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Chapter 25: The Differential Diagnosis of Purpura

Sarah Nakib; Monica Rani

INTRODUCTION TO CHAPTER

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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Table 25-1. Causes of petechiae (primary lesion is a macule ≤4 mm) with low platelets (<150,000/μL).
Disease/Disorder Clinical Features and Associations Initial Diagnostic Evaluation
Low platelets (<150,000/μL)

Mucocutaneous bleeding (eg, epistaxis, increased bleeding with menses or minor cuts)

Splenomegaly and lymphadenopathy may be present

CBC with differential and peripheral smear; If anemic, check reticulocyte count, LDH, haptoglobin, and bilirubin

If hemolysis is present, check PTT, PT/INR, fibrinogen, D-dimer, Coombs, ANA. If inconclusive, consider bone marrow biopsy
Immune thrombocytopenia (ITP)3

F > M

Primary: Insidious onset

Secondary: Associated with underlying disease, for example, viral infections

Platelets <100, 000

Diagnosis of exclusion

Primary: Isolated decreased platelets

Secondary: May relate to viral process (can obtain serology for HIV, HCV, HBV, EBV, or PCR for parvovirus and CMV), H. pylori, ANA, pregnancy, APLA, or TSH
Thrombotic thrombocytopenic purpura (TTP)1

Pentad: thrombocytopenia, hemolytic anemia, changes in mental status, fever, and renal dysfunction. Do not need all 5 for diagnosis

It may be idiopathic, familial, drug-induced, from pregnancy, HIV, autoimmune disease, or hematopoietic stem cell transplant

HUS: Thrombocytopenia, MAHA, and renal dysfunction; usually in children with prodrome of bloody diarrhea

 Labs that can be observed: ↓ Platelets and MAHA, schistocytes on p.smear, ↑reticulocyte count, positive hemolysis labs (↑LDH, ↑indirect bilirubin, ↓haptoglobin), nlPTT & INR, ↑creatinine, ↓ADAMSTS13 (in idiopathic TTP)
Disseminated intravascular coagulation (DIC)4

Spectrum in hematologic abnormalities can have bleeding and/or thrombosis. May also have soft-tissue bleeding in muscle and joints

May be related to trauma, shock, infection, malignancy, and obstetric complication

 ↑PT/INR, ↑PTT, ↓ fibrinogen, positive fibrin degradation product/D-dimer, and hemolysis ...

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