Diagnosing Dengue: What to Ask and Assess

1. History Questions (Anamnesis)

Travel & Exposure History

• Has the patient been to or lives in a dengue-endemic region? (Asia, Pacific Islands, Africa, South/Central America, Caribbean, or locally - Texas/Florida in the US)
• What was the date of travel or potential mosquito exposure? (Incubation period is 4-10 days)
• Was there exposure to standing water, lack of screens, or crowded living conditions?
• Any known dengue outbreak in the area?

Vaccination / Prior Dengue

• Has the patient had dengue before? (Second infection with a different serotype = higher risk of severe disease)
• Has the patient received a dengue vaccine (Dengvaxia)? (Can affect serology interpretation)
• Previous flavivirus vaccines or infections? (Yellow fever, Japanese encephalitis, West Nile, Zika - can cause cross-reactive antibodies)

Symptom Timeline

2. Symptoms to Screen (WHO 2009 Classification)

Dengue Without Warning Signs

• Nausea or vomiting
• Skin rash
• Aches and pains (muscle, joint, bone - "breakbone fever")
• Leukopenia (lab finding)
• Positive tourniquet test

Warning Signs (Dengue With Warning Signs) - Ask Specifically

• Abdominal pain or tenderness (especially right upper quadrant - hepatomegaly)
• Persistent vomiting (>3 episodes in 24 hours)
• Mucosal bleeding - gum bleeding, nosebleeds, blood in stool/urine
• Lethargy or restlessness (altered sensorium)
• Difficulty breathing (pleural effusion)
• Rapid deterioration despite fever breaking

Severe Dengue Features - Red Flags

• Signs of shock: dizziness on standing, cold extremities, weak/rapid pulse, reduced urine output
• Significant bleeding: hematemesis, melena, menorrhagia
• Organ dysfunction: jaundice (hepatitis, AST/ALT ≥1000), confusion (encephalitis), chest tightness (cardiac involvement)

3. Physical Examination Checkpoints

• Fever (typically >38.5°C, sudden onset)
• Retro-orbital pain on eye movement (classic)
• Facial erythema / flushing, injected oropharynx
• Macular or maculopapular rash (may appear days 3-5)
• Petechiae (especially legs)
• Tourniquet test: Inflate BP cuff to midpoint of systolic and diastolic for 5 minutes - ≥10 petechiae per sq inch = positive (suggests thrombocytopenia/capillary fragility)
• Hepatomegaly (liver >2 cm below costal margin = warning sign)
• Signs of fluid leakage: ascites, pleural effusion, periorbital edema

4. Laboratory Investigations

• IgG persists for life; useful for confirming past infection
• IgM can cross-react with Zika, West Nile, yellow fever - interpret carefully

Mandatory CBC

• Leukopenia (low WBC - characteristic finding)
• Thrombocytopenia (falling platelet count is a key warning sign)
• Rising hematocrit/hemoconcentration (signals plasma leakage - hallmark of critical phase)

Additional Labs Based on Severity

• LFTs (AST/ALT ≥1000 = severe dengue)
• Urea/creatinine (renal involvement)
• Coagulation profile if bleeding is present
• Chest X-ray / ultrasound to detect pleural effusion or ascites

5. Differential Diagnosis - Ask to Exclude

• Malaria (travel, cyclical fever, blood smear)
• Chikungunya (similar presentation - fever, rash, but SEVERE joint pain predominates)
• Zika virus (milder, rash prominent, neurological complications, sexual/vertical transmission risk)
• Typhoid fever (relative bradycardia, abdominal symptoms)
• Influenza / COVID-19 (respiratory predominance)
• Leptospirosis (water/animal exposure, jaundice, renal failure)
• Rickettsial disease (tick exposure, specific rash pattern)

History Taking for Suspected Dengue

Presenting Complaint

• "What brought you in today?" - establish the chief complaint
• "When exactly did the fever start?" - note the precise day of illness (critical for test timing)

History of Presenting Illness

Fever

• When did the fever start? How high has it been?
• Is it continuous or does it come and go?
• Did it break recently? (defervescence = entry into the dangerous critical phase)

Pain

• Do you have a headache? Where exactly? (retro-orbital pain = classic)
• Pain behind your eyes, especially when moving them?
• Severe muscle pain? Joint pain? Bone pain? ("breakbone fever" - very characteristic)
• Abdominal pain? Where? Is it tender? (warning sign)

Rash

• Have you noticed any skin changes or rash?
• When did it appear relative to the fever?

Bleeding

• Any bleeding from the gums or nose?
• Blood in urine, vomit, or stools?
• In females: unusually heavy or early menstrual bleeding?

GI Symptoms

• Nausea or vomiting? How many times a day? (persistent vomiting = warning sign)
• Any diarrhea?

Neurological / General

• Feeling unusually drowsy, confused, or restless? (warning sign)
• Fits or seizures?

Respiratory / Fluid

• Difficulty breathing or chest pain? (pleural effusion)
• Swelling of the abdomen or feet? (ascites, edema - plasma leakage)

Urinary Output

• Have you been passing urine normally? (reduced output = shock warning)

Past History

• Have you had dengue before? When? (prior infection = higher risk of severe disease)
• Any chronic illnesses - diabetes, hypertension, heart disease, asthma, sickle cell? (increase risk of severe dengue)
• Peptic ulcer or bleeding disorders? (risk of hemorrhage)

Drug History

• Are you taking any painkillers? Specifically aspirin or ibuprofen? (must stop - increase bleeding risk)
• Any anticoagulants (warfarin, heparin)?
• Any traditional/herbal remedies taken?

Travel & Exposure History

• Where do you live? Any recent travel?
• Have you been to a dengue-endemic area? (Asia, Africa, Caribbean, Latin America, Pacific)
• When did you return? (incubation is 4-10 days)
• Are there mosquitoes around your home? Standing water nearby?
• Do you know of others around you with similar illness? (cluster = local outbreak)

Vaccination History

• Have you received the dengue vaccine (Dengvaxia)?
• Yellow fever, Japanese encephalitis, or Zika-related vaccinations? (affects serology interpretation)

Quick Summary of Warning Signs to Screen in History

ABG Normal Values

Note: PaO2 decreases with age. Estimate by: PaO2 = 109 - 0.43 × (age in years). Also decreases with altitude (by ~20 mmHg at 1500 m elevation).

Quick Interpretation: What Abnormal Means

5-Step Interpretation Approach

• pH ↓ + PaCO2 ↑ = Respiratory acidosis
• pH ↓ + HCO3- ↓ = Metabolic acidosis
• pH ↑ + PaCO2 ↓ = Respiratory alkalosis
• pH ↑ + HCO3- ↑ = Metabolic alkalosis

Compensation Formulas