Immunology / Serology
Immunology / Serology
This domain tests immunoassay design, antibody-antigen reaction principles, and the statistical meaning of a serologic result - why a positive test is not the same as a diagnosis.
Core rules
- Sensitivity = TP/(TP + FN); specificity = TN/(TN + FP). Positive predictive value depends on prevalence; it falls sharply when a test is applied to a low-prevalence population.
- Sandwich ELISA detects antigen; indirect ELISA detects antibody; competitive ELISA suits small molecules. Enzyme conjugate plus substrate generates the signal.
- Prozone (antibody excess) causes false-negative agglutination; postzone (antigen excess) does the same. Dilute and repeat.
- IFA titer is the reciprocal of the highest dilution showing the characteristic pattern; a fourfold rise between acute and convalescent sera supports recent infection.
- Complement fixation consumes complement only when specific antibody binds antigen; residual hemolysis indicates a negative result.
Worked example
Screening ELISA: sensitivity 99%, specificity 99%, prevalence 1%. In 10,000 people, 100 are infected and 99 are detected. Of 9,900 uninfected, 99 are false positives. PPV = 99/(99 + 99) = 50%. The same assay at 20% prevalence gives PPV ≈ 96%. This is why confirmatory testing (e.g., Western blot or NAT) follows a reactive screen.
Traps that cost marks
- Quoting sensitivity as if it were predictive value.
- Missing prozone in a patient with very high antibody titers (e.g., undiluted sera in syphilis testing).
- Reading a single IgG titer as proof of acute infection.
- Ignoring cross-reactivity and heterophile antibodies when interpreting a positive result.