Thanks for your query at DoctorSpring. I can understand your concern.
Itching in the anal area could be due to a few causes like:-
1. Any eczema, allergy etc
2. Any infection in the area.
Once the cause is treated, the itching would subside. Its important to rule out the cause of itching.
At times, itching can even occur without any cause, its referred to as pruri ani. This kind of itching can be quite chronic.
I would recommend that you take an anti histaminic tablet like levocetrizine 5 mg at night daily to control the itching.
Also you can apply a steroid antibiotic antifungal combination cream in the anal area.
Also I would suggest that you visit a dermatologist again and request him to rule out any local cause like infection, eczema etc.
If the dermatologist gives you an all clear, then it would be advisable to visit a surgeon who could do a proctoscopy, anal examination with an instrument to rule out any fistula, fissure etc.
There are no other tests that are required as of now.
Hope my recommendations are useful to you.
Please let me know if you have any queries.