Phosphorus Consumption Within 1 Hour Prior to Blood Work and Associated Serum Levels of Phosphate, Calcium, and PTH in Adult Patients Receiving Hemodialysis Treatment

Background: While dietary intake is known to influence serum markers of chronic kidney disease–mineral and bone disorder (CKD-MBD), the effects of recent food and beverage intake, particularly phosphorus consumption on these serum markers (phosphate, calcium, and parathyroid hormone [PTH]), are unknown in hemodialysis patients. An understanding of these effects could have direct and important implications on the management of CKD-MBD. Objective: To determine whether serum phosphate, calcium, and PTH levels were higher in hemodialysis patients who had consumed dietary phosphorus within 1 hour prior to their routine dialysis-related blood work (non–phosphorus-fasted) compared with patients who did not (phosphorus-fasted). Design: Observational, cross-sectional study. Setting: Kingston Health Sciences Center—Kingston General Hospital Site and its affiliated satellite hemodialysis units. Patients: Two hundred fifty-four adult patients receiving outpatient hemodialysis treatment for end-stage kidney disease were recruited. Measurements: The main measurements for this study included an assessment of dietary phosphorus intake as well as serum phosphate, calcium, PTH, albumin, Kt/V, and urea reduction ratio. Methods: A direct patient interview was performed to assess dietary phosphorus intake within 1 hour prior to routine dialysis-related blood work. The Canadian Nutrient File was then used to estimate dietary phosphorus based on the specific foods and beverages (including portion sizes and brands where applicable) identified in the interview. Serum measures of phosphate, PTH, calcium, albumin, and dialysis adequacy (Kt/V and urea reduction ratio) were obtained from participants’ routine dialysis-related blood work. Results: Non–phosphorus-fasted participants had nonsignificantly higher serum PTH levels compared to phosphorus-fasted participants (61.2 ± 64.7 vs 47.9 ± 39.7, P = .05). Non–phosphorus-fasted participants with PTH levels at the Kidney Disease Improving Global Outcomes (KDIGO) “target” (between 15 and 60 pmol/L) had significantly higher serum phosphate levels relative to phosphorus-fasted participants (1.6 ± 0.3 vs 1.4 ± 0.4, P = .006). In non–phosphorus-fasted participants, there was a nonsignificant association between the number of items containing inorganic phosphate additives and higher levels of serum phosphate and lower levels of serum calcium. Limitations: Some limitations include the cross-sectional nature of this study, self-reporting biases and estimates (as opposed to direct measurements) related to the dietary assessment, and the use of single (and not serial) assessments of serum measures. Conclusions: Dietary phosphorus intake in close proximity to blood work may contribute to subtle alterations in some key serum CKD-MBD parameters in adult outpatient hemodialysis patients but may not meaningfully alter CKD-MBD management.