The Cost of False Negatives

Last week I ranted about false positives of skin prick tests and guess what? This week, I’m still ranting about skin prick tests, but this time about false negatives. False negatives are really rare. So much so that many sources online say, “If no wheal appears, it is unlikely that you are allergic to the test food.” I’ve never seen any false negative rate over 5% and my kid’s allergist said that false negatives are usually actually due to error in the test, rather than the wheel just not appearing.

C'mon skin tests! You have ONE job! Negatives are supposed to mean 100% safe to eat!

With all of that said, my kid has had not one, but two false negatives to milk. After her reaction to peanut butter, we saw the allergist and because I was so worried about the other top 8 allergens, did a skin prick test to them. Egg and peanuts were positive, but everything was else was negative. Fast forward a few weeks and she is eating yogurt while we are eating shrimp tacos and she has very clear anaphylaxis- repetitive vomiting, coughing, nose running. She had had smaller amounts of yogurt before, but for whatever reason this was the time that made her react. With her first negative test, we didn’t think yogurt was the cause.

A few weeks later, she’s eating cheese for the first time and wouldn’t you know we have another round of anaphylaxis? I guess George Bush said it best. "Fool me once, fool on you. Fool me twice, ain't gonna fool me again!" It had to be the milk, right? We take her back to the allergist and she has another negative skin prick test. Her allergist repeats it at the end of the appointment herself and she does get a wheel- it’s small, but it’s there.

So, what’s the cost of a false negative? Anaphylaxis. For us, twice. And not trusting skin testing. If the skin testing can’t at least tell you what food is safe, then really what is the point?

Earlier this summer, she had another negative skin test to milk. We are scheduled for an oral food challenge. To say I am concerned is an understatement. FARE says skin tests are inexpensive, give immediate results, and are extremely helpful. I’ve been a little disillusioned with them. I guess we’ll just see how the baked milk challenge goes.