How do you differentiate between bacterial and viral infections using a Full Blood Count (FBC)?

From the Guidelines

A full blood count (FBC) can help distinguish between bacterial and viral infections by examining specific patterns in white blood cell counts, with bacterial infections typically showing an elevated total white blood cell count (leukocytosis) with a marked increase in neutrophils (neutrophilia), and viral infections generally showing a normal or decreased total white cell count with relative lymphocytosis (increased percentage of lymphocytes).

Key Findings

• In bacterial infections, the total white blood cell count is often elevated, with a significant increase in neutrophils, usually above 10,000-15,000 cells/μL, and the presence of immature neutrophils (band cells or "left shift") 1.
• Viral infections, on the other hand, typically present with a normal or decreased total white cell count and relative lymphocytosis (increased percentage of lymphocytes) 1.
• The neutrophil-to-lymphocyte ratio (NLR) can be a useful indicator, with higher ratios (>3.5) suggesting bacterial infection and lower ratios more consistent with viral etiology.

Important Considerations

• These patterns are not absolute and can vary depending on the specific infection and patient context, such as typhoid fever presenting with normal or low white cell counts, or certain viral infections causing neutrophilia 1.
• Other parameters like C-reactive protein (CRP) and procalcitonin, though not part of the FBC, can provide additional information when combined with blood count results, helping to differentiate between viral and bacterial infections 1.
• Clinical context remains essential for accurate interpretation, as FBC findings should always be considered alongside patient symptoms, exposure history, and other diagnostic tests 1.

From the Research

Determining Bacterial and Viral Infections through FBC

To determine whether an infection is bacterial or viral through a full blood count (FBC), several parameters can be considered:

• White blood cell (WBC) count: A high WBC count is often indicative of a bacterial infection, as seen in studies 2, 3, 4.
• Granulocyte count: Elevated granulocyte counts are also associated with bacterial infections 2.
• Lymphocyte count: Lymphocyte counts do not have a clear association with the aetiology of infection and are not useful for distinguishing between viral and bacterial infections 2.
• Neutrophil left shift: The presence of a neutrophil left shift can indicate an increase in neutrophil consumption, which is often seen in bacterial infections 4.

Cut-off Values for Distinguishing Bacterial and Viral Infections

Specific cut-off values for WBC and granulocyte counts can help distinguish between bacterial and viral infections:

• WBC count: Cut-off levels of 15.0 and 20.0 x 10(9)/l have specificities of 86% and 95%, respectively, for distinguishing bacterial from viral cases 2.
• Granulocyte count: Cut-off levels of 10.0 and 15.0 x 10(9)/l have specificities of 84% and 97%, respectively, for distinguishing bacterial from viral cases 2.

Limitations of FBC in Diagnosing Infections

While FBC can provide useful information for diagnosing infections, it is essential to consider the clinical context and other laboratory results:

• A single FBC result may not be sufficient for diagnosing a bacterial infection, as left shift and WBC count can change over time 4.
• The presence of other underlying conditions, such as primary haematological disorders, can affect the interpretation of FBC results 5.