Greeting Doc.. for the past 1 week now.. my body...

Dear Customer
Welcome, and thank you for trusting us
Regarding your question

How to diagnose the cause of skin itching?

There are many causes and those include:
Allergy,
Dermatological disease
Side effects of drugs
Systemic illness
Local infection
 

You need to do the tests:

CBC, ESR, CRP, AST, ALT, creatinin, urea
 
 

What is the treatment of skin itching ?

If there is itching and wide space of skin in this case you need to use dexamethasone injection and also you need to use topical hydrocortisone 1%
I also recommend to use antihistamine medications such as loratadin10mg 1x1 to ease symptoms.
If itching last more than one day in this case you need to consult dermatologist to help you and to follow up treatment.
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