A doctor uses several factors such as size, color, shape, location, texture and presence of weeping to diagnose a rash or red spots on the skin, states eMedicineHealth. Once the doctor narrows in on the type of rash or irritation, environmental factors are taken into account to find the cause.
Rashes or red spots on the skin are most commonly caused by an outside, environmental influence; however, a skin irritation may be caused by a virus, infection, disease or condition, says Healthline. A doctor can easily tell the difference between hives and a rash because hives usually disappear within eight hours.
A rash that is caused by an irritant rubbing against the skin is called contact dermatitis, reports eMedicineHealth. A doctor can usually diagnose this rash by speaking with the patient. Common causes include poison ivy, jewelry containing nickel, cosmetics and soaps. Diaper rash is a fairly easy rash to diagnose. Rashes can also be caused by food allergies or side effects to medications.
Scaly, dry rashes are often caused by conditions such as eczema or seborrheic dermatitis, explains MedicineNet.com. Blood tests and skin biopsies are helpful diagnostic tools. However, a doctor usually makes a determination based on physical examination.
Viral rashes usually accompany other symptoms such as fever, cough or headache. Herpes, shingles and chickenpox are all examples of viral rashes. Most viral rashes go away on their own or disappear within two weeks, according to Healthline.