Sex‐ and age‐specific daily and seasonal rhythms in haemoglobin and ferritin among Dutch blood donors

Abstract Background and Objectives Intra‐individual haemoglobin (Hb) and ferritin variation is often overlooked in clinical practice, yet may be relevant where levels are around the cut‐offs. We characterised Hb and ferritin fluctuations across the year and day in Dutch blood donors. Materials and Methods We included 337,218 capillary Hb (cHb) and 171,542 serum ferritin measurements routinely collected between 8:00 AM and 9:00 PM in new blood donors aged 18–65 years. Cosinor rhythms with 24‐h and 12‐month periods were fitted for males and females separately. Donor groups were stratified into <45 and ≥45 years of age as a proxy for the onset of menopause and age‐related changes in circadian physiology. Results cHb levels showed a 24‐h rhythm, peaking around 10:30 AM. The daily amplitude was larger for males (0.122 mmol/L) than females (0.093 mmol/L), but females ≥45 years had a larger amplitude (0.106 mmol/L) than those <45. The cHb peak time occurred earlier in donors ≥45 (around 9:40 AM) than those <45 (around 10:40 AM). cHb levels were higher in winter than in summer. The yearly amplitude was larger in males (0.036 mmol/L) than in females (0.020 mmol/L). Ferritin levels also peaked in winter with similar amplitude in males and females. Conclusion The cosinor behaviour of cHb levels during the day may indicate contribution of circadian physiology. An earlier cHb peak time in older donors agrees with the known association between ‘morningness’ and older age. These results support the interpretation of cHb and ferritin measurements in clinical practice and may fuel further research into underlying mechanisms.