What laboratory tests should be ordered for a patient presenting with excessive skin itching?

Laboratory Tests for Excessive Skin Itching

For patients with excessive skin itching, a systematic laboratory workup should include complete blood count with ferritin, liver function tests, renal function tests, and targeted additional testing based on clinical suspicion. 1

Initial Laboratory Evaluation

First-Line Testing (Recommended for All Patients)

• Complete blood count with differential 1, 2
• Ferritin levels (iron deficiency is a common cause of generalized pruritus) 1
• Liver function tests (LFTs) 1, 2
• Urea and electrolytes (renal function) 1, 2

Second-Line Testing (Based on Clinical Suspicion)

• Blood film, lactate dehydrogenase (LDH), and erythrocyte sedimentation rate (ESR) if hematological disorders are suspected 1
• JAK2 V617F mutation analysis if polycythemia vera is suspected (raised hemoglobin/hematocrit) 1
• Bile acids and antimitochondrial antibodies if liver disease is suspected 1
• HIV and hepatitis A, B, and C serology if infectious causes are suspected 1

Specific Scenarios Requiring Additional Testing

For Suspected Hematological Causes

• If polycythemia vera is suspected (raised hemoglobin/hematocrit with microcytosis, raised white cell/platelet count, low ESR):
  • JAK2 V617F mutation analysis 1
  • Serum erythropoietin level
  • Oxygen saturation measurement
  • Chest X-ray and abdominal ultrasound if JAK2 mutation is negative 1

For Suspected Malignancy

• Do not routinely screen for malignancy without specific symptoms 1
• Targeted investigations only if systemic symptoms of malignancy are present 1
• Consider skin biopsy in persistent unexplained pruritus to rule out cutaneous lymphoma 1

For Suspected Endocrine Disorders

• Do not perform routine endocrine investigations (including thyroid function tests) unless additional clinical features suggest diabetes or other endocrinopathy 1
• Consider vitamin D level assessment 1

Special Considerations

Neuropathic Pruritus

• Consider skin biopsy if small fiber neuropathy is suspected 1
• Refer to appropriate specialist if neuropathic cause is suspected 1

Drug-Induced Pruritus

• Review medication list thoroughly
• Consider trial cessation of suspicious medications if benefit-risk analysis is favorable 1

Common Pitfalls to Avoid

1. Overlooking iron deficiency: Always check ferritin levels, not just hemoglobin 1
2. Premature malignancy workup: Full investigation for malignancy is not routinely recommended without specific symptoms 1
3. Unnecessary thyroid testing: Endocrine investigations should not be routine unless specific symptoms are present 1
4. Missing hematological disorders: Consider blood film examination in addition to CBC when hematological causes are suspected 1
5. Failure to consider drug causes: Always review medications as potential causes of pruritus 1

Algorithm for Laboratory Evaluation

1. Start with first-line tests for all patients (CBC with differential, ferritin, LFTs, renal function)
2. Based on results and clinical presentation, proceed with targeted second-line testing
3. Consider specialist referral (hematology, hepatology, etc.) based on abnormal findings
4. For persistent unexplained pruritus, consider skin biopsy to rule out cutaneous lymphoma 1

Remember that generalized pruritus without rash can be the presenting symptom of various systemic diseases, with hematological causes accounting for approximately 2% of cases 1.