The generalized rash: part I. Differential diagnosis.

Physicians often have difficulty diagnosing a generalized rash because many different conditions produce similar rashes, and a single condition can result in different rashes with varied appearances. A rapid and accurate diagnosis is critically important to make treatment decisions, especially when mortality or significant morbidity can occur without prompt intervention. When a specific diagnosis is not immediately apparent, it is important to generate an inclusive differential diagnosis to guide diagnostic strategy and initial treatment. In part I of this two-part article, tables listing common, uncommon, and rare causes of generalized rash are presented to help generate an inclusive differential diagnosis. The tables describe the key clinical features and recommended tests to help accurately diagnose generalized rashes. If the diagnosis remains unclear, the primary care physician must decide whether to observe and treat empirically, perform further diagnostic testing, or refer the patient to a dermatologist. This decision depends on the likelihood of a serious disorder and the patient's response to treatment.