From the Guidelines
Blood cultures should be obtained in patients with severe pneumonia requiring hospitalization, especially those admitted to the intensive care unit, patients with immunocompromising conditions, and those who have failed outpatient antibiotic therapy. The role of blood cultures in diagnosing pneumonia is to identify the presence of bacteremia and guide targeted antibiotic therapy, with the overall yield being approximately 5-14% in hospitalized patients 1. According to the guidelines for the management of adult lower respiratory tract infections, two sets of blood cultures should be performed in all patients with community-acquired pneumonia who require hospitalization 1.
The rationale for selective use of blood cultures is that bacteremia in pneumonia indicates a more severe infection and may require longer duration of treatment or adjustment of antibiotics based on the identified pathogen and its susceptibility 1. However, empiric antibiotic therapy should not be delayed while waiting for blood culture results, as prompt administration of appropriate antibiotics is associated with better outcomes in pneumonia. Key points to consider when deciding to obtain blood cultures include:
- The patient's severity of illness and need for hospitalization
- The presence of immunocompromising conditions
- The failure of outpatient antibiotic therapy
- The potential for bacteremia and its impact on treatment and outcomes
It is also important to note that blood cultures may isolate uncommon etiologic organisms, such as gram-negative organisms or agents demonstrating unusual antibiotic resistance patterns, which can inform antibiotic use 1. Additionally, in the elderly, obtaining blood cultures within 24 hours of admission has been associated with reduced 30-day mortality 1. Overall, the use of blood cultures in pneumonia diagnosis should be guided by the individual patient's clinical presentation and risk factors.
From the Research
Role of Blood Cultures in Diagnosing Pneumonia
- Blood cultures are of limited utility in nonsevere community-acquired pneumonia, but are routinely recommended for severe community-acquired pneumonia or health care-associated pneumonia due to perceived greater bacteremia risk, particularly with multidrug-resistant organisms 2.
- The yield and utility of routine blood cultures in pneumonia are extremely low, irrespective of severity and risk 2.
- Factors that predict positive blood cultures among patients hospitalized for pneumonia include higher pneumonia severity index (PSI) classes, smoking, and being of Malay ethnicity, while prior treatment with antibiotics, chronic obstructive pulmonary disease, and cough are less likely to result in positive blood cultures 3.
- A clinical scoring system can be used to predict negative blood cultures, which can help improve cost-effectiveness by selecting patients for whom blood cultures are performed 3.
- Positive blood culture rates are low and do not affect the clinical management of pneumonia patients, with the exception of patients at risk for multi-drug resistant pathogens, such as those with health-care-associated pneumonia (HCAP) 4.
- Blood cultures rarely alter therapy for patients presenting with pneumonia, with only 3.6% of patients having their therapy altered due to blood culture results 5.
- Organisms and resistance patterns differ by culture site, with respiratory cultures more often resistant to first-line therapy for community-acquired pneumonia than blood cultures 6.
- Patients with both respiratory and blood cultures positive have higher case-fatality, longer lengths of stay, and higher costs than patients with only one culture site positive 6.