IgG subclass testing (IgG subclasses (IgG1, IgG2, IgG3, IgG4))
A blood test that breaks IgG antibody down into its four subclasses (IgG1–IgG4), used selectively when total antibody levels look normal but a specialist still suspects an antibody problem.
✓ Medically reviewed by a GMC-registered consultant · last reviewed September 2026 · next review September 2027 · how we review
In short
- It splits IgG into four subclasses (IgG1–IgG4) and is a second-line test, used only after total antibody levels are checked.
- A low subclass is often not significant — many healthy people have one — and the result is debated even among specialists.
- Levels vary, so a low subclass usually needs repeating, and what matters more is whether vaccine responses are normal.
- A subclass result is not a diagnosis on its own; it must be interpreted by a clinical immunologist alongside your infection history.
A plain-English summary. The detail — including risks and recovery — is below.
At a glance
A general guide. Your specialist will give you advice for your situation.
Can add detail when total antibody levels are normal but infections remain a concern
It is the wrong first test — total immunoglobulin levels should be checked first.
Blood is taken from a vein in a few minutes. No special preparation is usually needed.
A consultant immunologist who interprets subclasses in full clinical context.
Blood is taken from a vein in a few minutes. No special preparation is usually needed.
Press on the site briefly. Mild bruising is normal and you can carry on as usual.
Results return and are compared with age-appropriate ranges.
The test is usually repeated at least once, and a vaccine-response check is often added.
What is IgG subclass testing?
IgG, the main protective antibody, comes in four types called subclasses: IgG1, IgG2, IgG3 and IgG4. They have somewhat different jobs — for example IgG2 is important against certain bacteria with sugary coats. This test measures all four at the same time.
It is not a first-line test. A clinical immunologist usually checks total immunoglobulin levels (IgG, IgA, IgM) first. IgG subclasses are measured only when those are normal but there is still genuine concern about how often or how severely you get infections.
There is real and ongoing debate among immunologists about what a low subclass actually means. Many people with a low subclass have no extra infections at all, while a few with a clear infection history do. Levels also vary over time, so a low subclass usually needs repeating at least once before it is taken seriously.
Crucially, a subclass level on its own rarely settles anything. What matters more is whether your immune system can actually make protective antibodies — so this test is usually interpreted alongside a vaccine-response check, not in isolation.
Types, options & approaches
There may be different ways to do this. The right approach depends on the clinical question and your circumstances.
Options at a glance
These are the main approaches described in this guide. The right option depends on the diagnosis, your goals and what your clinician thinks is safe.
IgG1
The most abundant subclass. A low IgG1 is more likely to be meaningful and may be seen alongside low total IgG.
IgG2
Important for defence against bacteria with polysaccharide (sugar) coatings, such as some pneumococci. Often the subclass of most interest with recurrent chest and sinus...
IgG3
Involved in responses to viruses and proteins. There is very limited evidence that an isolated low IgG3 reliably causes disease.
IgG4
Naturally present at very low levels, and undetectable in a notable proportion of healthy people — so a 'low' or absent IgG4 alone is usually not a cause for concern.
Preparing for your test
- Have your total immunoglobulin (IgG, IgA, IgM) results to hand, as subclasses are interpreted alongside them.
- Bring a clear infection history: how often, how severe, and any unusual or repeated bacterial infections.
- List your medicines and supplements, including steroids and any immune-affecting drugs.
- Mention any recent infection or vaccination, which can affect levels.
- If this is a repeat test, bring the previous subclass results so trends can be compared.
- Ask whether a vaccine-response test will be done alongside, as the two are usually read together.
- For children, remember subclass levels are naturally lower and often normalise with age.
What happens
A nurse or phlebotomist takes a blood sample from a vein in your arm; it takes only a few minutes. The laboratory measures all four IgG subclasses together.
Results are compared with age-appropriate reference ranges, which matters because subclass levels in children are normally lower and tend to rise to adult levels over time. Because levels vary, a low subclass is usually repeated at least once before it is considered significant.
Your immunologist interprets the subclasses alongside your total antibody levels, your infection history and — importantly — how well you respond to vaccines. A low subclass with a normal vaccine response is interpreted very differently from a low subclass with a poor response.
Is this test right for me?
A good consultation should explore whether it's the right choice for you now — including reasons to wait or consider something else.
May not be suitable if…
- It is the wrong first test — total immunoglobulin levels should be checked first.
- On its own it cannot diagnose an antibody deficiency; functional (vaccine-response) testing is needed.
- In young children, a low subclass is often normal for age and may not warrant action.
- Testing during or just after infection or vaccination can mislead.
- It is not a useful general health or 'tiredness' test.
Delay or rearrange if…
- Total immunoglobulin levels have not yet been checked and interpreted.
- You currently have an active infection or have just recovered from one.
- You have been vaccinated in the last few weeks.
- You are on steroids or other immune-affecting medicines not yet discussed.
- A repeat is being considered but the timing is too soon to be meaningful.
Alternatives to discuss
- No subclass testing if total levels and infection history do not justify it.
- Vaccine-response testing, which assesses antibody function more directly.
- Watchful waiting with a clear plan if symptoms are mild.
- A standard NHS pathway via your GP or referral to specialist immunology.
- Reviewing and treating other causes of frequent infections first.
Before you decide
Use this as a shared-decision checklist. The aim is not just “can this be done?”, but whether it is right for you, now, with the risks and alternatives clearly understood.
What matters most to me?
Think about symptoms, daily life, work, caring responsibilities, sport, fertility, travel, appearance and anxiety — the right choice depends on your priorities, not just the medical facts.
What are all my options?
Ask about waiting, monitoring, medicines, rehabilitation, a smaller or larger procedure, a different test, NHS referral, or a second opinion where that would help.
What would make me pause?
Active infection, pregnancy, unstable medical problems, smoking, medicines that increase bleeding, poor support at home, or feeling pressured are all reasons to slow down and get tailored advice.
What happens if I do nothing today?
For some problems, waiting is safe; for others, delay can make treatment harder. A good consultation should explain the trade-off in plain English.
Benefits
- Can add detail when total antibody levels are normal but infections remain a concern
- Can support a fuller picture of antibody function when combined with vaccine-response testing
- May help explain certain recurrent bacterial infections in selected patients
- Can guide whether further specialist assessment or treatment is needed
- A normal result can help reassure and avoid over-investigation
Risks & complications
- Minor bruising or soreness where blood was taken
- A low subclass that turns out not to be significant
- Needing to repeat the test because levels vary
- Anxiety from being told a result is 'low' when it may not matter
- Feeling faint during the blood test
- Being labelled with an 'IgG subclass deficiency' that does not explain symptoms
- Uncertainty because specialists themselves debate the meaning of results
- Infection at the needle site
- Unnecessary treatment if a borderline subclass result is over-interpreted without functional testing
The biggest risk is over-interpretation. A low IgG subclass — especially IgG3 or IgG4 — is often found in people with no immune problem, and IgG4 is normally undetectable in many healthy individuals. A subclass result should never be treated as a diagnosis on its own. Ask whether your vaccine responses are normal, because that matters far more than the subclass number.
Published figures to discuss
This is a low-risk blood test, so there are no procedural complication rates. The clinically important issue is over-interpretation: low subclasses are common in healthy people, IgG4 is often undetectable normally, and isolated IgG3 deficiency has limited proven significance. Because the meaning depends so heavily on context and on vaccine responses, no reliable single percentage is quoted here.
| Figure | Reported range | How to interpret it | Source / confidence |
|---|---|---|---|
| Low IgG subclass in otherwise healthy people | Common enough that isolated low results are not diagnostic | The result should usually be repeated and interpreted with infection history and vaccine responses. | Clinical utility of measuring IgG subclasses (Laboratory Medicine, PMC)pmc.ncbi.nlm.nih.govSource-linked context |
| Normal total IgG with poor polysaccharide vaccine response | Recognised pattern in specific antibody deficiency | Subclass testing alone can miss functional antibody problems. | Guide sourcesClinical context |
| Starting immunoglobulin replacement based only on IgG subclass | Usually inappropriate | UK guidance requires infection burden and functional antibody assessment, often after prophylactic antibiotics have been considered. | Clinical utility of measuring IgG subclasses (Laboratory Medicine, PMC)pmc.ncbi.nlm.nih.govSource-linked context |
| Transient abnormal subclass result | Recognised, especially around infection or laboratory variation | Repeat testing avoids over-diagnosis and unnecessary treatment. | Clinical utility of measuring IgG subclasses (Laboratory Medicine, PMC)pmc.ncbi.nlm.nih.govSource-linked context |
These are literature figures, not a personalised prediction. Your own risks and likely benefits depend on your circumstances, your health, and how your care is carried out and followed up.
What happens afterwards
There is no physical recovery from having blood taken. 'Afterwards' is about waiting for results and understanding the careful, often cautious, way they are interpreted.
- A small bruise or tender spot where blood was taken
- No change in how you feel — the test does not treat anything
- Being asked to repeat the test because levels fluctuate
- Being told a low subclass is probably not significant
- Finding a low IgG4 that is normal for many healthy people
Aftercare
- Keep the dressing on for an hour or so if the site is sore.
- Do not assume a 'low' subclass means a serious problem — wait for specialist interpretation.
- If a repeat test or vaccine-response check is advised, arrange it.
- Bring your total immunoglobulin results and infection history to the discussion.
- Do not start any treatment based on a subclass level alone.
- Keep copies of results so trends can be tracked over time.
- Ask specifically how your vaccine responses look, as these guide the meaning.
- Total immunoglobulin results gathered
- Infection timeline written down
- Medicine and supplement list ready
- Previous subclass results brought if a repeat
- Note of recent infections or vaccinations
- Vaccine-response test arranged if recommended
- Questions for the results discussion prepared
⚠ Get urgent help if…
- A high temperature with shivering, confusion or feeling very unwell — seek urgent help; do not wait for results
- Repeated serious bacterial infections such as pneumonia, severe sinus or ear infections
- A chest infection with breathing difficulty or chest pain
- An infection that is spreading or not responding to treatment
- Coughing up blood or persistent unexplained symptoms
- Feeling faint or unwell after the blood test itself
Who to contact: your clinician, clinic or test provider first (keep their number to hand). For urgent advice when you can't reach them, call NHS 111. In an emergency, call 999.
General guidance — it doesn't replace the specific advice your specialist gives you.
Results & realistic expectations
A normal set of subclasses is reassuring, but the more important question is usually how well your antibodies work, which a vaccine-response test answers. A low subclass is not a diagnosis: many healthy people have one, IgG4 is often undetectable normally, and isolated low IgG3 has very limited proven link to disease.
When a low subclass does matter, it is usually because it sits alongside a clear infection history and a poor vaccine response. Your immunologist weighs all of this together. A result that does not change your care — for example a low subclass with normal antibody function and no unusual infections — is generally not treated, and it is fair to ask what your result actually means for you.
Subclass levels vary over time and with age. In children they are naturally lower and usually rise to normal adult levels by around 10–12 years, so a 'low' subclass in a child is often a stage of development rather than a lasting problem. Because levels fluctuate, a low result is repeated, and interpretation can change as you grow or as your situation changes.
Related tests, treatments or support
IgG subclass testing is rarely done alone. It is interpreted alongside total immunoglobulin levels and, most importantly, a vaccine-response test that shows whether your immune system can make protective antibodies. Complement and autoantibody tests may be added if a broader immune assessment is needed.
Follow-up & long-term care
If subclasses are normal, or low but not significant, no specific treatment is usually needed. A clinically meaningful pattern — low subclass plus poor vaccine response and a real infection history — leads to specialist immunology review and a discussion of options. A good service explains the uncertainty honestly and who is responsible for the next step.
- Repeat testing because levels vary, before any result is acted on
- Re-checking in children as levels often normalise with age
- Vaccine-response testing to assess antibody function
- Ongoing specialist review only if a significant problem is confirmed
Repeat, follow-on and what comes next
- A low subclass is usually repeated at least once because levels vary over time.
- In children, levels often normalise with age, changing the interpretation.
- A subclass result rarely stands alone — vaccine-response testing usually decides its meaning.
- A 'deficiency' label may be revised once functional testing is done.
Ask what happens if the result is unclear or needs repeating, and what is included if further tests or follow-up are needed.
What good aftercare looks like
- A consultant immunologist who interprets subclasses in full clinical context.
- Honest explanation of the genuine uncertainty around subclass results.
- A plan for repeating the test and for vaccine-response assessment.
- Clear advice that a low subclass alone does not warrant treatment.
- Onward referral only when a meaningful, confirmed pattern is present.
What affects the cost
Costs vary a great deal between people and providers, and we don't publish prices. What matters is understanding what drives the cost and making sure your quote is complete. The main things that affect it:
- Whether subclasses are tested alone or as part of a wider antibody assessment
- Whether vaccine-response testing is done alongside (often needed for interpretation)
- The consultant immunologist's fee for interpreting results in context
- Likely repeat testing, since a single low result is usually confirmed
- Phlebotomy and laboratory processing fees
- Any follow-up consultation to discuss the (often nuanced) result
- Whether all four subclasses and the relevant total immunoglobulins are included
- Whether vaccine-response testing is included or extra
- The laboratory fee and the consultant's interpretation fee, separately
- The cost of the repeat test that a low subclass usually requires
- Whether a follow-up appointment to discuss results is included
- Who is responsible for interpreting and acting on the result
On the NHS? IgG subclass testing is available on the NHS as a second-line test when clinically indicated; it is also offered privately, but a low subclass is not a diagnosis and needs specialist interpretation.
You're entitled to your total cost in writing — including reports, follow-up and what happens if the result is inconclusive — before you decide.
Consent traps and marketing red flags
These are not small details. They are often where patients lose choice, time to reflect, or realistic expectations.
Consent traps
- Presenting a low subclass as a diagnosis without explaining the uncertainty.
- Not pairing the result with vaccine-response testing.
- Treating a low IgG4 or isolated low IgG3 as significant when it often is not.
- Not accounting for the child's age when interpreting low subclasses.
- Starting treatment on a single, unrepeated low result.
Marketing red flags
- Offering subclass testing direct to consumers as part of an 'immune panel'.
- Implying any low subclass means a treatable deficiency.
- Not mentioning that results are debated and often not significant.
- Skipping the vaccine-response testing that gives the result meaning.
- Promising the test will explain general tiredness or frequent colds.
Choosing a specialist safely
- Check the specialist is on the GMC Specialist Register for this area.
- Make sure they work at a CQC-registered service, and look for membership of the relevant Royal College or professional body.
- You're entitled to time to consider and to have your questions answered before you agree — the specialist who looks after you should explain it, not a salesperson.
- Be wary of pressure: time-limited offers or deposits taken before you've had time to think are red flags, not bargains.
- You're entitled to your total cost in writing — including any follow-up — before you decide.
Questions to ask your medical professional
Take this to your consultation. A good specialist will welcome every one of these.
- How do my subclasses compare with the right range for my age?
- Is this result significant given my infection history?
- Are my vaccine responses normal, and how does that change the meaning?
- Should this test be repeated before any conclusions are drawn?
- What would actually change in my care because of this result?
- Given the uncertainty, what do you recommend and why?
- Are you on the GMC Specialist Register for this area, and which Royal College or professional body are you a member of?
- Will you be the specialist who carries out my test, and who looks after me afterwards?
- What are the risks for someone like me, and how often do your own patients have a problem or need it repeated or redone?
- What does a realistic result look like — and what can this test not achieve?
- What are my options, including waiting, doing nothing for now, or choosing a different approach?
- Can I have written information, results and aftercare instructions in a format I can use, including any accessibility or communication support I need?
- What is the total cost in writing, including any follow-ups, and how much time do I have to decide?
Frequently asked questions
Why wasn't this test done first?
I was told a subclass is low — is that serious?
Why does my child have low subclasses?
Why might I need the test repeated?
What matters more than the subclass number?
Will a low subclass mean I need treatment?
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How we made this page
Medically reviewed by a GMC-registered consultant. Written in plain English, checked against NHS, NICE, GMC and relevant Royal College / specialist-society guidance, and kept under review. No clinic paid to appear on this page, and we publish no pricing. This is general information to help you prepare — it is not a substitute for advice from your own clinician. How we review our guides →
Source hierarchy: UK regulator and NHS/NICE guidance first, then relevant Royal College or specialist-society guidance, then peer-reviewed evidence for procedure-specific figures where available.
Sources & standards: Immunodeficiency UK — IgG subclass deficiencies Clinical utility of measuring IgG subclasses (Laboratory Medicine, PMC) North West London Pathology NHS — Immunoglobulin G subclasses (IgG1-4) Lab Tests Online UK — Quantitative Immunoglobulins British Society for Immunology — Clinical immunology guidelines
Reviews reflect patients' experience of care, not clinical outcomes. For procedure volumes and outcome data see PHIN.
Last medically reviewed 2026-09-21. Spotted something wrong or out of date? Report an error in this guide.
Related guides: Immune system blood tests · Immunoglobulin level testing · Vaccine response testing · Complement testing · Autoantibody / autoimmune testing