Just some questions to narrow down the possible causes of your rat's hair loss and scabs.
1. What are you feeding?
2. What type of bedding are you using?
3. Hair loss can also be caused from scratching at external parasites like mites, lice or fleas. Mange is also a possibility-- also caused by a parasite. Burrowing mites cannot always be detected by a skin scraping. You may need to dose with the Ivermectin. Generally, 3 doses separated by a week is recommended. The following is the location of the RMCA Drug Chart--look in the list-- Ivermectin is listed-- dose and treatment.
4. Another possibility is a fungal infection like ring worm which is common in guinea pigs-- it looks like a bald patch with dry scaly skin.
5. The last and hopefully not the cause of hair loss can be caused by auto-immune problems like Cushings Disease: <http://www.io.com/~lolawson/cushings/>
"Anipryl is the only drug licensed for use in dogs that suffer from Cushing's disease, a neurodegenerative hormonal disorder of the pituitary gland and hypothalamus in the brain. Dogs with Cushing's disease have decreased hypothalamic dopamine levels and Anipryl works to restore the natural balance of important brain chemicals, especially dopamine, which in turn helps to alleviate the clinical signs of Cushing's disease."
(We haven't tried this with rats, but it would probably be worth asking a vet about!)
Treat with for the most common causes and eliminate the easy problems first. Let rat-help know if this continues to be a problem. It can be very upsetting to see our beloved pets looking so miserable.