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How do I determine the cause?

Constructing and Ranking a DDx


Figure 29-1.

Approach to the patient with rash or skin lesion.

In clinical practice, rashes are diagnosed through pattern recognition (sometimes called system 1 reasoning) probably more than any other complaint. This is an effective way of making a diagnosis when the diagnosis is obvious or when the observer is very experienced. The risk with this type of reasoning is that diagnostic hypotheses are heavily influenced by recent experience, rare diagnoses tend not to be recognized, and physicians often reach premature closure on an incorrect diagnosis.

The differential diagnosis of a rash should be based on the morphology of the lesion. This can be considered the first pivotal point in the differential diagnosis. To correctly categorize a lesion’s morphology, the physician must first identify the primary lesion, the typical element of the eruption. This process can be complicated as the primary lesion is often affected by secondary changes such as excoriation, erosion, crusting, or coalescence. Because the differential diagnosis of 1 lesion can be extensive, once the morphology of the primary lesion is identified, the next pivotal points are determining the global reaction pattern and observing the distribution of lesions. What follows are some important definitions, followed by a differential diagnosis of some of the most common reaction patterns.

  1. Macule: lesion without elevation or depression, < 1 cm

  2. Patch: lesion without elevation or depression, > 1 cm

  3. Papule: any solid, elevated “bump” < 1 cm

  4. Plaque: raised plateau-like lesion of variable size, often a confluence of papules

  5. Nodule: solid lesion with palpable elevation, 1–5 cm

  6. Tumor: solid growth, > 5 cm

  7. Cyst: encapsulated lesion, filled with soft material

  8. Vesicle: elevated, fluid-filled blister, < 1 cm

  9. Bulla: elevated, fluid-filled blister, > 1 cm

  10. Pustule: elevated, pus-filled blister, any size

  11. Wheal: inflamed papule or plaque formed by transient and superficial local edema

  12. Comedone: a plug of keratinous material and skin oils retained in a follicle; open comedone has a black inclusion, closed comedone appears flesh-colored or pinkish

Primary lesions are further grouped into reaction patterns. Papulosquamous eruptions present with papules and plaques associated with superficial scaling. Folliculopapular eruptions begin as papules arising in a perifollicular distribution. Dermal reaction patterns result from infiltrative and inflammatory processes involving the dermal and subcutaneous tissues. Petechia and purpura occur when there is leakage of blood products into surrounding tissues from inflamed or damaged blood vessels. Blistering disorders present with vesicles and bullae. A differential diagnosis of rash is listed below and Figure 29-1 presents an algorithm of a possible approach to patients with rashes and skin lesions.

  1. Blistering disorders (vesicles, pustules, and bullae)

    1. Autoimmune

      1. Bullous pemphigoid

      2. Epidermolysis bullosa acquisita

      3. Pemphigus vulgaris


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