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Purpura is extravasation of red blood cells into the skin or mucous membrane. For this reason, purpuric lesions do not blanch on diascopy (pressing on the lesion with a glass slide or finger). The differential diagnosis for purpura is broad, but it can be quickly narrowed by classifying the lesions based on their morphology, as well as other clinical and laboratory findings.1,2 The clinical descriptive terms for purpura are listed below, and their respective tables describing the differential diagnosis are referenced.

  • Petechiae: Flat lesions, macules ≤4 mm (Figure 25-1), typically initially bright red and then fade to a rust color (Tables 25-1 and 25-2).

  • Ecchymosis: Flat lesions, macules and/or patches, >5 mm (Figures 25-2 and 25-3), typically initially red or purple, but may fade to yellow, brown, or green (Table 25-3).

  • Palpable purpura: Elevated, round or oval, red or purple papules and/or plaques (Figure 25-4), sometimes barely palpable (Table 25-4).

  • Retiform purpura: Stellate or branching lesions, with angular or geometric borders (Figure 25-5). These are often palpable plaques, but can present as nonpalpable patches as well (Tables 25-5 and 25-6).

Table 25-1.Causes of petechiae (primary lesion is a macule ≤4 mm) with low platelets (<150,000/μL).

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