Erythema multiforme is a skin lesion. It has a variety of manifestations from a simple change in skin color to a bulging or blistering of the skin surface. The rash is characterized by large and small patches of grainy redness that sometimes itch and sometimes do not. Diagnosis needs to be made based on test results. The pathway to diagnosis is aided by routine blood sampling tests (blood rheology is an eight-index test that reflects the consistency, viscosity, plasma viscosity, blood cell aggregation, and blood cell coagulation), urine tests, and chemical tests for bacteria on the skin surface. The average platelet volume, platelet distribution width, whole blood viscosity, plasma viscosity, erythrocyte specific volume, and fibrinogen in normal individuals with significantly similar results compared to healthy controls are normal values. Before the examination, pregnant patients should pay attention to the following: fasting, prohibit the consumption of foods high in sugar and fat, prohibit the consumption of alcohol, spicy and stimulating foods, avoid fish and shrimp and other easily allergenic and indigestible foods, and should cooperate with the doctor to clean the infected area during the examination and listen to the doctor’s arrangements and requirements.