How useful is blood work in finding the cause of pruritus that has few apparent primary skin changes and is unresponsive to topical steroids, pramoxine, lidocaine and doxepin (Prudoxin, Zonalon)? What tests should be ordered? — Melissa Raue, PA-C, Pound Ridge, N.Y.

A thorough history and review of systems focusing on the onset, duration, and nature of the pruritus may help to establish its cause. Chronic, progressive, generalized pruritus in the absence of primary skin lesions may be secondary to an underlying systemic disease. Conservative symptomatic treatment is reasonable for the initial management of “pruritus of unknown origin.”

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If there is no response after two weeks, however, laboratory evaluation should be considered to exclude a possible systemic etiology, neurologic disorders, lymphoreticular neoplasms, visceral malignancies, and other conditions. Initial screening laboratory studies should include complete blood cell counts with differential WBC count and platelets; serum chemistries with particular attention to blood urea nitrogen, creatinine, alkaline phosphatase, bilirubin, and glucose; thyroid function tests (i.e., thyroxine, triiodothyronine resin uptake, and thyroid-stimulating hormone); stool examination for occult blood; and a chest roentgenogram.

Depending on the results from the initial laboratory screening, additional tests to consider are serum iron and ferritin; serum protein electrophoresis and serum immunoelectrophoresis; skin biopsy for routine staining (i.e., hematoxylin and eosin stains) and special stains (i.e., toluidine blue or Giemsa to exclude mastocytosis); skin biopsy for direct immunofluorescence (to exclude dermatitis herpetiformis and bullous pemphigoid); stool examination for ova and parasites; urine collection for 5-hydroxyindoleacetic acid and mast-cell metabolites (i.e., histamine, histamine metabolites, and prostaglandin D2 metabolites); and additional radiologic studies. — Philip R. Cohen, MD (178-4)

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