Ask HN: My child food allergy from prick test. Can't find science based info | Hacker News Reader

Ask HN: My child food allergy from prick test. Can't find science based info

2 pointsdandare4 comments

So my child was diagnosed with a food allergy from a prick test, but I am struggling to find good science based information on what this actually means.

For instance the University of Michigan website https://www.uofmhealth.org/conditions-treatments/food-allergy/diagnosis-testing says

> A positive SPT is reliable about 50 percent of the time, but a negative SPT result is about 95 percent predictive. [...] Your allergist will use your medical history, a physical exam and his or her own specialized training to interpret your results.

This sounds very close to a quackery. Can you recommend science based resources on food allergies diagnosed from skin prick test?

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What prompted you to take your child to the allergist? Are there any symptoms?

It doesn’t hurt to eliminate a food to see if it makes a difference, that is probably what your doc told you to do.

My take is that food allergies usually have extreme effects. For all the people who are gluten free because they think it’s cool, people who have celiac violently void both sides of their digestive track when they get eat wheat.

Myself I had an anaphylactic attack when I was 4 or so and remember getting prick tests from my family doctor which were inconclusive. I”ve never had a dangerous allergic reaction but I have hay fever, break out in hives from a certain perfume, had asthma in my 30s, got allergy shots, and had the asthma resolve and haven’t used an inhaler or other drugs for it for a long time.

The science is statistics.

Fifty percent of positive results are false positives.

5% of negatives are false negatives.

Most results are negative.

Lets assume 99% for this example but for most uncommon allergies it is probably lower.

So out of 100 tests, the results are expected to be accurate 99 * 0.95 + 1.0 * 0.50.

That's essentially 95% of the time.

In addition, the harm is in not treating an undiagnosed allergy. So there's no direct harm from treating a false positive. The harm is in not treating an a false negative.

The other thing about the prick test is that it is fast and easy.

The alternative is controlled diet.

It's way more work.

Good luck.

Why quackery? As the page says the test itself is not enough for a diagnosis: the result will be interpreted based on whether your child had symptoms of food allergy in the first place (rash, bloating etc.), which is what, I believe, medical history / physical exam is referring to. If yes: that's the likely culprit; if not (and you just got this test randomly for no reason?), the chance of a false positive is 50%.

From what I know its that prick tests are done after someone has allergy problems to find out what foods or substances trigger the allergy. That can then be used to narrow things down significantly.

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