Circadian rhythm of serum phosphate, calcium and parathyroid hormone levels in hemodialysis patients

Physiological Rhythms and Biological Variation of Biomolecules: The Road to Personalized Laboratory Medicine

The concentration of biomolecules in living systems shows numerous systematic and random variations. Systematic variations can be classified based on the frequency of variations as ultradian (<24 h), circadian (approximately 24 h), and infradian (>24 h), which are partly predictable. Random biological variations are known as between-subject biological variations that are the variations among the set points of an analyte from different individuals and within-subject biological variation, which is the variation of the analyte around individuals’ set points. The random biological variation cannot be predicted but can be estimated using appropriate measurement and statistical procedures. Physiological rhythms and random biological variation of the analytes could be considered the essential elements of predictive, preventive, and particularly personalized laboratory medicine. This systematic review aims to summarize research that have been done about the types of physiological rhythms, biological variations, and their effects on laboratory tests. We have searched the PubMed and Web of Science databases for biological variation and physiological rhythm articles in English without time restrictions with the terms “Biological variation, Within-subject biological variation, Between-subject biological variation, Physiological rhythms, Ultradian rhythms, Circadian rhythm, Infradian rhythms”. It was concluded that, for effective management of predicting, preventing, and personalizing medicine, which is based on the safe and valid interpretation of patients’ laboratory test results, both physiological rhythms and biological variation of the measurands should be considered simultaneously.

Differences in Phosphate and Parathyroid Hormone Concentrations over the Day among Patients on Hemodialysis

BACKGROUND

Elevated serum phosphate and parathyroid hormone (PTH) concentrations are associated with cardiovascular events, bone disease, and mortality in patients on maintenance hemodialysis. Although circadian changes are known in people with CKD, it is unknown whether differences occur in these parameters over the course of a day in people receiving hemodialysis.

METHODS

We used clinical data from Fresenius Medical Care US dialysis clinics to determine how the time of day when measurements were collected (hemodialysis treatment start time) may be associated with serum phosphate and PTH concentrations. We used harmonic regression to assess these associations while accounting for demographic data and treatment parameters.

RESULTS

A total of 96,319 patients receiving maintenance hemodialysis were included in this analysis. Patients had a mean age of 64±14 years, 43% were women, and dialysis start times ranged from 3:00 am to 7:59 pm. The mean serum phosphate concentration was 5.2±1.5 mg/dl, and the median PTH was 351 pg/ml (interquartile range [IQR], 214-547). In fully adjusted models, serum phosphate had a nadir at 11:00 am of 4.97 (IQR, 4.94-5.01) mg/dl and a peak at 7:00 pm of 5.56 (IQR, 5.50-5.62) mg/dl. Serum PTH had a nadir at 9:00 am of 385 (IQR, 375-395) pg/ml and a peak at 7:00 pm of 530 (IQR, 516-547) pg/ml.

CONCLUSIONS

Among patients receiving maintenance hemodialysis, concentrations of PTH and phosphate before a dialysis session vary with the time of day that these values are measured. Consideration of whether these values were obtained at peak or nadir times of the day may be important in treatment decisions.

Randomized Trial of Acute Changes in Plasma Phosphate After Phosphorus-Standardized Meals in Peritoneal Dialysis

Introduction

Hyperphosphatemia is associated with increased morbidity and mortality in patients with chronic kidney disease. The aim of this study was to assess whether a meal with high phosphorus content would affect plasma phosphate in the hours that follow among subjects with end-stage kidney disease on peritoneal dialysis.

Methods

This was a single-blinded randomized cross-over trial of 12 subjects on maintenance peritoneal dialysis, in which subjects were randomized to consume a meal with either high or low phosphorus content on 2 separate trial days. On each trial day, plasma phosphate was measured immediately before consumption of the standardized meal and after 1, 2, 3, and 5 hours.

Results

The mean fasting plasma phosphate at baseline was 1.69 ± 0.22 mmol/l. Plasma phosphate was similar between the 2 meals at baseline, as well as at 1, 2, 3, and 5 hours after consumption. The largest observed difference in plasma phosphate between the 2 meals was 0.15 mmol/l, which occurred 5 hours after consumption (high-phosphorus meal 1.75 ± 0.32 mmol/l vs. low-phosphorus meal 1.60 ± 0.14 mmol/l (P = 0.06)). Using summary analyses for repeated measures, we observed a significant difference in the plasma phosphate between the 2 meals (P = 0.03).

Conclusion

Our results show that in subjects with end-stage kidney disease, a meal with high phosphorus content has only a negligible effect on plasma phosphate compared to a meal with low phosphorus content. Thus, large increases in plasma phosphate cannot be accounted for by a high intake of phosphorus in the hours before blood sampling.

The role of phosphate in kidney disease

The importance of phosphate homeostasis in chronic kidney disease (CKD) has been recognized for decades, but novel insights - which are frequently relevant to everyday clinical practice - continue to emerge. Epidemiological data consistently indicate an association between hyperphosphataemia and poor clinical outcomes. Moreover, compelling evidence suggests direct toxicity of increased phosphate concentrations. Importantly, serum phosphate concentration has a circadian rhythm that must be considered when interpreting patient phosphate levels. Detailed understanding of dietary sources of phosphate, including food additives, can enable phosphate restriction without risking protein malnutrition. Dietary counselling provides an often underestimated opportunity to target the increasing exposure to dietary phosphate of both the general population and patients with CKD. In patients with secondary hyperparathyroidism, bone can be an important source of serum phosphate, and adequate appreciation of this fact should impact treatment. Dietary and pharmotherapeutic interventions are efficacious strategies to lower phosphate intake and serum concentration. However, strong evidence that targeting serum phosphate improves patient outcomes is currently lacking. Future studies are, therefore, required to investigate the effects of modern dietary and pharmacological interventions on clinically meaningful end points.