattern of recurring respiratory or digestive infections, IgA deficiency is a reasonable thing to test for.
What to Expect From the Process
From start to finish, the testing process is not complicated or invasive. The initial immunoglobulin panel requires a standard blood draw with no fasting or special preparation. If your IgA comes back low, expect at least one repeat test to confirm the result, and possibly the additional workup described above depending on your symptoms and overall immunoglobulin profile. The entire process from first blood draw to confirmed diagnosis can take several weeks to a few months, largely because of the need for repeat testing and the time between appointments.
If you’re being tested because of celiac disease concerns specifically, make sure your total IgA level is checked at the same time as your celiac antibodies. A falsely reassuring negative celiac screen in someone with undiagnosed IgA deficiency is a well-documented pitfall, and catching it early saves time and unnecessary confusion.