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Cap Rast done less than 72 hours after antihistamine

9 replies [Last post]
By Enlightened on Thu, 04-07-05, 01:34

Hi-My DS saw his allergist on Monday for a yearly visit. I forgot and gave him Triaminic Cold and Allergy on Friday night. I mentioned this to the doctor, and he said we couldn't the scratch test done, but he'd order blood tests done. It didn't hit me till now-wouldn't the antihistamine affect the blood test results also?

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By Adele on Thu, 04-07-05, 02:54

Hello Enlightened - hopefully someone will enlighten both of us...as I was wondering the same thing. I had a regular RAST test done a few weeks ago, still waiting for the results. It occurred to me a few days ago, that I probably took a Tylenol PM (which contains 25mg of benedryl) the night before the test. So now I'm wondering if the results will be accurate. Wish I could help you, but perhaps someone can help both of us! Cheers. Adele

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By jtolpin on Thu, 04-07-05, 13:12


With the SPT, you have to be off antihistamines for X hrs (72? ok)

For the RAST, you can remain on antihistamines...



[b]* ENRICHED * [/b]

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By Adele on Thu, 04-07-05, 13:45

Thanks! Adele

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By kadely on Thu, 04-07-05, 15:31

I know that antihistamines can effect the results of the cap rsat, but what if you have a scratch test then within 72 hours you have blood work? Do you think that could make the numbers higher or not?

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By bookworm on Thu, 04-07-05, 16:51

CAP-RAST measures the level of IgE antibodies in your blood. This WILL NOT be affected by antihistamines.

Scratch testing measures an allergic response in the body caused by IgE antibodies. The job of an antihistamine is to subdue the allergic response by blocking downstream effects of the IgE reaction, but it does not affect the presence of the IgE itself. Our allergist recommends being off antihistamines for 2 weeks (rather than 72 hr) before scratch testing to avoid masking a reaction.

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By kadely on Thu, 04-07-05, 17:05

Thanks bookworm, that makes a lot of sense to me. So, having blood work right after a scratch test wouldn't effect the results? My son's tests came back over 100 for peanuts. What does this actually mean? Does this mean he is more vulnerable?

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By bookworm on Fri, 04-08-05, 14:57

Not sure about having bloodwork shortly after a scratch test. I think it's possible that the exposure of the scratch test could allow the body to become further sensitized to the antigen, and potentially the IgE level could increase further as a result of that. I'm not a medical dr or immunologist, so can't say for sure if that happens, or how quickly. But even if the scratch test *does* have the potential to trigger the body's production of more IgE against peanut, it probably is biologically relevant (meaning, the IgE level is what it is, no matter what caused it to get that high). If the RAST came back >100, there's no doubt that it should be taken very seriously.

Can I assume that the scratch test *did* come back positive - is that what led to the RAST test?

Anybody else have thoughts on this?

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By samirosenjacken on Fri, 04-08-05, 15:04

I talked with Dr. wood at Johns Hopkins about this and he said antihistimines cannot effect a blood cap rast.


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By mcmom on Sat, 04-09-05, 19:19

My friend just had blood drawn for her 1 year old - he had his first taste of pb and broke out - she gave him Benedryl and their ped drew blood the same day. Blood test was negative, but is it specifically the *cap rast test* that isn't affected by antihistamines? Are there other tests that could be affected? She was wondering, and I was concerned too (breaking out seemed like a classic allergic reaction, plus child has eczema and a few of the other allergy markers.) I know when I scheduled my son for blood tests, I was told he had to be free of antihistamines for 72 hours.

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