Are blood investigations necessary for patients with eczema?

Blood Investigations for Eczema

Blood investigations are not routinely required for most patients with eczema, but should be considered in specific clinical scenarios such as treatment-resistant cases, suspected comorbidities, or when considering systemic therapy.

When to Consider Blood Investigations

Blood tests are generally unnecessary for typical cases of eczema that respond to standard therapy, but may be indicated in the following situations:

1. Treatment-resistant eczema

   • When eczema does not respond to appropriate topical treatments
   • When there is unexpected deterioration in previously stable eczema

2. Suspected complications or comorbidities

   • Suspected secondary infection (bacterial or viral)
   • Suspected underlying autoimmune conditions
   • Suspected thyroid dysfunction

3. Before initiating systemic therapy

   • Baseline investigations prior to immunosuppressive treatments

Recommended Blood Investigations When Indicated

When blood tests are deemed necessary, the following may be considered:

• Full blood count and white cell differential 1

  • To detect eosinophilia (may suggest helminth infections)
  • To detect leucopenia (may suggest systemic lupus erythematosus)

• Erythrocyte sedimentation rate (ESR) 1

  • Usually normal in typical eczema
  • May be raised in urticarial vasculitis or autoinflammatory syndromes

• Thyroid function tests and thyroid autoantibodies 1

  • Particularly if autoimmune etiology is suspected
  • Thyroid autoimmunity is more prevalent in chronic urticaria

• Specific IgE testing 1

  • Only when food or aeroallergen triggers are strongly suspected
  • High negative predictive value (>95%) but low specificity (40-60%)
  • Not recommended as a routine test

Important Caveats

• Total IgE levels alone have limited diagnostic value 1, 2

  • May be elevated in atopic conditions but not specific for diagnosis
  • Not helpful in determining specific allergies

• Testing for non-IgE antibodies (IgG) is not beneficial 1

  • Does not yield information helpful for eczema management

• Patch testing 1

  • Not a blood test, but important when allergic contact dermatitis is suspected
  • Gold standard for investigating contact allergens

Clinical Approach Algorithm

1. For mild to moderate eczema responding to treatment:

   • No blood investigations required
   • Focus on proper skin care, emollients, and topical treatments

2. For moderate to severe eczema not responding to standard therapy:

   • Consider full blood count, ESR
   • Consider thyroid function tests if autoimmune etiology suspected
   • Consider specific IgE testing only if strong history of allergic triggers

3. For suspected secondary infection:

   • Bacteriological swabs rather than blood tests
   • Viral screening if herpes simplex infection is suspected

Conclusion

The diagnosis of eczema is primarily clinical, based on characteristic features and distribution of skin lesions. Blood investigations should be targeted and guided by clinical presentation rather than performed routinely. The focus of management should remain on appropriate skin care, avoidance of triggers, and topical treatments, with blood tests reserved for specific clinical scenarios that would influence treatment decisions.