Serum phosphorus levels in dialysis patients vary depending on when blood specimens are collected, researchers reported at the European Renal Association-European Dialysis and Transplant Association 53rd Congress in Vienna. Levels are higher when blood is collected after the weekend (interdialytic period) compared with blood samples taken on other days.

The timing of blood collection also affects the relationship between phosphorus level and mortality, investigators found.

The results are from COSMOS, a prospective 3-year, observational, open-cohort study including 6,797 dialysis patients from 227 centers in 20 European countries. Adriana Dusso, MD, of the Hospital Universitario Central de Asturias in Spain, and colleagues divided patients into 2 groups depending on whether the patients’ centers drew blood specimens midweek (MW) or post-weekend (PW). Mean serum phosphorus levels were significantly higher in the PW than MW group (5.5 vs 5.2 mg/dL). In addition, the PW group had a higher proportion of patients with serum phosphorus levels above the upper targets recommended by guidelines. Despite these differences, the prescription of phosphate binders was similar between the groups: 85% for the PW and 84.8% for MW arm.

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In addition, the timing of blood collection influenced the relative risk of mortality in relation to serum phosphorus. The lowest mortality ranges for serum phosphorus were 3.5–4.9 mg/dL when blood was sampled MW and 3.8–5.7 mg/dL when sampled PW. The association between high serum phosphorus and mortality risk was stronger for MW serum phosphorus measurements, according to the investigators. For serum phosphorus in the 5.5–6.3 mg/dL range (quintile 4), MW collection was associated with a significant 35% increased relative risk of mortality versus PW collection.

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The study found no differences in serum calcium or parathyroid hormone levels between the PW and MW groups.

Dr Dusso and her collaborators concluded in their study abstract “that future clinical guidelines should consider the time of blood withdrawal when recommending serum target ranges for P and therapeutic strategies for P control.”