Severe Acute Respiratory Syndrome (SARS)

WHO Case Definition for SARS

The World Health Organization established a tiered surveillance case definition during the 2003 outbreak. This classification is a testable framework because vignettes may present incomplete data and ask you to categorize the patient.

Note the WHO definition is hierarchical. A suspect case becomes probable based on imaging or autopsy. A confirmed case requires laboratory evidence. Many exam questions test whether you can distinguish a "probable" from a "confirmed" case when the vignette gives you only radiology without lab confirmation.

01Pathophysiology

SARS-CoV is an enveloped, positive-sense, single-stranded RNA virus belonging to the genus Betacoronavirus. The virus enters the human respiratory tract via respiratory droplets and close-contact transmission. The critical molecular event is the binding of the viral spike (S) glycoprotein to the angiotensin-converting enzyme 2 (ACE2) receptor, which is expressed abundantly on type II pneumocytes in the lower respiratory tract. This receptor tropism explains why SARS predominantly causes lower respiratory disease rather than upper airway symptoms. The relative scarcity of ACE2 receptors in the nasopharynx is the reason patients typically do not present with rhinorrhea or pharyngitis, which is a classic distinguishing feature from common upper respiratory infections.

After viral entry and replication within the alveolar epithelium, the host immune response triggers a cascade of proinflammatory cytokines, including IL-1, IL-6, IL-8, TNF-alpha, and interferon-gamma. This cytokine storm leads to recruitment of neutrophils and monocytes into the alveolar space, causing diffuse alveolar damage (DAD). Histologically, the lungs show hyaline membrane formation, desquamation of pneumocytes, and interstitial edema. This is the pathologic correlate of the ground-glass opacities seen on chest imaging and the progressive hypoxemia observed clinically.

The lymphopenia that defines SARS on laboratory testing is caused by two mechanisms: (1) direct viral infection and apoptosis of lymphocytes, particularly CD4+ and CD8+ T cells, and (2) cytokine-mediated redistribution and destruction of lymphocytes. Elevated LDH reflects widespread tissue injury, while the rise in hepatic transaminases and creatine kinase indicates extrapulmonary viral dissemination and multiorgan involvement. Gastrointestinal symptoms, particularly watery diarrhea, result from viral replication in enterocytes of the small and large intestine, where ACE2 is also expressed. This explains why stool specimens can be RT-PCR positive and why fecal-oral transmission was documented in certain outbreak clusters (notably the Amoy Gardens outbreak in Hong Kong).

The progression to acute respiratory distress syndrome (ARDS) occurs in approximately 20 to 30 percent of patients, typically during the second week of illness, and represents the leading cause of death. The case fatality rate was approximately 9.6% globally, but rose substantially in patients over 60 years of age (approaching 50% in some cohorts), making advanced age the strongest predictor of mortality.

02Clinical Manifestation and Classification

SARS follows a triphasic clinical course. Understanding this timeline is critical because exam vignettes often place the patient at a particular phase and ask you to predict the next clinical event or choose the appropriate intervention.

Severity Stratification

Additional clinical findings worth noting for vignette recognition: Absence of upper respiratory symptoms (no coryza, no sore throat) in the presence of a febrile lower respiratory illness is a red flag for SARS and other atypical pneumonias. Relative bradycardia (pulse-temperature dissociation) may also be seen. In pediatric patients, the disease tends to follow a milder course compared to adults.

03Diagnostic Workup

The best initial test in a patient presenting with febrile respiratory illness and an epidemiologic link is a chest radiograph combined with basic laboratory studies (CBC with differential, LDH, hepatic panel). The chest radiograph is essential not only for diagnosis but also for WHO case classification: a suspect case with radiographic pneumonia becomes a probable case.

The most accurate (confirmatory) test is RT-PCR for SARS-CoV RNA. However, there is an important caveat that is highly testable: RT-PCR sensitivity is low in the first 5 days of illness because the viral load in upper respiratory specimens has not yet peaked. During Phase 1, a single negative RT-PCR does not rule out SARS. This is why the WHO requires PCR positivity on at least two specimens or two separate runs to confirm the diagnosis. Lower respiratory tract specimens (bronchoalveolar lavage, induced sputum) have higher sensitivity than nasopharyngeal swabs.

Serology serves as the retrospective confirmatory method. A fourfold rise in IgG titer between acute and convalescent sera (collected at least 21 to 28 days apart) confirms the diagnosis. Serology is not useful for early clinical decision-making but is essential for epidemiologic surveillance and for confirming cases where PCR was negative or indeterminate.

HRCT of the chest is more sensitive than plain radiography for detecting early ground-glass changes and should be considered when clinical suspicion is high but the initial chest radiograph appears normal. The classic CT appearance is multifocal ground-glass opacities with peripheral and lower lobe predominance, which may progress to consolidation.

A key teaching point: the diagnostic workup and the infection control response must happen simultaneously. You do not wait for laboratory confirmation before initiating airborne and contact isolation. If the vignette asks "what is the next best step" in a febrile healthcare worker returning from an outbreak area, the answer is isolate the patient first, then order tests.

04Management and Treatment

Step 1: Suspect and Isolate. The moment a vignette describes a febrile patient with lower respiratory symptoms and a relevant epidemiologic link (travel to an affected area, contact with a SARS patient, healthcare worker in an outbreak facility), the first action is to place the patient in airborne isolation in a negative-pressure room with full personal protective equipment for all entering personnel. This is the "next best step" answer in nearly every early-management question.

Step 2: Diagnostic Workup. Obtain chest radiograph, CBC, comprehensive metabolic panel including LDH and hepatic enzymes, blood cultures, sputum for Gram stain and culture, nasopharyngeal aspirate for SARS-CoV RT-PCR, and paired serum for serology (store acute-phase serum for later comparison). Administer empiric antibiotics to cover community-acquired pneumonia, as SARS cannot be clinically differentiated from bacterial pneumonia on presentation alone.

Step 3: Supportive Care and Monitoring. Administer supplemental oxygen to maintain saturation at or above 93%. Serial monitoring of pulse oximetry and chest radiography every 24 to 48 hours during the first two weeks is essential because clinical deterioration typically occurs during Phase 2. If the patient's oxygen requirement is increasing, escalate to high-flow nasal cannula or noninvasive ventilation with extreme caution (these are aerosol-generating procedures and require enhanced infection control measures).

Step 4: ARDS Management. If the patient meets Berlin criteria for ARDS ($Pa{O}_2\mathrm{/}Fi{O}_{2}PaO\_2/FiO\_2$ ≤300 mmHg with bilateral opacities and the respiratory failure is not fully explained by cardiac failure or fluid overload), initiate lung-protective mechanical ventilation. Low tidal volumes (6 mL/kg predicted body weight), plateau pressures below 30 cmH2O, and a conservative fluid strategy are the evidence-based standard. Prone positioning for 12 to 16 hours per day should be implemented if the $Pa{O}_2\mathrm{/}Fi{O}_{2}PaO\_2/FiO\_2$ remains below 150 mmHg. Intubation is a high-risk aerosol-generating procedure; the team performing it must use powered air-purifying respirators (PAPRs) or equivalent.

Step 5: Notification. SARS is an internationally notifiable condition under the International Health Regulations (IHR 2005). Reporting to local public health authorities and the WHO is mandatory. Contact tracing and quarantine of exposed individuals for 10 days (the maximum incubation period) must be initiated immediately.

Contraindications and Cautions: Ribavirin is contraindicated in pregnancy (Category X) due to teratogenicity and in patients with significant renal impairment (creatinine clearance <50 mL/min) because it is renally cleared and causes dose-dependent hemolytic anemia. Corticosteroids should be avoided in the early viremic phase (Phase 1) as they may impair viral clearance and prolong the period of infectivity.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The most common way students get SARS questions wrong is by failing to prioritize infection control as the first management step. When a vignette describes a febrile patient with respiratory symptoms and a travel or contact history to an affected area, many examinees jump to ordering RT-PCR or starting ribavirin. The correct "next best step" is always to isolate the patient first. Testing comes after isolation, not before.

A second frequent error involves the timing sensitivity of RT-PCR. Students often assume that a negative RT-PCR on day 2 or 3 of illness rules out SARS. It does not. The viral load in upper respiratory specimens peaks during Phase 2 (days 8 to 14), so early testing can yield false negatives. The correct interpretation of a negative early PCR is that the test should be repeated on a different specimen type or at a later time point, and the patient should remain in isolation until SARS is adequately excluded.

A third trap is confusing SARS with MERS or COVID-19 based on the shared coronavirus etiology. All three cause severe lower respiratory illness and can progress to ARDS, but the epidemiologic link is the discriminator. SARS is linked to southern China and the 2003 outbreak period (or laboratory exposure); MERS is linked to the Arabian Peninsula and camel contact; COVID-19 has global distribution. If the vignette gives you a geographic or contact history, use it as the primary differentiating clue.

Finally, students frequently select ribavirin or corticosteroids as the primary treatment. Neither has proven efficacy for SARS in controlled trials. The examinable answer for management is supportive care plus strict infection control. Drug-based therapy is adjunctive at best, and the question stem is typically testing whether you know that no antiviral has been validated, rather than whether you can name one.

The core competency being tested in SARS questions is the ability to recognize a clinical pattern (febrile lower respiratory illness without upper respiratory symptoms, lymphopenia, bilateral ground-glass opacities, and an epidemiologic link), act immediately on infection control, classify the patient using the WHO framework, and sequence the diagnostic workup correctly while understanding the limitations of early testing.