A WBC out of control. Usually defines as a WBC > 50,000, mostly mature cells. Some will call it leukemoid if > 30,000.
Bands with a normal WBC count is important: as the bands goes up so does mortality (PubMed).
The differntial diagnsosis fairly small: miliary Tuberculosis, a big infarction (often bowel), an abscess the size of my head, or severe (often Clostridium difficile) colitis are the top 4. Big tumor and widely metastatic tumors occasionally do it. Sepsis rarely gets that high, CML is rare. Occasionally WBC will go up acutely after splenectomy or if you are too stupid to stop G-CSF.
"Infection was the most common cause of LR (n = 83, 47.9%; 95% confidence interval, 40.7-55.4), followed by ischemia/stress (27.7%), inflammation (6.9%), and obstetric diagnoses (6.9%). Higher WBC counts were significantly associated with positive blood cultures (P = .017) or a positive Clostridium difficile toxin (P = .001). Antibiotics were prescribed for 140 patients (80.9%). Sixty-six patients (38.1%) died during hospitalization (PubMed)."
"A total of 758 (20%) patients with solid tumors and extreme leukocytosis were identified. The etiology of the leukocytosis was hematopoietic growth factors in 522 (69%) patients, infection in 112 (15%) patients, high-dose corticosteroids in 38 (5%) patients, newly diagnosed leukemia in 9 (1%) patients, and paraneoplastic leukemoid reaction in 77 (10%) patients.
The patients diagnosed with a paraneoplastic leukemoid reaction typically had neutrophil predominance (96%) and radiographic evidence of metastatic disease (78%), were clinically stable, and had a poor prognosis....Infection was an uncommon cause of extreme leukocytosis in patients with solid tumors. Patients with paraneoplastic leukemoid reactions typically were clinically stable despite having large tumor burdens. (PubMed)"
It is rare for steroids to cause a WBC greater than 21,000.
Some tumors, like bladder cancers, can make GCSF.
Relevant links to my Medscape blog