Symptom · Catching everything going
Four colds a winter with two young children is normal life, not a broken immune system. What is not normal is being ill most months, taking weeks to shake what other people clear in days, needing repeated courses of antibiotics, or getting the same sinus or chest infection back within weeks of finishing treatment. That pattern usually has a measurable cause, and it is rarely the immune system itself: low vitamin D, low iron or zinc, undiagnosed diabetes or insulin resistance, an underactive thyroid, coeliac disease, or an infection that never fully cleared. A panel separates those from the small number of people who have a genuine immune problem and need a specialist.
Venous draw at 103 UK clinics or at home. Report within 48 hours of the lab receiving your sample. No GP referral.
Last reviewed 17 September 2026
The short answer
A blood test for recurrent infections should cover a full blood count with the white cell differential, vitamin D, ferritin with iron studies, zinc, HbA1c with fasting glucose, a full thyroid panel, hs-CRP, liver and kidney function, and total protein with albumin. If those are clean and the infections are genuinely frequent or severe, the next step is immunoglobulins (IgG, IgA, IgM) and an HIV test, which is where a real immune deficiency shows. Most people who feel they catch everything have a nutrient or metabolic explanation, not an immune one, and that is good news because it is fixable.
See a GP rather than starting with a private test if
You have needed two or more courses of antibiotics in a year for serious infections, two or more episodes of pneumonia, an infection needing hospital admission, deep abscesses, persistent thrush or diarrhoea, unexplained weight loss, or infections that are unusually severe or with unusual organisms. Those are the recognised warning signs of primary immunodeficiency and warrant proper immunology assessment on the NHS. The same applies to a child with recurrent infections and poor growth: that is a paediatric assessment, not a blood panel bought online.
What to test
| Marker | Why it matters here | What to look for | Panel |
|---|---|---|---|
| Vitamin D | Vitamin D regulates the antimicrobial peptides that line the airways. Deficiency is associated with more respiratory infections, and it is the single commonest abnormal result in people who catch everything through a UK winter | Below 30 nmol/L deficient; below 50 insufficient. Aim above 75 if you are prone to chest and sinus infections | All panels |
| Full blood count with differential | The white cells are the immune system's headcount. A persistently low neutrophil or lymphocyte count is the first sign of a real problem; a raised count suggests an infection that is still active | Neutrophils below 1.5, lymphocytes below 1.0, or any persistent abnormality. Also anaemia, which is common alongside | All panels |
| Ferritin and iron studies | Iron is needed for immune cells to divide and function. Deficiency is associated with more frequent infections, and it is very common in menstruating women and anyone with a restricted diet | Ferritin below 30 is deficiency. Read against hs-CRP, because inflammation from an active infection pushes ferritin up and can hide a real deficiency | All panels |
| Zinc | Needed for T cell development and wound healing. Deficiency causes more frequent and longer infections, poor taste, slow healing and mouth ulcers. Common in vegetarians, older adults and heavy drinkers | Low or bottom-of-range zinc, especially with mouth ulcers or slow-healing skin | Ultimate and Signature |
| HbA1c and fasting glucose | Undiagnosed diabetes is a classic cause of recurrent skin, urine, chest and fungal infections; high glucose impairs neutrophil function directly. Recurrent thrush or boils should always prompt this test | HbA1c 42 to 47 is prediabetes; 48 or above is diabetes. Anything above 38 with recurrent infections is worth acting on | All panels; fasting insulin in Ultimate and Signature |
| Thyroid (TSH, free T4, free T3, antibodies) | An underactive thyroid slows everything including immune response and mucociliary clearance, and produces the fatigue that makes every infection feel worse and last longer | TSH above range or above 2.5 with symptoms; positive antibodies | All panels |
| hs-CRP, total protein and albumin | hs-CRP shows whether something is still smouldering between illnesses. A low albumin or total protein can reflect poor intake, gut losses or, rarely, a problem making antibodies | hs-CRP persistently above 3 mg/L when you feel well; low albumin; a low total protein with a normal albumin can point to low immunoglobulins | All panels |
| Immunoglobulins (IgG, IgA, IgM) and coeliac screen | Low immunoglobulins are how primary antibody deficiency, the commonest significant immune disorder in adults, is found. Selective IgA deficiency also causes recurrent sinus and chest infections and is linked to coeliac disease | Any immunoglobulin below the reference range needs immunology referral, not self-management | Signature immune and digestive profiles |
An HIV test is part of any proper workup for recurrent infections and is free and confidential from your GP, a sexual health clinic or by post. Thresholds are UK laboratory conventions and NICE guidance where it exists. Guides: vitamin D, ferritin, HbA1c, thyroid, hs-CRP, immune system testing.
Read it in context
Pattern 1
Vitamin D at 24, ferritin at 22, zinc at the bottom of range, white cells entirely normal. This is the commonest result by far, and it explains a winter of back-to-back colds without any immune disorder at all. Correcting all three over three months usually changes the following winter noticeably.
Pattern 2
HbA1c at 50, recurrent thrush, boils or urine infections. This is undiagnosed type 2 diabetes presenting as infection rather than thirst, and it is the single most important thing on this page to catch. It needs a GP appointment, and treating it stops the infections. Diabetes blood test.
Pattern 3
A persistently low neutrophil or lymphocyte count, or low IgG, IgA or IgM. Uncommon, but this is what you are testing to avoid missing. It is not something to manage with supplements: it needs an immunology referral, and treatment for antibody deficiency is highly effective once diagnosed.
Which panel
Start here
Advanced
74 biomarkers · £269
Full blood count with differential, vitamin D, ferritin with iron studies, HbA1c and glucose, full thyroid with antibodies, hs-CRP, total protein, albumin, liver and kidney. The nutrient and metabolic causes, which is most of them.
View AdvancedUltimate
114 biomarkers · £349
Adds zinc, fasting insulin and HOMA-IR, cystatin C, the complete hormone set and tumour markers. Worth it if diet is restricted, healing is slow, or weight and blood sugar are part of the picture.
View UltimateSignature
230 biomarkers · £799
Processed by Randox. Adds the immune profile with immunoglobulins and the digestive profile including coeliac screening. The panel when the common causes have been ruled out and the infections are genuinely frequent.
View SignatureClinic draw £19. At-home £39. Test when you are well, at least two weeks after an infection, or hs-CRP and ferritin will be raised and harder to read. Keep eating gluten before a coeliac screen. Pay monthly available.
Honest comparison
If you have a toddler in nursery, work in a school, commute on the Tube, or have just come through a bad winter, you may simply be meeting more viruses than most people, and a panel will tell you your bloods are fine. That is a legitimate reason to buy one, if the reassurance is what you want, but it is worth going in knowing that the likeliest result is normal. The same applies to sleep, alcohol and stress, which measurably reduce immune function and do not show on any of these markers. And the things that reduce infections most reliably are not blood-related at all: the annual flu vaccine, a pneumococcal vaccine if you are eligible, stopping smoking, sleep, and hand hygiene. A panel is worth doing when the pattern is genuinely abnormal, when you want the deficiencies found rather than guessed at, or when you have been told "it is just one of those things" twice and want that checked properly. What to ask your GP for.
After your result
Nutrient and metabolic findings explained with what to do about them, and anything in the white cells, immunoglobulins or glucose that needs a doctor flagged clearly and prominently. Reviewed by a medical professional before it reaches you.
Vitamin D, iron and zinc are corrected with supplements at the right dose. A raised HbA1c, a low white cell count or a low immunoglobulin needs a GP, and in the last case an immunology referral. The report tells you which results to show them.
Vitamin D, iron and zinc all take about twelve weeks to correct. Testing in early autumn and again in late winter is the pattern that actually changes how a winter goes. Retest plans.
FAQs
Two to four colds a year is normal for an adult, and more if you have young children or work with them. The pattern that warrants investigation is infections that are unusually frequent, unusually severe, need repeated antibiotics, keep coming back to the same site, or take far longer to clear than other people's.
A full blood count with the white cell differential and immunoglobulins (IgG, IgA, IgM) are the core tests, alongside an HIV test. Low neutrophils, low lymphocytes or low immunoglobulins are the findings that matter and they need specialist assessment. Most people with frequent infections have entirely normal results on these.
It is associated with more respiratory infections, and supplementation appears to help most in people who are genuinely deficient rather than those already replete. It is the commonest abnormal result in UK adults who catch everything through winter, and it is cheap and simple to correct.
Yes, and recurrent thrush, boils, urine infections or slow-healing skin are a recognised way undiagnosed type 2 diabetes first presents. High blood glucose impairs how neutrophils work. An HbA1c is essential in anyone with this pattern.
When you are well, ideally at least two weeks after an infection has cleared. Testing mid-illness raises hs-CRP, white cells and ferritin, which makes iron status and inflammation much harder to interpret. Morning appointment, fasted from 10pm.
Take a child with recurrent infections to a GP rather than buying a private panel. Children meet far more viruses than adults and most frequent illness is normal, but where it is not, the assessment includes growth, development and vaccination history, and that is a paediatric job. TrueVitals tests adults aged 18 and over.
Advanced and Ultimate reports are delivered within 48 hours of the laboratory receiving your sample. Signature takes around 12 working days.
The nutrient, metabolic and immune causes in one venous draw, at 103 UK clinics or at home, with a report within 48 hours that says what to correct and what needs a doctor.
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