What tests should be requested for a pruritus (itching) screen?

Diagnostic Tests for Pruritus Screening

For a comprehensive pruritus screen, full blood count and ferritin levels should be checked in all patients with chronic generalized pruritus without rash, followed by targeted investigations based on clinical suspicion. 1

Initial Laboratory Investigations

First-line Tests (Recommended for All Patients)

• Full blood count (FBC) and ferritin levels 1
• Liver function tests (LFTs) 1
• Urea and electrolytes 1

Second-line Tests (Based on Clinical Presentation)

• Blood disorders screening:

  • Blood film
  • Lactate dehydrogenase (LDH)
  • Erythrocyte sedimentation rate (ESR) 1

• If iron deficiency is suspected:

  • Serum iron
  • Total iron binding capacity
  • Tissue transglutaminase (TTG) antibodies (if unexplained iron deficiency) 1

• If polycythemia vera is suspected (especially with aquagenic pruritus):

  • JAK2 V617F mutation analysis 1
  • Serum erythropoietin level (if JAK2 negative) 1

Additional Targeted Investigations

For Liver Disease

• Consider bile acids and antimitochondrial antibodies if LFTs abnormal 1
• Refer to hepatology if significant hepatic impairment is detected 1

For Infectious Causes

• HIV serology
• Hepatitis A, B, and C serology
• Consider screening for parasitic infections (malaria, strongyloidiasis, schistosomiasis) if travel history suggests 1

For Endocrine Disorders

• Thyroid function tests (only if clinical features suggest thyroid disease) 1
• Fasting glucose or HbA1c (if diabetes suspected) 2

For Malignancy

• Targeted investigations only if systemic symptoms of malignancy present 1
• Consider skin biopsy for persistent unexplained pruritus to rule out cutaneous lymphoma 1

Important Clinical Considerations

When to Expand Testing

• Expand testing when pruritus is:
  • Persistent and unexplained
  • Associated with systemic symptoms
  • Not responding to initial management

Common Pitfalls to Avoid

1. Over-investigation: Full investigation to rule out malignancy is not routinely recommended without specific symptoms 1
2. Under-investigation: Missing iron deficiency (check ferritin even with normal hemoglobin) 1
3. Overlooking drug causes: Always review medications as potential causes of pruritus 1
4. Unnecessary endocrine testing: Routine endocrine investigations are not recommended unless clinical features suggest endocrinopathy 1

Special Considerations

• In older patients with chronic generalized pruritus without primary skin lesions, consider evaluation for malignancy 3
• For pruritus lasting less than 1 year with few primary skin changes, a thorough laboratory evaluation is particularly important 2
• Consider neuropathic causes if standard investigations are negative 2

Diagnostic Algorithm

1. Initial evaluation: Complete history (including medication review) and thorough skin examination
2. First-line tests: FBC, ferritin, LFTs, urea and electrolytes
3. If primary skin lesions present: Focus on dermatological causes
4. If no primary skin lesions: Proceed with systematic investigation of systemic causes
5. If initial tests normal: Consider second-line tests based on clinical suspicion
6. If all tests normal: Consider neuropathic or psychogenic causes, or chronic pruritus of unknown origin

Remember that approximately 15% of people with chronic pruritus have systemic diseases with secondary itch, such as uremic pruritus and cholestatic pruritus 2, making a systematic approach to laboratory testing essential for proper diagnosis and management.