Serological Techniques and Immunodiagnosis
Introduction
This domain covers agglutination, precipitation, ELISA, lateral flow, immunofluorescence, Western blot, and complement fixation, plus test performance. The exam tests when a test is screening or confirmatory and how sensitivity, specificity, and predictive values differ.
Core rules
- Agglutination detects clumping of particulate antigen; precipitation detects soluble antigen-antibody complexes.
- ELISA can be direct, indirect, sandwich, or competitive; enzyme label converts substrate to a measurable signal.
- Lateral-flow tests are rapid and field-friendly but need confirmation when used for screening.
- Sensitivity is the ability to detect true positives; specificity is the ability to detect true negatives.
- Positive predictive value depends on prevalence: PPV = true positives / (true positives + false positives).
Worked example
A test has sensitivity 95%, specificity 90%, and prevalence 10% in 1000 people. True positives = 0.95 × 100 = 95; false positives = 0.10 × 900 = 90. PPV = 95/(95+90) = 51.4%. So a positive result is still a coin toss in this low-prevalence group, and a confirmatory test is needed.
Traps
Do not confuse sensitivity with PPV. Very high antigen levels can cause a hook effect and false-negative sandwich ELISA. IgM detection is useful early but may be negative in reinfection or delayed sampling. Always include controls and follow the stated cut-off.