If you get sick often, the tests your doctor may consider include a complete blood count (CBC) with differential and, if your pattern of infections warrants it, immunoglobulins (IgG, IgA and IgM), which measure your antibodies. Your doctor may also look for conditions that weaken your defenses, such as diabetes (with a hemoglobin A1c test) or HIV, which the CDC recommends everyone ages 13 to 64 be tested for at least once. Vitamin D, B12 and iron tests can find treatable problems, but they do not measure your immune system. Tests do not "boost" immunity; they look for a cause. Seek care promptly for severe or repeated infections, a fever that won't go away, or unexplained weight loss.
How many infections are too many?
According to the CDC, adults in the U.S. average two to three colds a year, and children often have more. The Immune Deficiency Foundation (IDF) explains that up to two sinus infections a year is considered normal for adults, and that a single pneumonia, unless it is very severe, usually does not trigger an immune evaluation. The IDF and the American Academy of Allergy, Asthma & Immunology (AAAAI) consider these warning signs:
- pneumonia, sinus infections or ear infections that keep coming back;
- infections that don't fully clear or clear very slowly;
- infections that need a hospital stay, IV antibiotics or several courses of antibiotics;
- infections from germs that don't usually make people sick, or repeated deep abscesses;
- a family history of immune deficiency.
Complete blood count with differential
A CBC measures red blood cells, white blood cells, platelets and hemoglobin. The differential counts each type of white blood cell; neutrophils, for example, are the body's main defense against infection.
- What it can show: anemia, or a low white blood cell count, which can come from bone marrow disease, chemotherapy, some medicines, cancer or HIV.
- What it can't show: according to the IDF, no CBC result can diagnose an immune deficiency on its own, and a normal CBC does not rule one out. Because neutrophil numbers change over time and with infections, the test sometimes has to be repeated.
Immunoglobulins and vaccine response
Immunoglobulins are antibodies. According to the IDF, when a doctor suspects an antibody deficiency, the first test ordered is a serum immunoglobulin test (IgG, IgA and IgM).
- What it can show: low levels. One of the main symptoms is frequent infections, especially repeated infections from the same germ. Besides genetic causes, low levels can come from kidney disease, malnutrition, absorption problems or complications of diabetes.
- What it can't show: it counts antibodies but does not measure how well they work. That is what a vaccine response test is for: antibodies to a vaccine are measured, a dose is given if they are low, and they are measured again about four weeks later.
Common variable immune deficiency (CVID), an uncommon antibody deficiency, is usually diagnosed in a person's twenties or thirties, often in adults with repeated lung, ear or sinus infections.
Tests that look for causes of weaker defenses
Secondary immune deficiencies, caused by something outside the immune system, are much more common than primary ones (IDF). Your doctor may consider:
- Hemoglobin A1c. It shows your average blood sugar over the past two to three months (5.7% to 6.4%: prediabetes; 6.5% or higher: diabetes). Diabetes raises the risk of certain infections. The result may not be accurate with anemia, kidney failure or liver disease. The CDC recommends testing from age 45, or earlier with risk factors.
- HIV. HIV destroys cells of the immune system. The CDC recommends that everyone ages 13 to 64 be tested at least once. After a recent exposure, a negative result may need repeating: lab tests can detect infection 18 to 45 days after it happens.
- Hepatitis B and C. The CDC recommends that all adults be screened at least once in their lifetime (hepatitis C since 2020; hepatitis B, with a three-test panel, since 2023). These tests don't measure your defenses.
- Your medicines. Corticosteroids, chemotherapy and some medicines for rheumatoid arthritis or inflammatory bowel disease weaken defenses; the effect often goes away once the medicine is stopped. Don't stop any medicine without talking to your doctor.
Vitamin D, B12 and iron: useful, but they don't measure immunity
- Vitamin D. It is mainly tested when a bone problem or other condition could be caused by very low levels, or when risk is high, for example after weight-loss surgery. The U.S. Preventive Services Task Force (USPSTF) finds the evidence insufficient for routine testing in adults without symptoms, and the Endocrine Society suggests against routine testing without an established reason. There is no consensus on an "optimal" level.
- Vitamin B12. Low B12 can cause fatigue and tingling or numb hands and feet; long-term metformin or antacids raise the risk. If the result is normal but symptoms continue, you may need other tests.
- Iron. Iron tests include serum iron, transferrin, total iron-binding capacity and ferritin, which measures stored iron. Low ferritin may mean iron deficiency anemia, which without treatment can lead to infections and other problems. Ferritin also rises with inflammation or infection, so it is read together with the other iron tests. Heavy periods raise the risk of low iron.
Red flags: when to seek care
Call your doctor promptly if you:
- have a fever that stays at or keeps rising above 103°F (39.4°C), lasts longer than 48 to 72 hours, or comes and goes for a week or more;
- lose more than 10 pounds (4.5 kg) or 5% of your body weight over 6 to 12 months without knowing why;
- have repeated yeast infections or oral thrush;
- take corticosteroids or get chemotherapy and have a fever of 100.5°F (38°C) or higher.
Call 911 for a fever with a stiff neck and headache, confusion, trouble breathing, or if the person is hard to wake. Sepsis, the body's extreme response to an infection, is an emergency; its signs include fast breathing and heart rate, shortness of breath, confusion, extreme pain, fever or shivering, and clammy or sweaty skin.
How to prepare for your visit
- Write down the past year's infections: dates, type, antibiotics (by mouth or IV) and hospital stays.
- Bring your medicine list, vaccine records and past lab results.
- Find out whether anyone in your family has an immune deficiency.
- Ask which result would change the plan, and whether you should see an allergist/immunologist, who has specialized training to diagnose these conditions.
What the research says
Each citation links to the original source.
- Diabetes and infections, cohort study
- Population: 102,493 adults aged 40 to 89 with diabetes in England, compared with 203,518 without diabetes.
- Details: infections from 2008 to 2015 in primary care, hospital and death records.
- Result: higher rates of all infections; infection-related hospital stays were 3.71 times as frequent with type 1 diabetes and 1.88 times with type 2. People with diabetes are "at increased risk of serious infection."
- Limits: observational and in England; it does not show that lowering A1c reduces infections.
- Citation: Carey IM, Critchley JA, DeWilde S, et al. Risk of Infection in Type 1 and Type 2 Diabetes Compared With the General Population: A Matched Cohort Study. Diabetes Care. 2018;41(3):513-521. https://doi.org/10.2337/dc17-2131
- Vitamin D, USPSTF recommendation
- Population: community-dwelling, nonpregnant adults without signs or symptoms of deficiency.
- Details: the evidence review behind the recommendation covered 46 studies with 16,205 participants on screening and treatment; infection was among the outcomes.
- Result: the evidence is "insufficient" to weigh the benefits and harms of routine screening; only one trial addressed infection.
- Limits: does not apply to people with symptoms or a reason for treatment; no study directly tested screening.
- Citation: US Preventive Services Task Force. Screening for Vitamin D Deficiency in Adults: US Preventive Services Task Force Recommendation Statement. JAMA. 2021;325(14):1436-1442. https://doi.org/10.1001/jama.2021.3069 · Evidence review: Kahwati LC, LeBlanc E, Weber RP, et al. Screening for Vitamin D Deficiency in Adults: Updated Evidence Report and Systematic Review for the US Preventive Services Task Force. JAMA. 2021;325(14):1443-1463. https://doi.org/10.1001/jama.2020.26498
- Common variable immune deficiency, European registry
- Population: 2,212 patients with CVID from 28 centers in the European Society for Immunodeficiencies registry.
- Details: retrospective analysis of how CVID presented and how it was treated with immunoglobulin.
- Result: the diagnostic delay "ranges between 4 and 5 years in many countries"; during treatment, higher IgG levels were linked to fewer serious bacterial infections.
- Limits: European, retrospective data from specialized centers; CVID is uncommon.
- Citation: Gathmann B, Mahlaoui N, CEREDIH, et al. Clinical picture and treatment of 2212 patients with common variable immunodeficiency. J Allergy Clin Immunol. 2014;134(1):116-126. https://doi.org/10.1016/j.jaci.2013.12.1077
Talk to Elizabeth Isalguez, MD, about frequent infections
At Organic Well in Coconut Grove, care is led by Elizabeth Isalguez, MD, an internal medicine physician. The evaluation looks specifically for the difference between an ordinary exposure load and a genuine problem with immune function, and recurrent severe or unusual infections need investigation and referral first. See also our immune health evaluation in Miami and advanced lab testing in Miami. Visits are by appointment at 3661 S. Miami Ave, Suite 1003.
Book a visit or call (305) 930-8891, Monday to Friday, 9 AM to 6 PM.
