Nathan has a 25% chance of not having the allergy. That's the number you must work with. If the test is 98% accurate and it says "No, he's allergy free." then there is still a two percent chance he could still have it.
The 2% is not at this point relevant. He has 25% * 98% = (25/100) * (98/100) = 0.2450 chance that the drug is right and the allergy is not there.
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