PATHCHAT Edition No. 38
Please contact your local Ampath pathologist for more information.
Introduction
🔹 What Are Primary Immunodeficiencies (PID)?
- PID consists of over 300 known genetic defects affecting the immune system.
- They can present at any age, though severe forms manifest in infancy.
- Due to variable presentation, PID diagnosis is often delayed or missed.
✅ Key Clinical Clues for PID Diagnosis:
- Severe infections requiring hospitalization or intravenous antibiotics.
- Persistent infections (never fully recover).
- Unusual pathogens or infection sites.
- Recurrent infections (multiple episodes of the same infection).
- Family history of immunodeficiency.
📌 A genetic diagnosis confirms PID and aids in family screening, prenatal testing, and selecting targeted therapies.
Ampath’s Next-Generation Sequencing (NGS) PID Panel
✅ Ampath has developed a comprehensive genetic PID panel covering 99 critical genes.
✅ This panel includes genetic testing for:
- Severe Combined Immunodeficiency (SCID).
- Hyper IgM and Hyper IgE syndromes.
- Common Variable Immunodeficiency (CVID).
- Immune dysregulation syndromes.
- Lymphoproliferative syndromes, including autoimmune lymphoproliferative syndrome.
- Chronic Granulomatous Disease (CGD).
- MHC Class I and MHC Class II deficiencies.
- Thymic defects associated with congenital abnormalities.
- Autoimmune and regulatory immune disorders.
- Innate immunity and complement deficiencies.
📌 This test provides a genetic diagnosis that can guide treatment decisions, bone marrow transplantation, and family genetic counselling.
Stepwise Approach to PID Diagnosis
✅ A structured, tiered diagnostic approach ensures cost-effective testing.
Step 1: First-Line Investigations (Basic Screening)
🔹 Rule Out Common Causes Before PID Testing:
- Exclude HIV (HIV ELISA or PCR).
- Assess for atopy (if indicated by history).
🔹 Initial Blood Tests:
- Full Blood Count (FBC) with Differential:
- Lymphopaenia → Suggests SCID.
- Small platelets → Seen in Wiskott-Aldrich syndrome.
- Serum Immunoglobulins (IgG, IgM, IgA, IgE):
- IgE should be tested in suspected hyper-IgE syndrome.
- Cystic Fibrosis (CF) Screening (if clinically indicated).
Step 2: Second-Line Investigations (Functional Immune Tests)
🔹 Assessing Antibody-Mediated Immunity:
- Specific Antibody Responses to Vaccines:
- Assess responses to polysaccharide antigens (e.g., pneumococcus, H. influenzae).
- Assess responses to protein antigens (e.g., tetanus, diphtheria).
- If antibody levels are low, revaccinate and retest after 4 weeks.
🔹 Assessing Lymphocyte Subsets (Flow Cytometry):
- B-Cells (CD19):
- Absent in X-linked agammaglobulinaemia (XLA).
- T-Cells (CD3, CD4, CD8):
- Reduced in SCID, combined immunodeficiencies, and some cases of CVID.
- Natural Killer (NK) Cells (CD16, CD56):
- Low in isolated NK-cell deficiencies (linked to recurrent herpesvirus infections).
🔹 Assessing Neutrophil Function:
- Neutrophil Oxidative Burst Test → Diagnoses Chronic Granulomatous Disease (CGD).
- Neutrophil Adhesion, Chemotaxis, and Phagocytosis Studies → Diagnoses leukocyte adhesion defects.
🔹 Complement Testing:
- Total Haemolytic Complement Assays (Classical & Alternative Pathways):
- Detect complement deficiencies.
Step 3: Third-Line Investigations (Advanced Functional Testing)
🔹 Lymphocyte Proliferation Studies:
- Tests response to mitogens (PHA, PMA, anti-CD3) and recall antigens (Candida, tetanus, varicella).
- Used to diagnose T-cell defects, including SCID and chronic mucocutaneous candidiasis.
🔹 Neutrophil Antibody Testing:
- Assesses autoimmune neutropaenia.
🔹 Lymphocyte Maturation Markers:
- Naïve vs. memory T-cells → Used for SCID and combined T-cell defects.
- Memory B-cells → Categorizes CVID subtypes.
🔹 TREC/KREC (T-cell and B-cell receptor excision circles):
- Newborn screening for SCID and XLA (performed on dried blood spots).
Step 4: Fourth-Line Investigations (Genetic and Molecular Testing)
✅ Genetic testing is essential for diagnosing severe immunodeficiencies and hereditary immune disorders.
🔹 Available Tests:
- NGS-based genetic PID panel (99 genes).
- Targeted gene sequencing for single-gene disorders.
- Cytokine and enzyme studies.
- T-regulatory cell and Th17 cell analysis.
- Surface marker studies for X-linked SCID.
- BTK mutation analysis (for diagnosing XLA).
📌 Genetic testing must be discussed with an immunology pathologist before ordering.
Genetic Testing for PID at Ampath
🔹 When to Consider Genetic Testing:
✅ For patients with severe or atypical PID requiring bone marrow transplantation.
✅ For screening family members or conducting prenatal testing.
✅ For confirming a suspected single-gene immunodeficiency disorder.
🔹 How to Request Genetic Testing:
✅ Specimen Requirement: One full EDTA tube (test code: PID).
✅ Genetic counselling is mandatory before and after testing.
✅ Estimated cost: R17,600 (subject to medical aid coverage).
✅ Turnaround time: 6 weeks.
📌 Consult Ampath’s immunology pathologists before ordering genetic tests to ensure appropriate selection and interpretation.
Key Takeaways for Clinicians
✅ PID diagnosis requires a structured, stepwise approach.
✅ Initial testing includes full blood count, immunoglobulins, and HIV exclusion.
✅ Functional immune assays guide the next steps in evaluating antibody, T-cell, neutrophil, and complement defects.
✅ Genetic testing is essential for severe PID cases and family screening.
✅ Ampath offers a next-generation sequencing (NGS) panel covering 99 PID-related genes.
✅ Genetic counselling is required before and after genetic testing.
📌 Early diagnosis and intervention in PID improve patient outcomes and reduce infection-related complications.