What will a food allergy blood test show?

No blood routine abnormality can directly diagnose food allergy. The most common clinical warning signal is that the eosinophil count and proportion exceed the normal reference range. If the doctor suspects allergy, he will usually order a total IgE test in addition to the blood routine. This item is not included in the ordinary blood routine package. At this time, you may also see an increase in the total IgE index, but these two are not "exclusive markers" of food allergy. In turn, normal indicators cannot completely rule out the possibility of food allergy.

In the past few years in the allergy clinic, I have met too many patients who asked if they had food allergies with blood tests. The one that impressed me the most was a 3-year-old girl. Her mother said that after eating half a piece of mango, her mouth became red and she vomited twice. The eosinophil ratio in the blood test was 8.2%, which is much higher than the normal upper limit of 5%. Later, combined with the specific IgE test, the mango allergy was confirmed. But there is also a young girl who is allergic to wheat. She gets stomachaches and rashes every time she eats pasta. Her eosinophils were within the normal range in three consecutive blood tests. This is not uncommon. After all, the intensity of the immune response is different for everyone, and not all allergies will be reflected in this item.

There is another pitfall that is easy to step into: when an acute food allergy attacks, the body is in a state of stress, and the blood routine may also show an increase in white blood cells and neutrophils. Many people will mistakenly think that it is a bacterial infection and require antibiotics. In fact, it is not necessary at all. Last month, there was a young man who developed wheals all over his body two hours after eating crayfish. He was also very depressed. The white blood cell count in his blood test was 12.7*10^9/L. He insisted on taking antibiotics at first, but later took anti-allergic drugs. The next day's re-examination showed that all the indicators fell back to the normal range. It was not an infection problem at all.

There is now a lot of controversy over this matter in clinical practice. Some doctors believe that if there is only a mild increase in eosinophils in the physical examination, and there are no suspicious allergic symptoms such as rash, itching, abdominal pain, or diarrhea, there is no need to specifically screen for food allergens. After all, there are many reasons for the increase in this indicator. Parasitic infections, asthma, eczema, and even seasonal allergic rhinitis attacks may cause the increase. Some healthy people will also have transient indicator abnormalities. There is no need to blindly avoid eating as soon as you see the arrow. Of course, some doctors will recommend a preliminary allergen screening if possible to rule out potential sensitization risks and avoid subsequent severe allergic reactions.

There was a patient who had a physical examination before and found that his eosinophil count was 1.2% higher. He immediately stopped eating eggs, milk, and seafood, and even dared not touch nuts. He lost 8 pounds in three months. Later, when he came for a follow-up examination, he found out that he was allergic to pollen and had rhinitis during that time. The high level of eosinophils had nothing to do with food, and he suffered for several months in vain. To be honest, if you really suspect that you have a food allergy, a blood routine is only the initial screening. If you really want to identify the allergens, you still need to combine specific IgE testing, skin prick tests, and even the gold standard food challenge test to determine. It is too easy to make mistakes by relying on a blood routine alone.