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Characteristic Fevers: CBC Findings, Diagnosis & Clinical Features

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Quick Reference Table

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1. Typhoid (Enteric) Fever

Fever Pattern

• Classic stepladder fever: temperature rises ~1°C per day over the first week, reaching 39-40°C, then plateaus as a continuous fever during weeks 2-3
• Relative bradycardia (Faget's sign) - pulse does not rise proportionally with fever; a strong diagnostic clue

CBC Findings

• Leukopenia (WBC often 3,000-4,000/µL) with relative neutropenia
• Thrombocytopenia (mild to moderate)
• Normocytic anemia (from bone marrow suppression)
• Eosinopenia (disappearance of eosinophils is a useful clue)

Other Diagnostic Clues

• Rose spots: faint, salmon-colored maculopapular rash on trunk/chest; appear in ~30% of patients at end of week 1; fade in 2-5 days
• Hepatosplenomegaly (soft, tender)
• Relative constipation in week 1, followed by "pea soup" diarrhea in week 2-3
• Elevated liver enzymes (hepatitis pattern)
• Mental status changes: "muttering delirium" or "coma vigil" in severe cases (neuropsychiatric typhoid)
• Late complications (week 3-4): intestinal perforation (1%), GI bleeding (6%)

Diagnosis

• Gold standard: Blood culture (sensitivity 40-60%), Bone marrow culture (80% sensitive, not reduced by prior antibiotics)
• Widal test: historically used but low specificity
• Rapid tests (Typhidot, Tubex): sensitivity ~70-80%, specificity ~80-90%
• Enteric fever must be considered in any febrile traveler from South Asia, Southeast Asia, or Africa - Harrison's 22E

Differential

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2. Malaria

Fever Pattern

• Plasmodium vivax / ovale: Tertian fever - paroxysms every 48 hours (classic "cold-hot-sweating" cycle)
• Plasmodium malariae: Quartan fever - paroxysms every 72 hours
• P. falciparum: Irregular, continuous or remittent (most dangerous; no classic periodicity early)

CBC Findings

• Anemia (hemolytic - normocytic/normochromic)
• Thrombocytopenia (common, often <100,000/µL)
• WBC usually normal or low (leukocytosis suggests complication/co-infection)
• Blood smear: ring forms, trophozoites, schizonts (definitive diagnosis)

Other Diagnostic Clues

• Paroxysm triad: rigors → high fever → drenching sweats (classic)
• Splenomegaly (progressive in chronic/recurrent malaria)
• Jaundice (hemolytic)
• Elevated LDH, indirect bilirubin
• Sickle cell trait is protective against P. falciparum
• Severe malaria (P. falciparum): cerebral malaria, pulmonary edema, renal failure, hypoglycemia, parasitemia >5%

Diagnosis

• Peripheral blood smear (thick and thin) - gold standard; repeat every 8-12h x3 if negative
• Rapid diagnostic test (RDT): detects HRP-2 antigen (P. falciparum) or LDH
• PCR: highest sensitivity, used for species confirmation

Differential

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3. Dengue Fever

Fever Pattern

• Biphasic / "saddle-back" fever: fever rises abruptly to 39-40°C for 2-7 days (febrile phase), then defervesces briefly, followed by a second peak
• The critical phase occurs around defervescence (days 3-7): plasma leakage, hemorrhage risk

CBC Findings

• Leukopenia (WBC often 2,000-4,000/µL) - present in febrile phase
• Thrombocytopenia (platelets <100,000 in ~50% of patients; <20,000 in severe DHF)
• Rising hematocrit (≥20% increase) = hemoconcentration from plasma leakage; warning sign of dengue hemorrhagic fever (DHF)
• Mildly elevated liver transaminases (~3x upper limit of normal)

Other Diagnostic Clues

• "Breakbone fever": intense myalgia, arthralgia, retro-orbital pain
• Maculopapular rash (50-80%) - appears at defervescence
• Positive tourniquet test (Rumpel-Leede test): >20 petechiae/square inch
• Hepatosplenomegaly (mild)
• DHF warning signs: abdominal pain, persistent vomiting, mucosal bleeding, rapid breathing, fatigue/restlessness
• ADE (antibody-dependent enhancement): secondary infection with a different serotype → higher risk of severe dengue

Diagnosis

• NS1 antigen: positive days 1-5 (early phase)
• IgM/IgG serology: IgM appears from day 4-5
• PCR: most sensitive in first 5 days

Differential

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4. Infectious Mononucleosis (EBV)

Fever Pattern

• Prolonged, low-to-moderate grade fever (38-39°C) for 1-3 weeks, often with exacerbation in the evening

CBC Findings

• Lymphocytosis: total WBC often elevated (10,000-20,000/µL) with >50% lymphocytes
• Atypical lymphocytes >10% (Downey cells / reactive T-cells) on peripheral smear - hallmark finding
• Mild thrombocytopenia (in some)
• Mild hemolytic anemia (in some)

Other Diagnostic Clues (from Lee's Otolaryngology & Sherris Microbiology)

• Classic triad: fever + pharyngitis (with grayish-white tonsillar exudate) + lymphadenopathy (posterior cervical)
• Palatal petechiae at the hard-soft palate junction
• Hepatosplenomegaly (spleen rupture risk with contact sports!)
• Amoxicillin rash: maculopapular rash if amoxicillin/ampicillin is given (90% of patients with IM)
• Elevated AST/ALT (hepatitis in ~80%)
• Associated with African Burkitt lymphoma, nasopharyngeal carcinoma, Hodgkin lymphoma (long-term)

Diagnosis

• Monospot test (heterophile antibodies): quick, but negative in children <4y and early disease
• EBV-specific antibodies: VCA IgM (acute), VCA IgG, EA-D (early antigen), EBNA (appears late)
• Peripheral smear showing atypical lymphocytes

Differential

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5. Rickettsial Fever (Rocky Mountain Spotted Fever / Ehrlichiosis)

Fever Pattern

• Abrupt-onset continuous high fever (39-40°C), often with headache and myalgia preceding rash

CBC Findings (Harrison's 22E)

• Leukopenia (especially in Ehrlichia/Anaplasma)
• Thrombocytopenia (common in RMSF and ehrlichiosis)
• Anemia
• Elevated liver enzymes (AST/ALT), hyponatremia

Other Diagnostic Clues

• Petechial/maculopapular rash: classically starts on wrists and ankles → spreads centripetally to trunk ("wrist-to-ankle-to-trunk"); involves palms and soles (distinguishes from most viral rashes)
• Tick bite history (often not recalled)
• Rash may be absent in 10-15% ("spotless RMSF") - high mortality risk if missed
• Eschar at tick bite site (in spotted fever group rickettsiae like R. africae)
• Mental status changes, seizures in severe disease

Diagnosis

• Serologic testing (IFA): diagnostic but retrospective (rises after 7-10 days)
• Treat empirically with doxycycline without waiting for confirmation
• Skin biopsy with immunofluorescence (early rapid test)

Differential

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6. Visceral Leishmaniasis (Kala-Azar)

Fever Pattern

• Double quotidian fever: two fever spikes per day (pathognomonic pattern)
• Prolonged fever (weeks to months)

CBC Findings (Frameworks for Internal Medicine)

• Pancytopenia (all 3 cell lines): anemia + leukopenia + thrombocytopenia - due to hypersplenism and bone marrow infiltration
• Elevated ESR, hypergammaglobulinemia
• Hypoalbuminemia

Other Diagnostic Clues

• Massive splenomegaly (often extends to umbilicus or beyond) - most prominent physical finding
• Progressive hepatomegaly (milder than spleen)
• Weight loss, cachexia over months
• Darkening of skin ("kala-azar" = "black fever" in Hindi)
• Endemic regions: Indian subcontinent, East Africa, Brazil, Mediterranean
• Diffuse lymphadenopathy

Diagnosis

• Bone marrow / splenic aspirate: Leishman-Donovan bodies (amastigotes in macrophages) - gold standard
• rK39 rapid test: highly sensitive/specific in endemic areas
• PCR

Differential

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7. Brucellosis

Fever Pattern

• Undulant fever: characteristic wave-like pattern with fever rising over several days, plateauing, then falling back to normal before recurring - may persist for weeks to months

CBC Findings

• Leukopenia with relative lymphocytosis
• Thrombocytopenia (in ~50%)
• Normocytic anemia

Other Diagnostic Clues

• Occupational exposure history: farmers, veterinarians, abattoir workers, laboratory personnel
• Ingestion of unpasteurized dairy
• Hepatosplenomegaly
• Sacroiliitis / spondylitis (back pain) - most common focal complication
• Orchitis/epididymitis in males
• Neurobrucellosis: meningitis, encephalitis
• Endocarditis (rare but most fatal form)

Diagnosis

• Blood/bone marrow culture (gold standard; takes 2-4 weeks)
• Serology: Standard Agglutination Test (SAT) - titer ≥1:160 significant
• Rose Bengal test: rapid screening

Differential

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8. Relapsing Fever

Fever Pattern

• Episodic: fever for 3-6 days → afebrile for ~7 days → fever returns
• Louse-borne (B. recurrentis): 1-2 relapses; tick-borne (B. hermsii): 3-10 relapses

CBC Findings

• Leukocytosis (distinguishes from most tropical fevers)
• Thrombocytopenia (severe, hemorrhagic risk)
• Normocytic anemia
• Spirochetes visible on peripheral blood smear during febrile episodes

Other Diagnostic Clues

• Jarisch-Herxheimer reaction upon treatment initiation (rigor, hypotension, fever spike)
• Jaundice (hepatic involvement)
• Petechiae, epistaxis (thrombocytopenia)
• Associated with louse infestation (louse-borne) or tick exposure (tick-borne)

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Summary: "Fever + CBC" Diagnostic Quick Clues

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Key Bedside Clues That Narrow Diagnosis

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