Abstract:

Background: Limited number of studies in humans revealed seasonal patterns of variability in white blood cells.

Material and methods: The study population included 151 743 consecutive patients  of the Department of clinical laboratory of the Hospital St Ivan Rilsky, Sofia  between January 2022 and April 2023.  After exclusion of the abnormal tests the re-maining cohort consisted of 31 910 patients – 18 020 women and 13 890 men

Results. We observed a December peak – a 16.6% increase in leukocytes, with the lowest values recorded in April and June. Neutrophils were highest in December compared to May and June – by 19.5%.  The annual fluctuation of lymphocytes was slightly different, with higher values a month earlier – from November to March-April, as the difference of June-December was 21%.  Monocytes had smallest numbers in June and showed increase with 22% in October and November.

In summary, we found a difference between the lowest and highest values of leukocytes, neutrophils, lymphocytes, monocytes and eosinophils as follows: 16.6 %, 19.5%, 21%, 22% and 28.5% (all within the reference ranges as per protocol). When comparing the values by sex, we found slightly higher (about 10%) monocyte and eosinophil levels in males than in females

Conclusions. There is a clear seasonal fluctuation with an almost perfect match of the annual peaks in values of different types of white blood cells in December while most values are lower in the summer, July being on the bottom. Despite some fluc-tuations with age and sex, both ageing and gender do not seem to affect the seasonal fluctuations in blood cell populations.

Keywords: seasons, neutrophils, lymphocytes, age, sex