Introduction: To our knowledge there are no studies that describe phosphorus behavior in the early days of post kidney and simultaneous kidney-pancreas transplant. We aim to describe the urinary handling of phosphorus as well as the bone and mineral metabolism immediately before and during the first 8 days of post transplantation.

Methods: Observational descriptive longitudinal study including 61 renal and simultaneous kidney-pancreas transplant adult recipients (either living or deceased donors) at Hospital CEMIC Buenos Aires. Baseline blood samples were drawn pre transplant and then after surgery along with urine samples at days 1,3,5, and 7 to measure  serum phosphorus, calcium, creatinine, pre-transplant intact parathormone (iPTH), urinary phosphorus and creatinine excretion, glomerular filtration rate (by urinary creatinine clearance), and fractional excretion of phosphorus.

Results: Mean age was 49 ± 13, 54% were male, 90% were kidney transplants, 75% received organs from deceased donors, average serum vitamin D was 22.1 ng/mL, 37% had hyperparathyroidism (iPTH>580 pg/mL) and 13% had hypoparathyroidism (iPTH<120 pg/mL) before transplant. We observed that creatininemia decreased (5.9 mg/dL at day 1 and 2.0 mg/dL at day 7 p 0.0001) and also serum phosphorus decreased (6.2 ± 1.9 mg/dL at day 1 and 4.2 ± 2.8 mg/dL at day 7 p 0.0001), while serum calcium and phosphaturia increased [0.5 g/day (0.1-1.3) at day 1 vs 0.63 g/day (0.4-0.88) at day 7 p 0,015] during the follow up. Interestingly, there were no changes in phosphorus fractional excretion (PEF) (table 1). In subgroup analysis recipients of living donors showed lower phosphatemia (4.8 mg/dL vs 6.7 mg/dL at day 1, p 0.000, and 2,2 mg/dL vs 4.8 mg/dL at day 7, p 0.000) probably related to the greater GFR and substantially higher phosphaturia [1.44 g/day (0.81-1.69) vs 0.29 (0.8-0.76) at day 1, p 0.000], on the other hand, PEF was lower when compared to deceased donors recipients (DDR). Among DDR (n:46), those without delayed graft function (n:29) (DGF) had higher GFR and phosphaturia during the whole study, [a greater phosphorus excretion was statistically significant at day 1 and 3: 0.54 g/day vs 0.10 g/day p 0.000 at day 1, 0.85 g/day vs 0.10 g/day at day 3, 0.77 g/day vs 0.47 g/day p 0.09 at day 5, 0.71 g/day vs 0.42 g/day p 0.07 at day 7 (table 2)], and there was no difference regarding PEF compared to those with DGF. Patients were divided by tertiles of iPTH, and there were no differences in any measured aspect and also no correlation between iPTH and phosphaturia.

Conclusion: In this study we observed how phosphaturia improves with the recovery of GFR while PEF remains unchanged. This finding suggests that the tubules and interstitium, together with the hormonal context might not play a significant role in phosphorus handling at this point when the organ is still regaining function.