Analytic Methods and Techniques
Understand method selection, validation, and interference. Key concepts: analytic sensitivity (limit of detection and quantification), specificity, linearity, and interference from hemolysis, icterus, and lipemia. Know the difference between a method's measuring range and its reportable range with dilution.
Immunoassay formats: competitive assays (small analytes, labeled antigen competes) versus sandwich assays (large analytes, two antibodies). Cross-reactivity, heterophile antibodies, and biotin interference are common question traps. Enzyme kinetics (zero-order kinetics during the measurement phase) underlies many assays.
Chromatographic and mass spectrometric methods: retention time, internal standards (stable isotope-labeled), ionization suppression, and the importance of specificity. Molecular methods (PCR, sequencing) appear in infectious disease and genetics questions; understand amplification, false positives from contamination, and limits of detection.
Method comparison and validation: correlation and regression, bias, and total error. Understand that a statistically significant difference may not be clinically significant.
Worked example: a patient's TSH is normal but free T4 is elevated and the patient takes high-dose biotin. Biotin interferes with streptavidin-biotin immunoassays; repeat after withholding biotin.
Trap: assuming a low result is real when a hook effect or heterophile antibody could be responsible-dilution testing distinguishes them.