What indicates a left shift in a complete blood count (CBC)?

Identifying a Left Shift in a Complete Blood Count

A left shift is occurring when the percentage of band neutrophils or metamyelocytes is ≥16% or when the total band neutrophil count is ≥1500 cells/mm³ in a complete blood count. 1

What is a Left Shift?

A left shift refers to the presence of increased immature neutrophils (primarily bands) in the peripheral blood, indicating that the bone marrow is releasing immature cells in response to increased demand. This is a key indicator of active bacterial infection.

Specific Laboratory Findings That Indicate a Left Shift:

1. Band neutrophil percentage:

   • ≥16% of total white blood cells 1
   • This has a likelihood ratio of 4.7 for detecting bacterial infection

2. Absolute band count:

   • ≥1500 cells/mm³ 1
   • This has the highest likelihood ratio (14.5) for detecting bacterial infection

3. Other supporting findings:

   • Increased percentage of neutrophils (>90%) has a likelihood ratio of 7.5 1
   • May occur with or without leukocytosis (elevated WBC count)

Clinical Significance of a Left Shift

A left shift is highly predictive of bacterial infection, even in the absence of fever. The presence of a left shift warrants careful assessment for bacterial infection in any patient 1.

• A left shift can be present even with a normal total WBC count (<10,000 cells/mm³) 1
• The dynamic changes in left shift and WBC count reflect the progression of bacterial infection 2
• The degree of left shift may correlate with infection severity and prognosis

Common Pitfalls in Identifying a Left Shift

1. Relying on a single time point:

   • Left shift changes dynamically during the course of infection
   • May not be present in extremely early or late phases of infection 2
   • Serial measurements provide more accurate information

2. Confusing with Pelger-Huët anomaly:

   • This hereditary condition can mimic a left shift
   • In Pelger-Huët anomaly, neutrophils appear hyposegmented but have mature condensed chromatin 3, 4
   • Family history is important to differentiate this condition from true infection 3

3. Overlooking the importance of manual differential:

   • A manual differential count is preferred over automated counts to accurately assess bands and other immature forms 1
   • Guideline recommends "preferably a manual differential to assess bands and other immature forms" 1

Algorithm for Evaluating a Left Shift

1. Obtain a CBC with manual differential to accurately assess band neutrophils and other immature forms
2. Calculate key metrics:
   • Percentage of band neutrophils (≥16% indicates left shift)
   • Absolute band count (≥1500 cells/mm³ indicates left shift)
   • Total WBC count (≥14,000 cells/mm³ indicates leukocytosis)
3. Interpret findings:
   • Left shift + leukocytosis: Strong indication of bacterial infection
   • Left shift without leukocytosis: Still highly suspicious for bacterial infection
   • No left shift but leukocytosis: Consider other inflammatory conditions
4. Consider serial measurements to track the progression of infection and response to treatment

In summary, a left shift is a valuable marker for bacterial infection and should prompt careful clinical evaluation, even in the absence of fever or elevated total WBC count. The most reliable indicators are an elevated band percentage (≥16%) or absolute band count (≥1500 cells/mm³).