From the Guidelines
For a patient with a persistent periodic rash, a comprehensive laboratory evaluation is recommended, starting with a complete blood count (CBC) with differential, comprehensive metabolic panel, erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), and antinuclear antibody (ANA) testing, as outlined in the most recent guidelines 1. The initial testing should aim to assess for infection, inflammation, or hematologic disorders, evaluate organ function, detect inflammation, and screen for autoimmune conditions.
- Additional tests may include:
- Thyroid function tests
- Serum immunoglobulin levels
- Complement levels (C3, C4)
- Specific autoantibody panels if autoimmune conditions are suspected
- Skin biopsy during an active rash episode for histopathological examination, as suggested by 1
- If the rash appears to be triggered by specific exposures, allergen-specific IgE testing or patch testing may be appropriate, as mentioned in 1.
- It is also important to consider testing for autoinflammatory diseases, such as those outlined in 1, including tests for IL-1RA or IL-36RA, CIAS1, and NLRP13, among others, if the clinical presentation suggests an autoinflammatory component.
- The choice of tests should be guided by the clinical presentation and history of the patient, with the goal of identifying underlying systemic diseases, autoimmune conditions, allergic reactions, or other triggers that may be causing the periodic nature of the rash, allowing for targeted treatment of the underlying cause rather than just symptomatic management, as recommended by 1.
From the Research
Laboratory Evaluation for Persistent Periodic Rash
The laboratory evaluation for a patient with a persistent periodic rash can involve various tests to determine the underlying cause of the rash.
- The choice of tests depends on the clinical presentation and suspected diagnosis.
- Some studies suggest that autoantibodies can help clinicians detect autoimmune diseases and their clinical manifestations, determine effective monitoring of prognosis and treatment response 2.
- However, autoantibody positivity alone does not make a diagnosis, and the absence of autoantibodies alone does not exclude diagnosis 2.
- Serum rheumatologic tests are generally most useful for confirming a clinically suspected diagnosis 3.
- Tests such as erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP) can be used to monitor inflammatory conditions, with ESR being helpful in monitoring chronic inflammatory conditions and CRP being more useful in diagnosis and monitoring responses to therapy in acute inflammatory conditions 4.
Recommended Tests
Some recommended tests for a patient with a persistent periodic rash include:
- Rheumatoid factor testing, which is appropriate when rheumatoid arthritis, Sjögren's syndrome, or cryoglobulinemia is suspected 3.
- Antinuclear antibody testing, which is highly sensitive for systemic lupus erythematosus and drug-induced lupus 3.
- ESR and CRP tests to monitor inflammatory conditions 4.
- Autoantibody tests, such as anti-double-stranded DNA antibodies, anti-Ro (anti-SS-A), and anti-La (anti-SS-B), which can help confirm the diagnosis of certain rheumatic diseases 2, 3.
Limitations of Laboratory Tests
It is essential to note that laboratory tests have limitations, and a careful history and physical examination along with serial ordering of a few selected tests appear optimal to establish a clinical diagnosis of a rheumatic disease 5.
- Combinations of blood tests in "rheumatic panels" may not be useful in diagnosis, as they can lead to misclassification errors and have a low positive predictive value 5.
- False positive test results can lead to wrong treatment and unnecessary anxiety for patients, highlighting the importance of interpreting test results in the context of clinical presentation and medical history 2.