Laboratory Workup for Week-Long Pruritic Rash
For a patient presenting with a week-long pruritic rash, order the following initial laboratory panel: complete blood count (CBC), ferritin, liver function tests (LFTs), and renal function (urea and electrolytes). 1
Essential First-Line Tests
The British Association of Dermatologists provides clear guidance on the core laboratory workup for pruritic conditions:
- Complete Blood Count (CBC): Essential to evaluate for hematological causes including polycythemia vera, lymphoma, and anemia 2, 1
- Ferritin levels: Iron deficiency is a common and treatable cause of pruritus; ferritin should be checked in all patients with chronic pruritic conditions 2, 1
- Liver Function Tests (LFTs): Critical for detecting cholestatic liver disease and other hepatic causes of pruritus 2, 1
- Renal function (Urea and Electrolytes): Uremic pruritus from kidney disease is a significant systemic cause 2, 1
Additional Tests Based on Clinical Context
If the history or examination suggests specific underlying conditions, expand the workup:
- Blood film, lactate dehydrogenase (LDH), and ESR: Order these if you suspect hematological disorders such as lymphoma or polycythemia vera 2
- JAK2 V617F mutation: Specifically indicated if polycythemia vera is suspected (elevated hemoglobin/hematocrit, microcytosis, elevated platelets) 2
- HIV and hepatitis serologies (A, B, C): Consider with relevant risk factors or travel history 1
Important Nuances About Iron Studies
When evaluating iron deficiency, recognize that ferritin is an acute-phase protein and may appear falsely "normal" in inflammatory states 2. If clinical suspicion remains high despite normal ferritin, check serum iron and total iron binding capacity 2, 1. Consider tissue transglutaminase (TTG) antibodies for unexplained iron deficiency to screen for celiac disease, but note that IgA deficiency can cause false-negative TTG results 2.
Critical Pitfalls to Avoid
- Do NOT routinely order thyroid function tests unless the patient has specific clinical features suggesting thyroid disease or diabetes 2, 1
- Do NOT order extensive malignancy screening without specific clinical indicators (weight loss, night sweats, lymphadenopathy) 2, 1
- Do NOT forget to check ferritin - this is recommended in all cases of chronic pruritus and is frequently overlooked 1
- Do NOT ignore medication history - drug-induced pruritus requires thorough medication review rather than additional laboratory tests 1
When to Consider Additional Workup
For persistent, unexplained pruritus despite negative initial workup, consider skin biopsy to exclude cutaneous lymphoma, particularly if the patient has normal-appearing skin but persistent symptoms 2, 1. This is especially important in patients over 60 years with diffuse itch of less than 12 months duration 3.