Blood Tests for Frequent Infections and Low Immunity

Catching one cold after another, needing antibiotics again and again, or feeling as though you never quite shake off the last bug is tiring and frustrating. For most people there is no serious cause, but sometimes frequent infections point to something a simple blood test can pick up, such as a low white cell count, raised blood sugar or low iron stores.

This guide explains which blood tests can help when you keep getting ill, what each one looks for, which panel to choose and when frequent infections need a GP rather than a private test. A result does not diagnose anything on its own. It starts a conversation with your practitioner or GP about what to do next.

Test it in Chiswick: our Omnia Essential blood test costs £89, with your result explained in person by a practitioner. No GP referral needed. Book this test or call 020 4515 0000.

Quick answer

The most useful first tests for frequent infections are a full blood count with a white cell differential (neutrophils and lymphocytes), HbA1c to check for raised blood sugar, ferritin for iron stores and vitamin D. If infections are repeated, severe or unusual, immunoglobulins (IgG, IgA and IgM) and an HIV test are the next step, and UK immunology guidance lists these alongside the full blood count as first-line checks. Tests are best done when you are well, not in the middle of an infection, because illness can change white cell counts for a while.

Blood tests that can help

Full blood count with differential

A Full Blood Count (£59) measures your white blood cells and breaks them down into types, including neutrophils, which fight bacteria, and lymphocytes, which handle viruses and make antibodies. A count that is persistently low in one of these groups can make infections more likely, while a raised count may simply reflect a recent infection. The same test checks haemoglobin and platelets. Ranges vary between laboratories, so read the range printed on your report.

Immunoglobulins (IgG, IgA and IgM)

Immunoglobulins are the antibodies your immune system makes to fight infection. NHS laboratory guidance describes measuring them as an essential investigation for recurrent infections, and low levels always need further investigation. You can test each one separately: Immunoglobulin G (IgG) (£49), Immunoglobulin A (IgA) (£49) and Immunoglobulin M (IgM) (£49). Low IgA on its own is fairly common, affecting about 1 in 700 people, and often causes no problems, so results are always interpreted together.

HbA1c (blood sugar)

The NHS lists repeated thrush and cuts or wounds that take longer to heal among the symptoms of type 2 diabetes, which often develops slowly and without obvious signs. An HbA1c test (£59) shows your average blood sugar over the past two to three months and is a sensible check if infections keep coming back, especially skin, urine or thrush infections. Our guide to HbA1c levels explains the bands.

Ferritin (iron stores)

Low iron is common, particularly with heavy periods or during pregnancy, and iron deficiency anaemia causes tiredness, lack of energy and breathlessness. Feeling run down for weeks can be hard to separate from feeling that you are always ill. A Ferritin test (£49) shows your iron stores. Ferritin can also rise during infection or inflammation, so your practitioner will read it alongside your other results.

Vitamin D

Vitamin D keeps bones, teeth and muscles healthy. Research into vitamin D and infections is ongoing, and the NHS says there is not enough evidence to take it solely to prevent or treat COVID-19. A Vitamin D test (£59) is still worthwhile if you rarely go outdoors, cover most of your skin or have dark skin, as these groups are at higher risk of low levels. NHS advice is that adults need 10 micrograms a day and everyone should consider a supplement in autumn and winter.

HIV test

Untreated HIV damages the immune system, and the NHS notes that many people have no symptoms for years. UK immunology guidance includes an HIV test in the first checks for anyone with recurrent infections, whatever their background. Our HIV Test (4th generation) costs £49 and is handled confidentially. UK guidance gives a window period of 45 days for this type of test, so if you had a possible exposure more recently, you may need a repeat test.

Which test should I book?

• A focused start: Inflammation Check, £79. A full blood count with differential, ferritin and two inflammation markers, CRP and ESR. A good first look if you feel generally unwell and keep picking up infections.
• A broader picture: Omnia Essential, £89. A full blood count with differential plus HbA1c, kidney function, liver function and albumin, covering the routine chemistry that sits alongside immune testing. You can add immunoglobulins, ferritin, vitamin D or an HIV test at the same visit.
• The fullest view: T-Cell Subsets, £329. A specialist count of the different types of lymphocyte. It is usually a second step, chosen with a practitioner or doctor when earlier tests show persistently low lymphocytes or infections are unusual.

What blood tests cannot tell you

No single blood test measures how strong your immune system is. Normal results are reassuring, but UK immunology guidance is clear that an immune deficiency cannot be ruled out on blood tests alone when the story strongly suggests one, so specialist referral may still be right.

Blood tests also cannot identify which bug caused each infection, and they do not measure the everyday factors that affect how often you get ill, such as poor sleep, stress, smoking or close contact with young children. If your results are normal, your practitioner can talk through these and help you decide whether further tests are worthwhile.

When to see a GP urgently

Call 999 or go to A&E if you or someone else has signs of sepsis, a serious reaction to infection:

• confusion, slurred speech or not making sense
• breathing very fast
• blue, pale or blotchy skin
• a rash that does not fade when you press it
• a very high or very low temperature, or feeling hot, cold or shivery

See your GP, rather than relying on a private test, if you have pneumonia most years, two or more sinus or ear infections a year, a repeated need for antibiotics through a drip, unusual infections, persistent thrush in the mouth, unexplained weight loss, or a family history of immune deficiency. These are recognised warning signs that may need referral to an immunology specialist. If you have had sepsis in the past 12 months and have signs of infection, call NHS 111.

Frequently asked questions

Can a blood test show if my immune system is weak?

Blood tests can show some causes of frequent infections, such as low white cells, low antibodies, raised blood sugar or HIV. They cannot give a single score for immunity, so results are always read alongside your symptoms and history.

Should I have a blood test while I have an infection?

It is usually better to wait until you have recovered. Infection can push white cells, ferritin and inflammation markers up or down for a while, and UK guidance advises immune tests when there is no active infection. If a result is unusual, your practitioner may suggest repeating it a few weeks later.

Can low vitamin D cause frequent colds?

Researchers are still studying vitamin D and infections, and the evidence that low levels cause frequent colds is not settled. A test is most useful if you are in a higher-risk group, and NHS advice on supplements applies to everyone in autumn and winter.

What does a low white blood cell count mean?

A low result can follow a recent viral illness, so it is often repeated once you are well. A count that stays low, or a low neutrophil count, needs review with a doctor. Our guide to the white blood cell count explains more.

Do I need to fast before these tests?

Most of these tests do not depend on what you have eaten, but some panels include glucose or cholesterol. Ask when you book and we will tell you whether your chosen test needs any preparation.

Immunity blood testing in Chiswick

Omnia Lifestyle is at 8 to 9 Bedford Corner, The Avenue, Chiswick, London W4 1LZ, a short walk from Turnham Green station (District line; Piccadilly line trains stop there early morning and late evening only) and on the 94, E3 and 272 bus routes. Limited parking is available on request when you book. Every clinic test price includes your sample taken at the clinic, analysis at the Randox accredited laboratory, a review of your result with a practitioner and a GP feedback report on request. No GP referral is needed and most results are back within a few working days.

Start with the Omnia Essential panel or the Inflammation Check, or compare every blood test and price before you book. For more background, read our guides to vitamin D levels and ferritin levels.

This article is general information, not medical advice. Discuss your results with your GP or practitioner.