IM4.{13,15-17} | Febrile Syndrome Investigations — Summary & Reflection

KEY TAKEAWAYS

IM-skills arc — investigations for febrile syndromes:

Step 1 (indication): every test must answer a specific question from the differential; imaging indications: localise focus (CXR/USS abdomen), characterise organomegaly/lymphadenopathy (CT), endocarditis (echo), guide biopsy, monitor treatment.

Step 2 (governing principles): pre-test probability × test sensitivity/specificity = post-test probability; CBC patterns: leucocytosis + neutrophilia = bacterial; leucopaenia + lymphocytosis = viral/TB; thrombocytopaenia = dengue/malaria/DIC/hypersplenism.

Step 3 (procedure): blood culture — 3 sets, 10 mL/bottle, separate sites, 60 mL total; Mantoux — intradermal 0.1 mL PPD, read induration at 72 hours; bone marrow — PSIS, Jamshidi needle, aspirate 0.5–2 mL; CSF — after fundoscopy excluded papilloedema.

Step 4 (interpretation): P. falciparum = multiple rings per RBC + banana gametocytes; Mantoux ≥10 mm (general) or ≥5 mm (immunosuppressed); bacterial meningitis CSF = turbid, neutrophils >80%, glucose <40 mg/dL; miliary TB Mantoux often falsely negative due to anergy.

Step 5 (applied): kala-azar = bone marrow Leishman-Donovan bodies; miliary TB = negative Mantoux ≠ no TB; bacterial meningitis with Gram-negative diplococci = meningococcus → IV ceftriaxone + dexamethasone.

Step 6 (self-assessment): blood culture volume > timing; Mantoux read induration not erythema; lymphoma workup = CECT + lymph node biopsy; bloody diarrhoea workup = stool microscopy + culture + E. histolytica antigen.

REFLECT

Recall Scenario B in the Applied Practice section: a patient with miliary TB whose Mantoux test was 0 mm — a false-negative from anergy. How does this case reshape your understanding of the Mantoux test as a tool to 'exclude' TB? Think about the principle that every investigation has a false-negative rate that depends on the clinical context. How will you apply this principle when a colleague says 'Mantoux is negative, so TB is excluded'? What alternative investigation would you recommend in an immunosuppressed patient where the Mantoux is likely to be falsely negative?