Paper I
Question
Laboratory diagnosis of brucellosis
Answer
Brucellosis is a zoonotic infection caused by Brucella species (B. abortus from cattle, B. melitensis from goats/sheep, B. suis from pigs), acquired through contact with infected animals/animal products or ingestion of unpasteurized dairy products, presenting classically with undulant fever.
Laboratory diagnosis:
- Culture — blood culture is the definitive diagnostic method, but the organism is slow-growing and fastidious, requiring prolonged incubation (up to 4–6 weeks with conventional methods; automated continuous-monitoring blood culture systems, e.g., BACTEC, shorten detection time); culture requires biosafety level 3 precautions, since Brucella is highly infectious to laboratory personnel via aerosols.
- Serology — the mainstay of diagnosis in practice, given the difficulty of culture:
- Standard Agglutination Test (SAT/Brucella agglutination test) — demonstrates a rising or significantly elevated titre of agglutinating antibody against standardized Brucella antigen; a titre ≥1:160 (in an appropriate clinical context) is generally considered significant.
- Rose Bengal Plate Test — a rapid, simple slide agglutination screening test using stained Brucella antigen.
- ELISA — detects IgM (recent/acute infection) and IgG (past or chronic infection) antibody, useful for distinguishing acute from chronic/relapsed disease.
- Coombs’ (antiglobulin) test — used to detect incomplete/non-agglutinating antibodies, important in chronic brucellosis where blocking antibodies can cause false-negative standard agglutination results.
- Molecular methods — PCR, where available, offers rapid and sensitive detection without the biosafety concerns of culture.
- Bone marrow culture — sometimes more sensitive than blood culture, particularly in patients who have already received antibiotics.