SIRS Diagnosis in Patients with Dual Viral Infections
No, you do not need to obtain a CBC to diagnose SIRS if 2 of the 4 SIRS criteria are already met—SIRS is a clinical diagnosis based on meeting at least 2 criteria, and the CBC is only required if one of those criteria is an abnormal white blood cell count or band count. However, obtaining a CBC is strongly recommended in patients with COVID-19 and influenza co-infection for risk stratification, assessment of disease severity, and detection of bacterial superinfection.
Understanding SIRS Criteria
The 4 SIRS criteria are:
- Temperature >38°C or <36°C
- Heart rate >90 beats/minute
- Respiratory rate >20 breaths/minute or PaCO2 <32 mmHg
- White blood cell count >12,000/μL or <4,000/μL, or >10% bands
If 2 criteria are already met through vital signs alone (e.g., fever and tachycardia), the SIRS diagnosis is established without laboratory testing. The WBC count is only one of four possible criteria, not a mandatory component.
Why CBC Should Still Be Obtained
Risk Stratification in Co-Infected Patients
- Co-infected patients with influenza and COVID-19 have elevated risk for poor outcomes (15.7% mortality/deterioration rate) compared to mono-infected patients 1
- Higher WBC counts are associated with worse outcomes and mortality in COVID-19 patients 2
- Elevated WBC with elevated CRP or procalcitonin indicates higher possibility of bacterial superinfection, particularly in critically ill patients 2
Diagnostic Utility Beyond SIRS
The CBC provides critical information for managing dual viral infections:
- Distinguishing true leukopenia from isolated lymphopenia is essential, as lymphopenia is common in COVID-19 while leukocytosis may indicate bacterial coinfection 2
- WBC count >9,000/μL is more common in influenza than COVID-19, helping differentiate the predominant pathogen 3
- Thrombocytopenia (platelet count <150,000/μL) is a secondary clinical criterion for multisystem inflammatory complications in COVID-19 patients 4
Screening for Severe Complications
In patients with positive COVID-19 and influenza tests, the CBC is part of tier 1 screening for multisystem inflammatory syndrome (MIS-C in children or hyperinflammation in adults):
- Tier 1 screening includes CBC with differential, complete metabolic panel, ESR, CRP, and viral testing 4
- Absolute lymphocyte count <1,000/μL is a suggestive laboratory feature requiring full diagnostic evaluation 4
- Platelet count <150,000/μL triggers progression to tier 2 evaluation 4
Clinical Algorithm for Co-Infected Patients
When 2 SIRS criteria are met with dual positive viral tests:
- Document which 2 criteria are met (vital signs alone establish SIRS diagnosis)
- Obtain CBC with differential regardless for risk stratification 2
- Add inflammatory markers (CRP, ESR, procalcitonin) to assess severity and bacterial superinfection risk 2
- Monitor for deterioration as co-infected patients show elevated risk for poor outcomes 1
- Perform comprehensive microbiologic workup before empirical antibiotics if WBC is elevated 2
Important Caveats
- Co-infection remains underdiagnosed (0.54% in one series) unless systematic screening is performed 5
- Similar clinical presentations of COVID-19 and influenza (fever, cough, dyspnea) make laboratory differentiation essential 1, 6, 3
- Treatment differs significantly between the two viruses, making accurate diagnosis paramount 6
- The most common complications in co-infected patients are pneumonia, atelectasis, and acute respiratory distress syndrome 1
Bottom line: While the CBC is not required to diagnose SIRS when 2 criteria are already met, it should be obtained in all patients with COVID-19 and influenza co-infection for proper risk assessment and management decisions.