Your health care provider can usually diagnose pityriasis rosea by the way the rash looks. A blood test may be needed to rule out a form of syphilis, which can cause a similar rash. Occasionally, a skin biopsy may be needed to confirm the diagnosis.
Habif TP. Psoriasis and other papulosquamous diseases. In: Habif TP, ed. Clinical Dermatology. 4th ed. Philadelphia, Pa: Mosby Elsevier; 2004: chap 8.
Lim HW. Eczemas, photodermatoses, papulosquamous (including fungal) diseases, and figurate erythemas. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007: chap 464.