When does White Blood Cell (WBC) count rise in response to an insult or infection?

From the Guidelines

White blood cells (WBCs) typically begin to rise within hours of an infection or inflammatory insult to the body, with a significant increase in WBC count (≥ 14,000 cells/mm³) or a left shift (percentage of band neutrophils or metamyelocytes, ≥ 16%; or total band neutrophil count, ≥ 1500 cells/mm³) often occurring within 12-24 hours of onset of symptoms 1. The initial response occurs rapidly, with neutrophils being mobilized from the bone marrow reserve pool within 30 minutes to 2 hours after the insult. This early response is followed by increased production of new WBCs in the bone marrow, which becomes evident in the peripheral blood count within 4-6 hours and peaks around 24-48 hours after the initial insult. Some key points to consider when evaluating WBC response to infection include:

• Different types of infections trigger different WBC responses: bacterial infections often cause neutrophilia (elevated neutrophils), viral infections typically increase lymphocytes, parasitic infections may elevate eosinophils, and certain chronic infections can increase monocytes.
• The magnitude and timing of the WBC elevation depend on several factors including the severity of infection, the patient's immune status, and any pre-existing conditions.
• A complete blood cell (CBC) count, including peripheral WBC and differential cell counts, should be performed for all suspected infections within 12–24 h of onset of symptoms (or sooner, if the resident is seriously ill) 1.
• The presence of an elevated WBC count or a left shift warrants a careful assessment for bacterial infection in any resident with suspected infection, with or without fever 1.

From the Research

WBC Response to Insult/Infection

• The white blood cell count (WBC) rises in response to insult or infection, and this increase can be used as a diagnostic tool 2, 3, 4, 5.
• Studies have shown that an elevated WBC count is associated with bacterial infections, sepsis, and other inflammatory conditions 3, 4, 5.
• The trajectory of WBC count over time can also provide valuable information about patient outcomes, with a rising WBC trajectory being associated with increased mortality in patients with septic shock 2.

Diagnostic Value of WBC

• WBC count is a valuable tool for diagnosing infections in the emergency department, particularly when combined with other biomarkers such as C-reactive protein (CRP) and procalcitonin (PCT) 3, 4, 5.
• Eosinopenia, which is a decrease in eosinophil count, has been shown to be a sensitive and specific marker for urinary and biliary tract infections 3.
• A careful analysis of WBC count, including parameters such as neutrophil and lymphocyte counts, can help identify patients with bacterial infections and sepsis 3, 4, 5.

Predicting Sepsis and Mortality

• WBC count, along with other biomarkers such as CRP and PCT, can be used to predict sepsis and mortality in patients with suspected infections 4, 5, 6.
• A study found that monocyte distribution width (MDW) and mean monocyte volume (MMV) were sensitive and specific markers for predicting sepsis in ICU patients 6.
• Another study found that a rising WBC trajectory was associated with increased mortality in patients with septic shock, highlighting the importance of monitoring WBC count over time 2.