Medical Microbiology and Infectious Disease Serology
Core concepts
Organise microbiology by specimen, organism and detection method. Know the Gram stain logic (morphology, arrangement, reaction), the culture media that select for specific pathogens (MacConkey, chocolate agar, mannitol salt, thiosulfate-citrate-bile-sucrose), and the identification tests (catalase, coagulase, oxidase, indole). Understand serological principles: sensitivity and specificity, positive and negative predictive values, the effect of prevalence, and the meaning of IgM versus IgG, avidity and the fourfold rise in paired sera.
Viral diagnostics require a grasp of antigen detection, nucleic acid amplification, and seroconversion windows. For hepatitis, map the serological panel: HBsAg, anti-HBc IgM, anti-HBs, HBeAg, and anti-HCV with confirmatory RNA. For HIV, know the screening/confirmatory algorithm and the window period. For tuberculosis, understand the role of acid-fast staining, culture, and interferon-gamma release assays.
Traps in questions
A high-sensitivity test in a low-prevalence population yields poor positive predictive value - the classic trap. IgM alone does not prove acute infection (persistence, cross-reactivity, rheumatoid factor interference). Anti-HBc IgM indicates acute or recent infection, whereas anti-HBs alone indicates immunity. Do not confuse colonisation with infection (e.g. MRSA on a nasal swab). Sterile-site isolates always require identification and susceptibility.
Worked example
A screening test with 95% sensitivity and 95% specificity applied to a population with 1% prevalence gives a positive predictive value near 16%. In a 30% prevalence population the same test gives a PPV near 89%. Always compute PPV from prevalence, not from sensitivity alone.
Rule of thumb
Interpret serology against the clinical syndrome and the pre-test probability; a single positive result rarely diagnoses infection.