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I have had a severe bacterial infection. I was prescribed ceftriaxone, and when the time came for injecting it the nurse asked me whether I have an allergy to ceftriaxone. I answered that it's my first time, so she did the test and it was negative.

I was expecting a second test because as far as I know from type 1 hypersensitivity the body - even if it is sensitive to an allergen - won't show a reaction the first time. The body makes the particular IgE and in the next exposure of same allergen it will have an allergic reaction.

So why is only one test done, considering the first two columns of the following picture?

Comparative features of 4 types of hypersensitivity reaction

From: Harsh Mohan's Pathology textbook 7E

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  • $\begingroup$ When were you expecting a second test? Directly afterwards? After taking the antibiotic? $\endgroup$ – YviDe Dec 23 '15 at 15:32
  • $\begingroup$ If you are not allergic, you are not allergic, and it's unnecessary to do the test more than once. If, on subsequent treatment with the same (or similar) antibiotic,you do develop an allergy, you'll know from then on, and they'll know not to give it to you. Finally, it's not necessary to test for every allergy that might possibly be present. Most allergies are not life threatening, and if you don't have a history of allergy, that's a good enough reason to believe it won't be a problem. $\endgroup$ – anongoodnurse Dec 23 '15 at 23:11
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    $\begingroup$ You are misinterpreting that table. Peak action time refers the time after exposure if you have already been exposed to the antigen. Raising the adaptive response to the first exposure to that antigen will then take 8 to 14 days. Even if you are genetically predisposed, you need to activate that B-Cell to make IgE which acts as the Mast Cell receptor and triggers the degranulation that triggers anaphylaxis. $\endgroup$ – AMR Dec 24 '15 at 6:16
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    $\begingroup$ I did read it and you are misinterpreting it. "...previously sensitized person" does not mean sensitized 5 minutes ago or an hour ago. It means weeks, months, or years prior. Reaction to humoral antibodies, means that you would have had to make the antibodies first from a prior exposure. That would be at the minimum 8 days to two weeks after exposure. $\endgroup$ – AMR Dec 29 '15 at 20:04
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    $\begingroup$ Was it a single bolus? You said it was by injection, so were they treating you over a period of more than two weeks? Or was it one and done? They aren't going to tell you to come in for a second test if you aren't receiving the drug, because you might develop a sensitivity to it. They would cross that bridge if and when they ever got to it. For the most part, you do not test healthy people for anything more than vital signs unless they are at risk or they have a complaint. Do you have other allergies, to other drugs or are you healthy? If you are healthy, then there is no point to test you. $\endgroup$ – AMR Jan 1 '16 at 4:18
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Because developing a hypersensitivity takes time. If you get exposed to an allergen, the body first has to produce those antibodies that then get used in the hypersensitivity response at the second, or subsequent, exposure.

If you are talking about taking a second test just minutes after the first one, how would your body even "know" that this is a second exposure? Two exposures within minutes are not distinguishable from one exposure for the body.

Usually, an allergic reaction shows the second time a patient takes a full course of antibiotics. It does not show at the second oral dose or injection, which are way too close to the first dose to have already lead to the developing of a hypersensitivity.

So, why test you in the first place? Because you might have been exposed previously without remembering it - people misremember what medication they got during their childhood all the time, I imagine, and your doctor/nurse seem to just routinely do the test before giving these antibiotics.

Sources/Reading

Drug allergies

Update on the Management of Antibiotic Allergy

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  • $\begingroup$ Personally, my amoxicillin allergy decided to show itself on day 8 of a 10 day treatment, the first time taking it. $\endgroup$ – YviDe Dec 23 '15 at 18:18
  • $\begingroup$ I'm not sure of something you assume in your answer (because the question is unclear): I don't think the OP is talking about the second dose (though he might be), but rather the second course of antibiotics (at least, I hope that's what was meant.) Also, you might want to read about amoxicillin-associated rashes. $\endgroup$ – anongoodnurse Dec 23 '15 at 23:16
  • $\begingroup$ @anongoodnurse I thought he was talking about the course of antibiotics being the second exposure, the first being the skin test. But it's unclear. What do you mean with those rashes? Or rather, what do you mean with them in relation to this question/answer? $\endgroup$ – YviDe Dec 23 '15 at 23:20
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There already has been an answer but I'd still like to clarify further. I'm assuming the OP was expecting a second skin test because type 1 hypersensitivity requires a repeat exposure. Getting exposed to the antigen will just get you sensitized but will not precipitate the reaction.

What I want to clarify is that the skin test doesn't test whether you are allergic to a medication but if you are already sensitized to it. It is important to get the difference.

If you have. never been exposed (hence sensitized) to ceftriaxone before, the test would be negative even if you are actually allergic. And as sensitization needs time, you wouldn't be getting tested right afterwards (as mentioned in the comments). Instead, you'll be given the dose.

So why skin tests? That's because the dose is low and intradermal - chances of a full blown anaphylaxis are lower (not zero) even if the pt. is previously sensitized.

So, to conclude, as mentioned in the previous answer, the skin test is done just once to check if you've already sensitized, since rarely do people precisely remember the medications they have been previously exposed to.

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