A reaction (infectious usually) following a blood transfusion
taken 2.5 yrs ago is unlikely and your urticaria
will be controlled on long acting non-sedating antihistamines that need to be taken for 6 weeks at least.
Blood tests such as CBC LFT TSH (thyroid
profile) followed by Hepatitis
infection screen (only if LFTs altered or clinically jaundiced) is indicated.
Allergy testing is unhelpful in 80% of patients with urticaria.