Can an Allergy Test give a False Positive Result?

There are numerous tests for food allergies, and if you suspect that you have a food allergy but haven’t yet identified the culprit, a food allergy test in the form of a skin prick test or a blood allergy test may be necessary to determine what food(s) you are allergic to.

But be aware that all food allergy tests, with the exception of food trials, can yield a false positive result. Rarely, a false negative allergy test can happen, but it’s not impossible.  However, false positives are the focus of this particular article.

I read a news article today about a football player who recently returned to his team after spending a great deal of time in the hospital. It all started with some Chinese food he ate that contained pine nuts,a  food he wasn’t aware he was allergic to.  After having an anaphylactic reaction, the team and coach responded with an epi-pen and a call to 9-1-1.  The defensive lineman, University of Arizona sophomore Justin Washington was rushed to the emergency room where he received treatment for his food allergic reaction and then had follow-up allergy tests yielding positive results to pine nuts, whey protein, milk, and eggs.  The last 3 were a surprise to Washington, as he’s consumed those foods without incident.

Those familiar with food allergies and allergy testing immediately responded with a very loud “FALSE POSITIVE” outcry, a fact not covered in the original story. However, those of us who live with food allergies on a daily basis know and understand the importance of identifying a true food allergy for the sake of quality of life and health care accuracy.  It is also equally important in order to show the world that food allergies are a real danger to the health of our children and many adults.  Articles like the above, showing someone diagnosed with a food allergy when one isn’t present undermines the credibility of all those with food allergies who desire understanding and require special accommodations in schools, the workplace, and on airplanes.

Blood Allergy Test False Positive

Venipuncture using a vacutainer. Photo taken b...

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Blood allergy tests will yield a false positive more often than the skin allergy testing. Allergists will often tell you that while you tested positive for a food item, it’s still unclear if you’re actually allergic as the result fell low on the positive scale. If the positive result lands extremely high, it is almost guaranteed that you are allergic to that particular item, but there is still a chance of it being a false positive without getting a food challenge (more on that below).

This has happened to both my youngest child and myself.  My youngest has an extreme dairy allergy, one that we know exists, but not from allergy testing.  He reacts to dairy foods every single time without fail – hives, wheezing, runny nose, itching in the ears, etc…

In addition to this food allergy, our son also has asthma.  We had avoided the flu shot for his entire life because we weren’t sure if, like his brother and myself, if he was allergic to eggs.  People with egg allergies cannot take the flu shot or the H1N1 shot because it contains egg. So, instead of risking a food trial to see if he could eat eggs, mostly because I just didn’t have the stomach for it, we decided to have him allergy tested.

Skin tests require that you be off of your antihistamines for a specified period of time. Since his asthma is also related to environmental allergies, I was reluctant to take him off of his medication.  So we decided to start with the blood allergy test in order to see if we could get definitive positive or negative confirmation on an egg allergy.  Fortunately, the egg tested negative, which left no question in our minds that he is not allergic to eggs. A negative allergy test result, in any scenario, is rarely a false negative. You can pretty much guarantee that a negative allergy test is a true negative.

That’s good news. No egg allergy means our asthmatic child can get the flu shot – a good thing, because a simple cold can land him in the emergency room with an asthma attack.

Of course, we weren’t surprised when the blood allergy test showed that he is most definitely, without a doubt, allergic to milk products.  Those results were very high on the positive scale, though I didn’t need a test to tell me that he is allergic to milk.  Milk causes an allergic reaction in our son that causes hives, sometimes all over his body but usually just on his face, and intense wheezing requiring an emergency room visit, a breathing treatment and a course of oral steroids for a week or more.

Peanuts and tree nuts were moderately shown on the blood test results as a positive as well, and since his brother and I are both allergic, we’re playing it safe and assuming that he is allergic to those foods.  Eventually, we will have him skin tested and possibly get a food challenge for all of us to be certain about particular food items and whether or not they are a real danger.

But then there was another positive that had me questioning the accuracy of food allergy blood tests.  Soy showed up as a positive, but on the low scale.  The doctor asked if he’s ever reacted to soy.  Aside from getting extremely loose stools if he drinks way too much soy milk in a day (which also happens if I let him have more than a single helping of apple sauce or raisins), he’s never reacted to soy.  The soy was a false positive.

Skin Allergy Testing Procedure

Photo of skin test about 15 minutes after the ...

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More accurate that the food allergy blood test, a skin allergy test is done in the doctor’s office and can take a long amount of time and preparation.  If the patient takes antihistamines and certain other drugs, his/her allergist will advise them to go off of the medications for a pre-determined amount of time before the test can be performed.  Remaining on anti-histamines before a skin allergy test can cause false negatives to certain allergens, giving the patient a false sense of security against real threats to his or her health.

In the office, the patient will remove his/her shirt and a grid will be made on the back (sometimes allergy testing is also done on the arm).  A histamine control will be performed, an intentional reaction that will cause a large hive on the back as a comparison against other potential allergens.  Other allergens will be introduced to the skin through a skin prick or puncture.  If the patient is allergic to the item, a hive will most likely develop and the allergist and/or his assistants will measure the hives and determine the accuracy of the positive result and the potential severity of the allergy.

If all pre-test instructions were followed and medications were avoided prior to the test, any negative result should be accurate.  As I said before, it is rare that a person who doesn’t react to an allergen in testing will actually be allergic to that item. A negative to peanuts will usually never occur in a peanut allergic individual.

A false positive can result, however, in people with sensitive skin or for other unknown reasons.  You can eat shrimp once a week without any reaction and still test positive to shellfish on your skin allergy test.  When this occurs, the patient is usually advised to ignore that particular positive result.  However, if the patient has never eaten shellfish before (perhaps a child who hasn’t yet been introduced to the food), the doctor may then either recommend avoiding that food item as a true allergy or order a food challenge to determine if the food item is a true threat to the patient.

Food Challenge – The Most Accurate Food Allergy Testing Procedure

Many people can get all kinds of positive results for food allergic reactions on a food allergy test.  Usually, this doesn’t mean that the patient is actually allergic to all of these foods, but rather they had a slew of false positives.  The problem with this is when a patient is reacting to something they are eating and they aren’t sure which ingredient is the offending culprit.  At that point, the positive results to things like corn, soy, wheat, artificial colorings and flavorings, and sunflower (found in many of our processed food items, making it hard to determine the true allergen) etc… need to be narrowed down using a food challenge in order to determine which of these foods is actually causing the reaction.

A food challenge is best performed in a doctor’s office.  They will give you the food item to eat and then observe your potential reaction. If a reaction occurs, they are equipped to treat the reaction immediately, providing the patient with the security and feeling of safety needed in such an anxious moment.

Some food challenges are done at home, but this is inadvisable unless directed by your doctor.

Summary of False Positive Results in Food Allergy Testing

1.  All tests can yield false positives. Unless you’ve reacted to the food item or had a food challenge, you cannot know for certain that you are allergic to a food item simply because the allergy test yields a positive result.

2.  Most negative results are accurate. False negative allergy test results are rare.

3.  Food challenges are the only 100% accurate way to determine if you are allergic to any food items.  These are best performed under the supervision of a doctor in his/her office.

Question:  Have you had or considered a food challenge because of the high number of positive results you got on your allergy tests? Have you ever had a false positive during a food allergy test?  A false negative?

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