Undifferentiated Fever and Fever of Unknown Origin — Written Clinical Reasoning

SAQ 1 — Integrated Diagnostic and Management Plan for Classic FUO in an Immunosuppressed Patient (20 marks, 30 minutes)

Prompt: Outline your integrated diagnostic and management plan for this patient with classic fever of unknown origin, including: (a) confirmation of the FUO definition and the leading diagnostic hypotheses; (b) the staged Tier 2 and Tier 3 investigation; (c) the role of the regional and exposure-specific serology; (d) the management of the adalimumab during the workup; (e) the threshold for empiric therapy; and (f) the high-yield exam traps in the immunosuppressed FUO patient. ^[1]

Model Answer

(a) Confirmation of the definition and the leading hypotheses (3 marks): ^[1]

This patient meets the modern (Durack and Street) definition of classic fever of unknown origin: temperature above 38.3 degrees Celsius on several occasions, duration of more than three weeks, and no diagnosis after a competent initial outpatient workup ^[2]. The leading diagnostic hypotheses, in order of clinical priority, are:

1. Lymphoma — the supraclavicular node, the B symptoms (drenching night sweats, weight loss), the splenomegaly, and the mild normocytic anaemia all point to a lymphoproliferative malignancy. The left supraclavicular (Virchow) node is a sentinel for upper abdominal and thoracic malignancy.
2. Tuberculosis reactivation — the prior treated TB, the adalimumab therapy (an anti-TNF biologic, the single most potent risk factor for TB reactivation in current practice), the recent travel to a TB-endemic region, and the apical scarring on the chest X-ray. The presentation may be extrapulmonary or miliary, with a normal chest X-ray.
3. Region-specific zoonotic infection — the rural New South Wales property with cattle and sheep (Q fever, brucellosis), the feral pig hunting (brucellosis, melioidosis in tropical areas, leptospirosis), and the travel to Vietnam (scrub typhus, melioidosis, enteric fever, malaria).
4. Adalimumab-related complication — drug fever is less common with biologics than with small molecules, but possible; the rare but recognised hepatosplenic T-cell lymphoma; and the broader opportunistic infection spectrum. ^[1]

(b) The staged Tier 2 and Tier 3 investigation (5 marks): ^[1]

The investigation proceeds in stages, each directed by the previous tier. The Tier 1 workup (full blood count, CRP, ESR, U&E, LFTs, blood cultures, urinalysis, CXR, HIV, hepatitis serology) is complete and non-diagnostic. The Tier 2 workup adds: ^[1]

• Directed imaging: CT of chest, abdomen and pelvis with contrast, to localise the supraclavicular node, the splenomegaly, any occult lymphadenopathy, and any focal infection. This is the central Tier 2 investigation and has displaced the older blind investigations from the diagnostic pathway ^[3].
• Autoimmune and vasculitic screen: ANA, ANCA, rheumatoid factor, anti-CCP, complement, immunoglobulins — to address the non-infectious inflammatory disease bucket, although his presentation points more to infection or malignancy.
• Tuberculosis workup: IGRA (interferon-gamma release assay) and three sputum samples for acid-fast bacilli and mycobacterial culture. The IGRA may be blunted by the adalimumab and a negative result does not exclude TB.
• Viral serology: EBV and CMV serology.
• Lymphoma markers: LDH, beta-2 microglobulin, serum protein electrophoresis, serum free light chains.
• Excisional biopsy of the supraclavicular node: preferred over core or fine-needle aspiration because the nodal architecture is often the diagnosis. The node is sent fresh for histology, immunohistochemistry, flow cytometry, and mycobacterial and fungal culture (given his TB and immunosuppression).

If the CT and the node biopsy are non-diagnostic, the Tier 3 workup adds: ^[1]

• FDG-PET-CT: the central Tier 3 investigation, localising metabolically active disease and directing the directed biopsy. The Dong meta-analysis reported a pooled FDG-PET-CT sensitivity of 0.982 and specificity of 0.859 in FUO ^[4]. The PET-CT identifies the highest-yield site for biopsy if the supraclavicular node is not diagnostic.
• Bone marrow biopsy: if haematological disease is suspected but imaging is non-diagnostic, or if miliary TB is suspected (the marrow may show granulomata).
• Liver biopsy: if the abnormal LFTs persist without another explanation, to assess for granulomatous disease, lymphoma, or miliary TB.

(c) The role of the regional and exposure-specific serology (3 marks): ^[1]

The exposure history is decisive and the serology panel is built around it: ^[1]

• Q fever (Coxiella burnetii) — phase I and phase II IgG and IgM, given the sheep and cattle exposure.
• Brucellosis (Brucella abortus and melitensis) — serology and blood culture on Castaneda medium, given the cattle and feral pig exposure.
• Melioidosis (Burkholderia pseudomallei) — serology and blood culture on Ashdown's medium, particularly given the tropical exposure potential (northern New South Wales and Vietnam).
• Scrub typhus (Orientia tsutsugamushi) — serology, given the travel to Vietnam and the eschar that may be overlooked.
• Leptospirosis — serology, given the rodent and fresh water exposure.
• Enteric fever (Salmonella typhi and paratyphi) — blood cultures and serology (Widal, though the latter has limited sensitivity), given the travel.
• Malaria — thick and thin blood films, given the travel to Vietnam (a malaria-endemic region, particularly for Plasmodium vivax). ^[1]

(d) Management of the adalimumab during the workup (2 marks): ^[1]

The adalimumab is held for the duration of the workup. The reasoning is threefold: (1) it may be contributing to the fever (drug fever is uncommon but possible); (2) it is the central risk factor for TB reactivation and opportunistic infection, and removing it may allow the immune system to express the infection more clearly; (3) it must not be resumed until the diagnosis is clear, because resuming an anti-TNF in the face of active TB or untreated lymphoma would be dangerous. The rheumatology team is involved early, and an alternative strategy for the psoriatic arthritis (such as NSAIDs or low-dose prednisolone) is considered only after the diagnosis is established, because empiric steroids would mask lymphoma and TB. ^[1]

(e) The threshold for empiric therapy (4 marks): ^[1]

Empiric therapy is discouraged as a diagnostic manoeuvre in the stable FUO patient, because it obscures the diagnosis — empiric antibiotics suppress the blood culture yield, empiric steroids mask lymphoma and reactivate TB. The exception is clinical deterioration, which justifies empiric therapy even before the definitive diagnosis is reached ^[5]. The thresholds are:

• New hypotension or rising lactate — empiric broad-spectrum antibiotics covering the most likely pathogens, after repeating the blood cultures. Given his exposures and his anti-TNF therapy, the cover includes TB (empirical quadruple therapy if TB is the leading hypothesis), melioidosis (meropenem or ceftazidime), and a broad-spectrum agent for healthcare-associated and nosocomial pathogens.
• A high-probability single diagnosis — if the clinical picture makes a single diagnosis highly probable (for example, miliary TB with characteristic imaging), targeted therapy may begin pending the definitive workup.
• Visual symptoms in suspected giant cell arteritis — high-dose glucocorticoids immediately, with temporal artery biopsy within 1 to 2 weeks. (Not directly relevant to this patient but a general principle.) ^[1]

The diagnostic workup (the node biopsy, the PET-CT, the serology) proceeds in parallel with the empiric therapy, because the deterioration does not abandon the diagnostic strategy. ^[1]

(f) High-yield exam traps in the immunosuppressed FUO patient (3 marks): ^[1]

The traps in the immunosuppressed FUO patient are: ^[1]

1. A negative IGRA does not exclude TB in the patient on anti-TNF therapy. The diagnosis requires sputum for AFB, biopsy of accessible tissue with mycobacterial culture, and a low threshold for empirical therapy in the deteriorating patient.
2. The blind bone marrow is lower-yield than the directed biopsy of a PET-avid lesion. In the patient with possible lymphoma and a non-diagnostic CT, the PET-CT identifies the highest-yield site for biopsy, and the blind bone marrow is reserved for when PET is not available or when miliary TB is suspected.
3. Empiric steroids are doubly dangerous in this patient because they mask lymphoma (which is high on the differential) and they reactivate TB (which is also high on the differential). The steroid trial as a diagnostic manoeuvre is reserved for the patient in whom vasculitis is the leading hypothesis and infection and malignancy have been excluded — not this patient.
4. The adalimumab must not be resumed until the diagnosis is clear. Resuming anti-TNF therapy in the face of active TB or untreated lymphoma would be dangerous, and the rheumatology team must be engaged in the decision.
5. The region-specific serology is non-negotiable in the patient with the exposure history. The standard "infection screen" will not detect Q fever, brucellosis, melioidosis, scrub typhus or leptospirosis, and these are common in the Australian rural population. ^[1]