Association of calcium, phosphate and parathyroid hormone with renal allograft function: a retrospective cohort study

Bone and Mineral Disorder in Renal Transplant Patients: Overview of Pathology, Clinical, and Therapeutic Aspects

Renal transplantation (RTx) allows us to obtain the resolution of the uremic status but is not frequently able to solve all the metabolic complications present during end-stage renal disease. Mineral and bone disorders (MBDs) are frequent since the early stages of chronic kidney disease (CKD) and strongly influence the morbidity and mortality of patients with CKD. Some mineral metabolism (MM) alterations can persist in patients with RTx (RTx-p), as well as in the presence of complete renal function recovery. In those patients, anomalies of calcium, phosphorus, parathormone, fibroblast growth factor 23, and vitamin D such as bone and vessels are frequent and related to both pre-RTx and post-RTx specific factors. Many treatments are present for the management of post-RTx MBD. Despite that, the guidelines that can give clear directives in MBD treatment of RTx-p are still missed. For the future, to obtain an ever-greater individualisation of therapy, an increase of the evidence, the specificity of international guidelines, and more uniform management of these anomalies worldwide should be expected. In this review, the major factors related to post-renal transplant MBD (post-RTx-MBD), the main mineral metabolism biochemical anomalies, and the principal treatment for post-RTx MBD will be reported.

Calcium and phosphate levels after kidney transplantation and long-term patient and allograft survival

Background

Non-traditional cardiovascular risk factors, including calcium and phosphate derangement, may play a role in mortality in renal transplant. The data regarding this effect are conflicting. Our aim was to assess the impact of calcium and phosphate derangements in the first 90 days post-transplant on allograft and recipient outcomes.

Methods

We performed a retrospective cohort review of all-adult, first renal transplants in the Republic of Ireland between 1999 and 2015. We divided patients into tertiles based on serum phosphate and calcium levels post-transplant. We assessed their effect on death-censored graft survival and all-cause mortality. We used Stata for statistical analysis and did survival analysis and spline curves to assess the association.

Results

We included 1525 renal transplant recipients. Of the total, 86.3% had hypophosphataemia and 36.1% hypercalcaemia. Patients in the lowest phosphate tertile were younger, more likely female, had lower weight, more time on dialysis, received a kidney from a younger donor, had less delayed graft function and better transplant function compared with other tertiles. Patients in the highest calcium tertile were younger, more likely male, had higher body mass index, more time on dialysis and better transplant function. Adjusting for differences between groups, we were unable to show any difference in death-censored graft failure [phosphate = 1.14, 95% confidence interval (CI) 0.92–1.41; calcium = 0.98, 95% CI 0.80–1.20] or all-cause mortality (phosphate = 1.10, 95% CI 0.91–1.32; calcium = 0.96, 95% CI 0.81–1.13) based on tertiles of calcium or phosphate in the initial 90 days.

Conclusions

Hypophosphataemia and hypercalcaemia are common occurrences post-kidney transplant. We have identified different risk factors for these metabolic derangements. The calcium and phosphate levels exhibit no independent association with death-censored graft failure and mortality.

Hyperparathyroidism and increased fractional excretion of phosphate predict allograft loss in long-term kidney transplant recipients

BACKGROUND

After kidney transplantation, fibroblast growth factor-23 (FGF-23) normally returns to baseline within 1 year whereas hyperparathyroidism persists in most kidney transplant (KT) recipients. As a result, serum phosphate remains relatively low in association with increased serum calcium and urinary phosphate excretion when compared to chronic kidney disease patients. The relationship between mineral metabolism and outcomes in long-term KT recipients has not been extensively studied. This study investigated whether the alteration in mineral metabolism influenced graft survival in long-term KT recipients.

METHODS

This study included 273 KT recipients after 1 year of transplantation. Mineral parameters were obtained at the time of enrolment and patients were followed prospectively for an average of 71 months.

RESULTS

Graft loss (death-censored) occurred in 41 (15%) patients. In univariate analysis, deceased donor transplantation, decreased serum albumin and estimated glomerular filtration rate, increased serum phosphate, parathyroid hormone (PTH), FGF-23 and fractional excretion of phosphate (FePi) predicted future allograft loss. After adjustments for cardiovascular disease risk factors, donor type, dialysis vintage, serum albumin and allograft function, only increased PTH and FePi remained associated with the outcome. Relationships between increased serum phosphate and FGF-23 with graft survival were lost after adjustments. Adjusted survival curves revealed the association between PTH > 90 pg/mL and FePi > 20% with worse graft survival.

CONCLUSIONS

Hyperparathyroidism and increased FePi predicted allograft loss in long-term KT recipients.

Natural History of Serum Calcium and Parathyroid Hormone Following Renal Transplantation

BACKGROUND

Hyperparathyroidism is common in end-stage renal disease. It often persists following renal transplantation (RTx) and remains elusive to manage due to the lack of evidence base. We therefore present observational data describing the natural history and management of hypercalcemia and hyperparathyroidism following RTx.

METHODS

Single-center experience of 216 adult patients undergoing kidney transplantation between January 1, 2011, and December 31, 2012. Data included calcium and parathyroid hormone (PTH) pretransplant and post-transplant at 1, 13, 26, and 52 weeks. Hyperparathyroidism management modalities were also noted.

RESULTS

Persistent hyperparathyroidism (secondary/tertiary) following transplantation was observed in 71 (32.9%) patients. Mean PTH level decreased in the first 3 months post-RTx (3.95 ± 0.14 vs 3.61 ± 0.13 pmol/L; P < .01). Thereafter it remained relatively static until 1 year post-RTx (3.39 ± 0.14 pmol/L). Mean adjusted calcium level rose in the 3 months post-RTx and then remained largely unchanged until 1 year (2.39 ± 0.2 mmol/L, 2.49 ± 0.21 mmol/L, 2.47 ± 0.23 mmol/L at pretransplant, 3 months, and 12 months, respectively). Cinacalcet use pretransplant was significantly associated with reduced post-transplant hyperparathyroidism (15% vs 4.7% respectively, P = .006). There was no association between PTH (3.62 ± 4.16 mmol/L) or adjusted calcium levels (2.51 ± .17 mmol/L) at 6 months and estimated glomerular filtration rate at 1 year (r² = 0.16 and r² = 0.23, respectively).

CONCLUSION

Tertiary hyperparathyroidism following RTx is common. However, no association was observed between either post-transplant PTH or adjusted calcium and graft function.

Phosphocalcic Markers and Calcification Propensity for Assessment of Interstitial Fibrosis and Vascular Lesions in Kidney Allograft Recipients

Renal interstitial fibrosis and arterial lesions predict loss of function in chronic kidney disease. Noninvasive estimation of interstitial fibrosis and vascular lesions is currently not available. The aim of the study was to determine whether phosphocalcic markers are associated with, and can predict, renal chronic histological changes. We included 129 kidney allograft recipients with an available transplant biopsy in a retrospective study. We analyzed the associations and predictive values of phosphocalcic markers and serum calcification propensity (T50) for chronic histological changes (interstitial fibrosis and vascular lesions). PTH, T50 and vitamin D levels were independently associated to interstitial fibrosis. PTH elevation was associated with increasing interstitial fibrosis severity (r = 0.29, p = 0.001), while T50 and vitamin D were protective (r = -0.20, p = 0.025 and r = -0.23, p = 0.009 respectively). On the contrary, fibroblast growth factor 23 (FGF23) and Klotho correlated only modestly with interstitial fibrosis (p = 0.045) whereas calcium and phosphate did not. PTH, vitamin D and T50 were predictors of extensive fibrosis (AUC: 0.73, 0.72 and 0.68 respectively), but did not add to renal function prediction. PTH, FGF23 and T50 were modestly predictive of low fibrosis (AUC: 0.63, 0.63 and 0.61) but did not add to renal function prediction. T50 decreased with increasing arterial lesions (r = -0.21, p = 0.038). The discriminative performance of T50 in predicting significant vascular lesions was modest (AUC 0.61). In summary, we demonstrated that PTH, vitamin D and T50 are associated to interstitial fibrosis and vascular lesions in kidney allograft recipients independently of renal function. Despite these associations, mineral metabolism indices do not show superiority or additive value to fibrosis prediction by eGFR and proteinuria in kidney allograft recipients, except for vascular lesions where T50 could be of relevance.

Mineral and bone disorder after kidney transplantation

After successful kidney transplantation, accumulated waste products and electrolytes are excreted and regulatory hormones return to normal levels. Despite the improvement in mineral metabolites and mineral regulating hormones after kidney transplantation, abnormal bone and mineral metabolism continues to present in most patients. During the first 3 mo, fibroblast growth factor-23 (FGF-23) and parathyroid hormone levels decrease rapidly in association with an increase in 1,25-dihydroxyvitamin D production. Renal phosphate excretion resumes and serum calcium, if elevated before, returns toward normal levels. FGF-23 excess during the first 3-12 mo results in exaggerated renal phosphate loss and hypophosphatemia occurs in some patients. After 1 year, FGF-23 and serum phosphate return to normal levels but persistent hyperparathyroidism remains in some patients. The progression of vascular calcification also attenuates. High dose corticosteroid and persistent hyperparathyroidism are the most important factors influencing abnormal bone and mineral metabolism in long-term kidney transplant (KT) recipients. Bone loss occurs at a highest rate during the first 6-12 mo after transplantation. Measurement of bone mineral density is recommended in patients with estimated glomerular filtration rate > 30 mL/min. The use of active vitamin D with or without bisphosphonate is effective in preventing early post-transplant bone loss. Steroid withdrawal regimen is also beneficial in preservation of bone mass in long-term. Calcimimetic is an alternative therapy to parathyroidectomy in KT recipients with persistent hyperparathyroidism. If parathyroidectomy is required, subtotal to near total parathyroidectomy is recommended. Performing parathyroidectomy during the waiting period prior to transplantation is also preferred in patients with severe hyperparathyroidism associated with hypercalcemia.