Elevated White and Red Blood Cell Counts: Causes and Clinical Approach

Primary Mechanism: Hemoconcentration

Volume depletion is the most frequent cause of concurrent WBC and RBC elevation, as both cell lines become concentrated when plasma volume decreases, often due to dehydration from any cause, infection with volume depletion, or diuretic use, according to Clinical Infectious Diseases 1

Bacterial infection is the leading cause of leukocytosis and should be the first consideration, with key diagnostic markers including absolute band count ≥1,500 cells/mm³, neutrophil percentage >90%, and left shift ≥16% bands, as indicated by Praxis Medical Insights 2, 3, 4
The American College of Physicians suggests that common bacterial sources include respiratory tract infections, urinary tract infections, skin/soft tissue infections, and gastrointestinal infections 2, 4

The Infectious Diseases Society of America recommends obtaining a CBC with manual differential to assess absolute neutrophil count, band forms, and immature cells, and to assess clinical context systematically, including fever, localizing symptoms, signs of sepsis, and volume status assessment 1, 2, 3, 4
The Centers for Disease Control and Prevention advise considering additional testing if indicated, such as blood cultures if fever or sepsis is suspected, site-specific cultures based on symptoms, and repeat CBC after hydration to assess for hemoconcentration 1, 4