Determining the Cause

Chief Complaint: Fever

A. History: Subacute

Next Pivotal Assessment	Findings

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1. Clinical Diagnosis

Doctors may suspect influenza based on:

Symptoms:
- Sudden onset of fever
- Cough
- Sore throat
- Muscle aches
- Headache
- Fatigue
- Sometimes vomiting or diarrhea (more common in children)
Epidemiologic clues:
- Influenza season
- Local outbreaks
- Known contact with a confirmed flu case

In healthy, non-hospitalized patients during flu season, a clinical diagnosis alone is often sufficient, and treatment may be started without lab confirmation.

2. Laboratory Diagnosis

Lab testing is more often used for hospitalized patients, those with severe or atypical illness, or when confirmation is important for public health or infection control.

a. Rapid Influenza Diagnostic Tests (RIDTs)

Detect viral antigens
Results in 10–15 minutes
Moderate specificity, but low to moderate sensitivity (false negatives are common)
Often used in clinics and urgent care

b. Rapid Molecular Assays

Detect viral RNA (e.g., via isothermal amplification)
Results in 15–30 minutes
Higher sensitivity and specificity than RIDTs

c. Reverse Transcription Polymerase Chain Reaction (RT-PCR)

Gold standard test
Detects viral RNA
High sensitivity and specificity
Can subtype influenza (e.g., H1N1 vs. H3N2)
Takes several hours; requires a specialized lab

d. Viral Culture

Grows virus in cell culture
Takes 3–10 days
Not used for routine diagnosis, but may be used for research or surveillance

e. Direct Fluorescent Antibody (DFA) or Immunofluorescence

Detects viral antigens in respiratory specimens
Less commonly used now due to lower sensitivity and labor-intensive nature

Specimen Collection

Nasopharyngeal swab is the preferred sample for most tests
Alternatives: throat swabs, nasal swabs, or aspirates, depending on the test and patient

Summary Table

Method	Speed	Accuracy (Sens./Spec.)	Use Case
Clinical diagnosis	Immediate	Variable	Outpatient, typical flu season
RIDTs	10–15 min	Low–moderate	Quick screen; caution with negatives
Rapid molecular assay	~30 min	High	More reliable in urgent care
RT-PCR	Hours	Very high	Hospitalized, severe, or atypical cases
Viral culture	Days	Very high	Surveillance, not routine diagnosis

Would you like a flowchart or guideline example on when to test and treat?

associated localizing symptoms

2. Subacute Fever (1–3 weeks)

Causes are broader; both infections and non-infectious diseases should be considered.

Infections:
- Tuberculosis
- Infective endocarditis
- Abscesses (e.g., intra-abdominal, dental)
- Typhoid fever
Autoimmune diseases: Still’s disease, temporal arteritis
Malignancies: lymphomas
Drug fever: from new medications started days to weeks earlier

Clues: Persistent but often non-specific symptoms, waxing/waning fever

3. Chronic Fever (>3 weeks)

This is often classified as Fever of Unknown Origin (FUO) if no cause is found after an initial workup.

Infectious: tuberculosis, occult abscesses, HIV, endocarditis
Inflammatory/autoimmune: lupus, vasculitis, rheumatoid arthritis
Neoplastic: lymphoma, renal cell carcinoma
Miscellaneous: drug fever, factitious fever (self-induced), deep vein thrombosis, thyroiditis

Clues: Often requires advanced workup, including imaging and sometimes biopsy

Summary Table

Duration	Likely Causes
< 7 days	Viral or bacterial infections, acute inflammation
1–3 weeks	TB, abscess, endocarditis, autoimmune diseases
> 3 weeks	FUO: infection, malignancy, autoimmune, other rare

Let me know if you’d like to explore how to use fever pattern (intermittent, remittent, etc.) to aid diagnosis too.