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Egyptian Board Clinical Pathology study materials
Egyptian Board Clinical Pathology study resources

Egyptian Board Clinical Pathology: Medical Microbiology and Infectious Disease Serology

Egyptian Board Clinical Pathology · Work through your exam, one topic at a time.

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Medical Microbiology and Infectious Disease Serology

Microbiology questions reward a specimen-to-result chain: was the right specimen taken at the right time, was it transported and plated properly, is the isolate an infection or a coloniser, and does the susceptibility result rest on a valid breakpoint?

Core rules

  • Blood cultures: take two to three sets before antibiotics; a single bottle is not a culture set.
  • Sputum is rejected for culture when Gram stain shows more than 10 squamous epithelial cells per low-power field, indicating oral contamination.
  • Urine culture: ≥10^5 CFU/mL in midstream urine suggests significant bacteriuria; ≥10^3 with pyuria and symptoms is meaningful; mixed growth suggests contamination.
  • Disk diffusion zones are read against a validated table; an MIC without a breakpoint is not an interpretation.
  • CLSI and EUCAST breakpoints are not interchangeable; state the standard and version used.
  • Serology: a positive IgM is not proof of acute infection (persistence, cross-reactivity, rheumatoid factor); a fourfold IgG rise or seroconversion is stronger evidence.
  • HIV: fourth-generation antigen/antibody screening, then a confirmatory assay (immunoblot or nucleic acid test); a single reactive screen is never reported as HIV-positive.
  • HBV: HBsAg positive with anti-HBc IgM indicates acute infection; HBsAg negative with anti-HBc and anti-HBs indicates resolved infection; isolated anti-HBc needs HBV DNA to exclude occult infection.

Worked example

A pregnant woman: HBsAg negative, total anti-HBc positive, anti-HBs positive, IgM anti-HBc negative. This is resolved past infection with immunity, not active infection and not a vaccination profile, since vaccination produces anti-HBs alone. No HBV DNA is required on this basis, and the neonate does not need immunoprophylaxis for this maternal pattern.

Traps

  • Reporting a positive Widal titre as typhoid; specificity is poor in endemic areas.
  • Treating coagulase-negative staphylococci in one of four bottles as bacteraemia.
  • Using serology alone to diagnose active tuberculosis or brucellosis.
  • Ignoring whether the specimen was taken before or after antibiotics.