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1️⃣ Infectious Disease Serology
| Test | Positive in Acute Infection | Positive in Chronic Infection | Negative in | Notes |
|---|---|---|---|---|
| HIV (ELISA, Western Blot, PCR) | HIV RNA, p24 antigen, IgM | HIV IgG | Uninfected individuals | HIV PCR is most sensitive in early infection |
| Hepatitis B (HBV Panel) | HBsAg, Anti-HBc IgM | Anti-HBc IgG, Anti-HBs (if recovered) | Non-exposed individuals | Anti-HBs alone = immunity from vaccine |
| Hepatitis C (HCV Antibody, HCV RNA) | HCV RNA | HCV RNA, HCV Ab | Non-exposed individuals | HCV RNA confirms active infection |
| Syphilis (RPR, TPHA, FTA-ABS) | RPR, TPHA (+), FTA-ABS (+) | TPHA (+), RPR varies | No prior infection | RPR may revert to negative after treatment |
| COVID-19 (PCR, IgM/IgG Antibodies) | PCR (+), IgM | IgG | No prior infection | PCR preferred for acute infection |
| Tuberculosis (Quantiferon Gold, TST) | NA | Positive in latent TB | No TB exposure | IGRA preferred in BCG-vaccinated individuals |
🔹 Key Takeaways:
- IgM antibodies indicate recent infection, while IgG antibodies suggest past exposure or immunity.
- PCR tests detect active infections early, especially in HIV, Hepatitis C, and COVID-19.
- A positive RPR in Syphilis requires confirmation with TPHA/FTA-ABS.
2️⃣ Autoimmune Disease Serology
| Autoimmune Disease | Key Autoantibodies | Sensitivity (%) | Specificity (%) | Notes |
|---|---|---|---|---|
| Systemic Lupus Erythematosus (SLE) | ANA, Anti-dsDNA, Anti-Smith | ANA: 98%, Anti-dsDNA: 70% | Anti-dsDNA: 95% | Anti-dsDNA correlates with disease activity |
| Rheumatoid Arthritis (RA) | RF, Anti-CCP | RF: 80%, Anti-CCP: 70% | Anti-CCP: 95% | Anti-CCP is more specific than RF |
| Sjögren’s Syndrome | Anti-SSA (Ro), Anti-SSB (La) | 60–80% | 90% | Anti-SSA/SSB present in dry eye, dry mouth syndrome |
| Scleroderma (Systemic Sclerosis) | Anti-Scl-70, Anti-centromere | 60% | 90% | Anti-Scl-70 = Diffuse Scleroderma, Anti-Centromere = Limited Scleroderma |
| Vasculitis (ANCA-Associated) | p-ANCA, c-ANCA | 75% | 90% | c-ANCA in Wegener’s, p-ANCA in microscopic polyangiitis |
🔹 Key Takeaways:
- ANA is highly sensitive for SLE but not specific.
- Anti-dsDNA is highly specific for SLE and correlates with disease activity.
- Anti-CCP is the best test for diagnosing RA due to high specificity.
3️⃣ Allergy & Hypersensitivity Testing
| Test | Findings | Clinical Relevance |
|---|---|---|
| Total IgE | Elevated in atopy, allergic asthma | High levels suggest allergic disorders |
| Specific IgE (RAST, ELISA) | Positive for specific allergens (e.g., peanuts, pollen) | Confirms Type 1 hypersensitivity reaction |
| Skin Prick Test | Immediate wheal & flare reaction | Identifies allergen triggers |
| Basophil Activation Test | Increased basophil response to allergens | Used for drug & venom allergy diagnosis |
🔹 Key Takeaways:
- Skin prick test is the best method for immediate hypersensitivity reactions.
- Elevated total IgE alone does not confirm allergy—specific IgE is required.
- RAST & ELISA tests measure allergen-specific IgE levels in blood.
4️⃣ Complement System & Immune Deficiency Tests
| Test | Findings | Clinical Relevance |
|---|---|---|
| C3, C4 Levels | Low in active SLE, immune complex diseases | Suggests complement consumption |
| CH50 Assay | Decreased in hereditary angioedema | Deficiency of C1 esterase inhibitor |
| CD4/CD8 Ratio | Low in HIV/AIDS (CD4 <200 = AIDS) | Used for staging HIV |
| Flow Cytometry | Abnormal T, B, or NK cell populations | Diagnostic for primary immune deficiencies |
🔹 Key Takeaways:
- Low complement levels suggest immune activation (e.g., SLE, sepsis).
- CD4 count is essential for monitoring HIV progression.
- Flow cytometry identifies immune deficiencies like SCID or CVID.
📝 Summary & Clinical Integration
| Category | Best Test for Diagnosis | Key Finding | Notes |
|---|---|---|---|
| Infectious Disease | PCR, Serology (IgM, IgG) | IgM = Acute, IgG = Chronic | PCR detects early infection |
| Autoimmune Disease | Autoantibodies (ANA, Anti-dsDNA, Anti-CCP) | Disease-specific autoantibodies | ANA alone is not diagnostic |
| Allergy & Hypersensitivity | Skin Prick Test, Specific IgE | IgE-mediated reactions | Total IgE alone is not enough |
| Immune Deficiency | Flow Cytometry, CD4/CD8 | Abnormal immune cell levels | Used in HIV, SCID, CVID |
🔍 Clinical Utility of Serology & Immunology Tests
✅ Differentiate between acute vs. chronic infections (IgM vs. IgG).
✅ Confirm autoimmune diseases using specific autoantibodies.
✅ Diagnose allergic reactions based on IgE and skin prick test results.
✅ Assess immune function in suspected immunodeficiency cases.