Which lab test indicates a left shift, characterized by an increased percentage of immature white blood cells, such as band neutrophils, in a complete blood count (CBC) with differential?

Complete Blood Count (CBC) with Differential

A complete blood count (CBC) with manual differential is the laboratory test that shows a left shift, which is defined by an elevated band neutrophil count (≥1,500 cells/mm³) or an elevated band percentage (≥16% of total white blood cells). 1, 2

Quantitative Definitions of Left Shift

The most reliable indicators of left shift, in order of diagnostic accuracy for bacterial infection, are:

• Absolute band count ≥1,500 cells/mm³ (likelihood ratio 14.5 for bacterial infection) 1, 3
• Band percentage ≥16% of total WBCs (likelihood ratio 4.7 for bacterial infection) 1, 2
• Neutrophil percentage >90% (likelihood ratio 7.5) 1, 4
• Total WBC count ≥14,000 cells/mm³ (likelihood ratio 3.7) 1, 4

Critical Technical Requirement

Manual differential count is mandatory for accurate left shift assessment. 1, 2, 3 Automated analyzers cannot reliably identify and enumerate band forms and other immature neutrophils. 2, 3 The Infectious Diseases Society of America explicitly recommends manual differential to assess bands and immature forms for all patients with suspected infection. 1, 3

Clinical Significance

Left shift indicates significant bacterial infection and warrants immediate evaluation, even when:

• Total WBC count is completely normal (<10,000 cells/mm³) 1, 2
• Fever is absent 1
• The patient is elderly with decreased basal body temperature 1, 2

Any patient with band count ≥1,500 cells/mm³ OR band percentage ≥16% requires urgent assessment for bacterial infection source, regardless of total WBC count or fever presence. 1, 3

Timing and Ordering

For suspected infection, CBC with manual differential should be performed within 12-24 hours of symptom onset (or sooner if the patient is seriously ill). 1, 3 The laboratory requisition must specifically request manual differential with band enumeration, not just automated immature granulocyte counts. 2, 3

Common Pitfalls to Avoid

• Do not rely on automated analyzer flags alone—they cannot accurately assess band forms. 2, 3
• Do not ignore left shift when total WBC is normal—this combination still indicates significant bacterial infection requiring evaluation. 1, 2, 3
• Do not order only automated differential—manual counting by a trained technologist is essential for band assessment. 1, 2
• Do not dismiss left shift in elderly patients—it has particular diagnostic importance in this population due to absent typical infection symptoms and decreased basal temperature. 1, 2

Non-Infectious Causes to Consider

While left shift most commonly indicates bacterial infection, other causes include:

• Myelodysplastic syndromes (dysplastic granulocytopoiesis) 2
• Medications: lithium, beta-agonists, epinephrine 2, 4
• Pelger-Huet anomaly (hereditary condition mimicking left shift) 5

However, the sudden and significant left shift characteristic of acute bacterial infection is distinct from these other causes. 6