The timing of blood withdrawal in hemodialysis (HD) patients, whether midweek or after the weekend, may influence phosphate values, hyperphosphatemia management, and mortality risk, according to investigators.

Jorge B. Cannata-Andia, MD, PhD, and colleagues studied 3 years’ worth of serum phosphate measurements from 6797 HD patients in COSMOS (Current Management of Secondary Hyperparathyroidism: A Multicentre Observational Study). Patients were divided into two groups according to whether blood was routinely collected at a midweek or post-weekend HD session at their center.

Mean serum phosphate levels were significantly higher after the 3-day weekend than the 2-day midweek interval: 5.5 vs 5.2 mg/dL, according to results published in Nephrology Dialysis Transplantation. In contrast, no meaningful differences were observed for serum calcium or parathyroid hormone values.

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“The COSMOS difference of 0.3mg/dL between midweek and post-weekend serum phosphate is clinically relevant, as differences of a similar magnitude have been used to evaluate the efficacy of interventions with phosphate binders,” Dr Cannata-Andia and the team stated.

More importantly, the target range of phosphate associated with the lowest all-cause mortality (10% or less increase in relative risk) differed according to the timing of blood collection. For the midweek group, the optimal range was 3.5 to 4.9 mg/dL. The upper limit for the post-weekend group was significantly higher: 3.8 to 5.7 mg/dL. The spread between the upper and lower limits was just 1.4 mg/dL for the midweek group, but 1.9 mg/dL for the post-weekend group.

All of these differences are clinically meaningful, the researchers emphasized. The proportion of patients with phosphate levels above target range was always higher for post-weekend collections, whether using a previously identified optimal COSMOS target range (3.6 to 5.2 mg/dL) or those from Kidney Disease Improving Global Outcomes (KDIGO) or Kidney Disease Outcomes Quality Initiative (KDOQI).

In the current study, midweek patients above the target phosphate range had 42% excess mortality, compared with just 24% for above-range post-weekend patients. The serum phosphate value associated with the lowest mortality risk was 4.2 mg/dL for midweek and 4.7 mg/dL for post-weekend patients.

Midweek measurements reflect phosphate levels over 6 of 7 days in the week and could be a better indicator of abnormality, according to the team. 

The investigators stated that the recommendation of a single range of serum phosphate regardless of the timing for blood sampling after the last dialysis session “may be insufficient as a target reference value and not accurate enough for clinical practice.”

“Thus, clinical guidelines should consider the timing of blood withdrawal when recommending optimal target ranges for serum phosphate and therapeutic strategies for phosphate control,” they concluded.

Among the study’s limitations, the team acknowledged that midweek and post-weekend measurements were not performed in the same patients. In addition, the team had no information on diurnal rhythm or fasting, both of which affect serum phosphate.

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Fernandez-Martın JL, Dusso A, Martinez-Camblor P, et al. Serum phosphate optimal timing and range associated with patients survival in haemodialysis: the COSMOS study. Nephrol Dial Transplant 2018;1–9. DOI: 10.1093/ndt/gfy093