Your doctor is absolutely correct.This is the consensus opinion of most dermatologist worldwide, that no blood tests prick tests or allergy tests are helpful in diagnosis of urticaria.
Next Steps
If your disease is exacerbated by any particular food or medicines specifically avoid those.
Health Tips
You need to take medication(anti histaminics) on a daily basis for a long period of time may be 1 year or even more .If levo cetirizine is causing drowsiness discuss with your dermatologist and switch to another alternate drug.