Circadian variation of granulocyte colony stimulating factor levels in man

Glucocorticoids dose‐dependently increase plasma levels of granulocyte colony stimulating factor (G‐CSF). Based on the marked circadian rhythm of cortisol levels, we hypothesized that plasma levels of G‐CSF may also show a diurnal rhythm. A prospective study was conducted in 12 healthy young volunteers. Blood samples were obtained every 2 h over 24 h. G‐CSF levels averaged 18.0 ng/l (CI 13.1–22.9) at 8.00 am, increased continuously and reached peak values at 10.00 p.m. Individual harmonic regression analysis showed a clear circadian rhythm. The individual differences between nadir and peak levels averaged 54% (CI 43–65%). This pronounced diurnal rhythm of G‐CSF levels may help understand the circadian changes in circulating stem cells, bone marrow DNA synthesis, or bone marrow toxicity induced by some chemotherapeutic agents.