Fever of Unknown Origin (FUO) - Next Investigative Steps

Step 1 - Complete the Core Baseline Labs (if not yet done)

Step 2 - Autoimmune / Inflammatory Panel

• ANA (Antinuclear Antibody) - screen for SLE (positive in >95% of SLE cases)
• Anti-dsDNA + Anti-Sm - SLE-specific antibodies
• Complement levels (C3, C4, CH50) - low in active SLE
• ANCA (p-ANCA, c-ANCA) - vasculitis workup
• Anti-CCP - more specific for RA than RF alone
• Serum immunoglobulins + SPEP - monoclonal gammopathy, hypergammaglobulinemia
• Creatine kinase (CK) - inflammatory myopathy

Key: SLE peaks in young females aged 15-40. AOSD (Adult-onset Still's disease) has a bimodal peak at ages 15-25, making it highly relevant here. AOSD presents with high quotidian fever, salmon-colored evanescent rash, arthritis/arthralgia, and dramatic ferritin elevation. - Frameworks for Internal Medicine

Step 3 - Infectious Workup (Second-Tier)

Step 4 - Imaging Studies

• Chest X-ray - First line; look for hilar lymphadenopathy (sarcoidosis, lymphoma, TB), infiltrates
• CT chest/abdomen/pelvis with contrast - Most valuable imaging for FUO; detects lymphadenopathy, occult abscesses, hepatosplenomegaly, masses
• Echocardiography (TTE) - If blood cultures are pending or murmur exists; rule out infective endocarditis
• Abdominal ultrasound - Hepatosplenomegaly, lymphadenopathy, abscess

CT scanning has significantly reduced the detection time for intra-abdominal causes of FUO (abscesses, lymphoma). - Goldman-Cecil Medicine

Step 5 - Nuclear Medicine / Advanced Imaging

• FDG-PET/CT - Highly sensitive; identifies occult infection foci, lymphoma, vasculitis, and inflammatory foci. Particularly useful when conventional workup is negative. - Grainger & Allison's Diagnostic Radiology
• Gallium-67 or labeled leukocyte scintigraphy - For occult infection/inflammation

Step 6 - Tissue Biopsy / Invasive Testing

• Bone marrow biopsy + culture - Mandatory if hematologic malignancy suspected, or when lymphoma/leukemia cannot be excluded; also reveals granulomas (TB, histoplasmosis, sarcoidosis)
• Lymph node biopsy - If lymphadenopathy detected
• Liver biopsy - If hepatomegaly with enzyme elevation; granulomatous hepatitis
• Lumbar puncture - If any neurologic signs are present

Specific Diagnoses to Prioritize in a 17-Year-Old Female with FUO

1. SLE - Young female, multisystem disease; start with ANA
2. Adult-onset Still's disease (AOSD) - Peak at age 15-25; check serum ferritin
3. Lymphoma (Hodgkin's) - Peak in adolescents; check LDH, PET/CT, lymph node exam
4. Infectious mononucleosis (EBV/CMV) - Even with normal CBC, serology may be positive
5. Extrapulmonary TB - Especially in endemic areas; Quantiferon gold
6. Inflammatory bowel disease (IBD) - May present with fever alone; colonoscopy if GI symptoms
7. Familial Mediterranean fever (FMF) - If recurrent episodic pattern and Mediterranean heritage; genetic testing (MEFV gene)
8. Drug fever - Review all medications, including over-the-counter and herbal

Summary Algorithm

Negative CBC, ESR, RF, Viral Panel, Urea, Procalcitonin
            ↓
1. Complete core labs: Ferritin, LDH, ALP, CRP, TFTs, Ca, Uric acid
2. Autoimmune panel: ANA, anti-dsDNA, C3/C4, ANCA, anti-CCP
3. Targeted serology: EBV, CMV, HIV, Toxoplasma, Quantiferon-TB
4. Blood cultures x3
5. Imaging: CXR → CT chest/abdomen/pelvis → Echo
6. If still undiagnosed: FDG-PET/CT
7. Invasive: Bone marrow biopsy, lymph node biopsy

• Harrison's Principles of Internal Medicine, 22E - Chapter 22 (Fever of Unknown Origin)
• Frameworks for Internal Medicine - FUO Chapter
• Goldman-Cecil Medicine - Chapter 259
• StatPearls FUO Workup - NIH

FUO - Next Steps When All Basic Labs Are Negative

Tier 1 - Critical Labs Still Outstanding

Normal CRP and ESR do NOT reliably exclude malignancy or autoinflammatory disorders. In AOSD specifically, ferritin is the key marker. - Frameworks for Internal Medicine

Tier 2 - Targeted Infectious Serology

Microbiologic serology should be targeted by history - travel, animal contact, sexual history. Do not "shotgun" serology without clinical direction. - Harrison's, 22E

Tier 3 - Imaging (Begin in Parallel)

3A - Chest X-ray (if not done)

• Hilar lymphadenopathy → sarcoidosis, lymphoma, TB
• Pulmonary infiltrates → TB, histoplasmosis, lymphoma

3B - CT Chest / Abdomen / Pelvis with Contrast

• The single highest-yield imaging in FUO
• Detects lymphadenopathy, hepatosplenomegaly, occult abscesses, masses, mediastinal disease
• Look specifically for: mediastinal/retroperitoneal lymph nodes (lymphoma), splenic lesions, sinusitis (skull base CT in adolescents)

3C - Echocardiography (TTE)

• Rule out infective endocarditis (vegetation may be present with negative blood cultures if prior antibiotics were used, or if fastidious organisms)

3D - Abdominal Ultrasound

• Hepatosplenomegaly, lymphadenopathy, pelvic pathology in young female (pelvic inflammatory disease, adnexitis, ovarian pathology)

In adolescents, sinus CT should always be considered as occult sinusitis is a relatively common but easily missed cause of FUO in this age group. - Pediatría Integral FUO guidelines

Tier 4 - Advanced Imaging (If CT is Non-Diagnostic)

• Sensitivity for malignancy in FUO: ~100%
• Sensitivity for NIID (vasculitis, sarcoidosis, AOSD): ~100%
• Overall diagnostic accuracy in FUO: 87.5% in published series
• Identifies the correct tissue to biopsy, avoiding blind biopsies
• Should be performed when CRP or ESR are elevated to maximize yield; even a negative PET/CT is diagnostically useful

FDG-PET/CT has replaced Gallium-67 as the gold-standard nuclear medicine investigation for FUO. - Grainger & Allison's Diagnostic Radiology; Rheumatology, 2-Volume Set (Elsevier 2022)

Tier 5 - Invasive Investigations

The diagnostic yield of bone marrow biopsy, liver biopsy, and colonoscopy is low in the absence of pointing diagnostic clues (PDCs). Order them when imaging or labs point toward a site. - Harrison's Principles, 22E

Priority Diagnoses for a 17-Year-Old Female with FUO and All Basic Labs Negative

Practical Decision Algorithm

All basic labs negative (CBC, ESR, CRP, RF, viral panel, urea, procalcitonin, LFTs, RFTs)
        ↓
STEP 1: Ferritin, LDH, ANA, anti-dsDNA, C3/C4, ANCA, TSH, SPEP, peripheral smear
        ↓
STEP 2: EBV/CMV IgM, HIV Ag/Ab, Quantiferon-TB Gold, Blood cultures x3,
        Toxoplasma, Bartonella, Parvovirus B19
        ↓
STEP 3: Imaging - CXR + CT chest/abdomen/pelvis + Echo + Pelvic US
        ↓ (if still negative)
STEP 4: FDG-PET/CT (most powerful single test at this stage)
        ↓ (if still negative or positive with target site)
STEP 5: Bone marrow biopsy ± Lymph node biopsy ± Liver biopsy

• Harrison's Principles of Internal Medicine, 22E - FUO Chapter
• Frameworks for Internal Medicine - FUO section
• Grainger & Allison's Diagnostic Radiology - Nuclear Medicine
• StatPearls: Fever of Unknown Origin
• Pediatric FUO guidelines - Pediatría Integral 2023

Low Cortisol + Low ACTH: Diagnosis & Treatment

Understanding the Pattern First

• Secondary AI: Pituitary failure → ACTH deficiency → adrenal atrophy
• Tertiary AI: Hypothalamic failure → CRH deficiency → low ACTH → low cortisol (most common cause: prolonged exogenous glucocorticoid use)

Step 1 - Confirm and Investigate the Cause

Inappropriately low ACTH in the setting of low cortisol is characteristic of diminished ACTH reserve. Pituitary MRI and evaluation for loss of other pituitary hormones is mandatory. - Harrison's Principles, 22E; Symptom to Diagnosis, 4th Ed.

Step 2 - Treatment Overview

A. Chronic Maintenance (Stable Outpatient)

NICE 2024 Guideline: For secondary/tertiary AI, hydrocortisone 15-25 mg/day orally in 2-3 divided doses is the first-line recommendation. - NICE 2024 Adrenal Insufficiency Guidelines

• Traditional doses were 20-30 mg/day but most current guidelines recommend the lower range of 15-20 mg/day to avoid iatrogenic Cushing's. - Goodman & Gilman's
• Dexamethasone is NOT used for chronic maintenance - dose titration is difficult and cushingoid effects are pronounced. - Tintinalli's Emergency Medicine

• Consider DHEA 50 mg/day for women with primary AI and persistent impaired wellbeing or decreased libido despite glucocorticoid replacement.
• In secondary AI, benefit is less established but can be tried in symptomatic women. - Symptom to Diagnosis, 4th Ed.

B. Sick Day Rules / Stress Dosing (Critical Education Point)

Patients and family members should be trained to administer parenteral hydrocortisone or dexamethasone 4 mg IM when vomiting prevents oral intake, then seek immediate medical attention. All patients should wear a medical alert bracelet. - Goodman & Gilman's

C. Adrenal Crisis - Emergency Treatment

1. Hydrocortisone 100 mg IV bolus immediately - do NOT wait for lab results
2. IV Normal Saline - often 1 L/hour initially for volume replacement
3. Hydrocortisone 50-200 mg/day by continuous IV infusion or 50 mg IV every 6 hours
4. Glucose - IV dextrose if hypoglycemic
5. Monitor and correct electrolytes (hyponatremia is common)
6. Identify and treat the precipitating cause (infection, trauma, surgery)

Treatment of adrenal crisis must begin before diagnostic tests are completed when the diagnosis is strongly suspected. - Rosen's Emergency Medicine

D. Important Co-management Points

• Do NOT use ACTH level to monitor adequacy in secondary AI - it is suppressed by design
• Clinical assessment: weight, BP, wellbeing, signs of Cushing's (moon face, striae, weight gain)
• Aim for the lowest effective dose that abolishes symptoms

Summary Table

• Goodman & Gilman's Pharmacological Basis of Therapeutics
• Harrison's Principles of Internal Medicine, 22E
• Symptom to Diagnosis, 4th Ed.
• Harriet Lane Handbook, 23rd Ed.
• Tintinalli's Emergency Medicine
• NICE 2024 Adrenal Insufficiency Guideline

Hydrocortisone vs. Fludrocortisone in Secondary/Tertiary AI

The Short Answer

Why Fludrocortisone is NOT Given

"Secondary adrenal insufficiency only results in glucocorticoid deficiency, as mineralocorticoid activity is primarily regulated by the renin-angiotensin-aldosterone system (RAAS) rather than ACTH." - Miller's Anesthesia, 10th Ed.

"In secondary adrenal insufficiency, the administration of a glucocorticoid alone is generally adequate because mineralocorticoid synthesis in the zona glomerulosa usually remains intact." - Goodman & Gilman's Pharmacological Basis of Therapeutics

"Replacement therapy for secondary or tertiary adrenal insufficiency: Hydrocortisone is used for treatment of these deficiencies." - Lippincott Illustrated Reviews: Pharmacology

What This Means Clinically

• No salt wasting
• No hyperkalemia (hyperkalemia is a feature of PRIMARY AI only)
• No postural hypotension from mineralocorticoid deficiency
• Hyponatremia can still occur - but this is from loss of glucocorticoid effect on free-water clearance (cortisol normally suppresses ADH), not from aldosterone deficiency

"In secondary adrenal insufficiency, mineralocorticoid production is maintained by the RAS. Thus, hyperkalemia is absent, but hyponatremia may occur as a result of loss of glucocorticoid effect on free-water clearance." - Textbook of Family Medicine, 9th Ed.

Contrast with Primary AI (Addison's Disease)

Treatment Summary for Secondary/Tertiary AI

• Hydrocortisone 15-25 mg/day orally in 2-3 divided doses (largest dose in the morning to mimic diurnal rhythm)
• Double/triple the dose during illness, surgery, or physical stress
• Carry an IM hydrocortisone 100 mg emergency kit

• Fludrocortisone (mineralocorticoid replacement is unnecessary)