What laboratory tests are indicated for an ongoing pruritic (itchy) rash?

From the Guidelines

For a pruritic rash, I recommend checking a full blood count and ferritin levels, as iron deficiency is a common cause of generalized pruritus, as suggested by the British Association of Dermatologists' guidelines 1. When investigating the cause of a pruritic rash, it is essential to consider various potential underlying conditions.

• A complete blood count (CBC) can help detect eosinophilia, which may indicate allergic reactions or parasitic infections.
• Liver function tests (LFTs) should be considered, especially if there is suspicion of iron overload or liver disease, as LFTs can help evaluate liver function and detect potential causes of pruritus 1.
• In cases where hematological involvement is suspected, initial investigations should include a full blood count, blood film, lactate dehydrogenase, and erythrocyte sedimentation rate (ESR) 1.
• If polycythemia vera (PV) is suspected, blood should be analyzed for the Janus kinase (JAK)2 V617F mutation, which is present in up to 97% of cases 1.
• A thorough history and physical examination should be performed to rule out malignancy, and tailored investigations should be conducted if systemic symptoms of malignancy are present 1.
• Urea and electrolytes should be checked as part of the investigation, especially if there is suspicion of uraemia or renal disease 1.
• Vitamin D supplementation may be beneficial for some patients with generalized pruritus, as suggested by the British Association of Dermatologists' guidelines 1. The specific combination of tests should be tailored to the patient's presentation, including the rash's appearance, distribution, and associated symptoms.
• Consider specific IgE antibody tests or skin prick tests if allergies are suspected.
• If the rash appears infectious, bacterial or fungal cultures may be appropriate.
• For persistent cases, a skin biopsy might be necessary to examine tissue histology.
• In autoimmune concerns, autoantibody panels including ANA and anti-dsDNA can be valuable. These tests help identify the cause of pruritus, which is essential for effective treatment and improvement of morbidity, mortality, and quality of life.

From the Research

Labs to Check for Ongoing Pruritic Rash

To diagnose the underlying cause of an ongoing pruritic rash, several laboratory tests can be performed. The following are some of the key labs to check:

• Complete blood count (CBC) with differential 2, 3, 4
• Hepatic function testing 2, 3, 4
• Renal function testing 2, 3, 4
• Thyroid function testing 2, 3, 4
• Diabetes screening 2
• Erythrocyte sedimentation rate (ESR) 4
• HIV screening 4
• Hepatitis serologies 4
• Chest radiography 4
• Iron studies 4
• Fasting glucose or A1C level 4

Additional Testing

Additional testing may be necessary based on the patient's history and physical examination findings. For example:

• If the patient has a history of liver disease and diffuse itch less than 12 months of duration, there should be a heightened concern for underlying malignancy 2
• If the patient has chronic pruritus of unknown origin, increased blood eosinophils may serve as a biomarker of T helper cell type 2 polarization and response to immunomodulator therapies 2
• If the patient has localized cutaneous pruritus, neuropathic causes such as postherpetic neuralgia and notalgia paresthetica should be considered 3, 5