Tubular maximum phosphate reabsorption capacity in living kidney donors is independently associated with one-year recipient GFR

Uncertainty in measurement and the renal tubular reabsorption of phosphate

OBJECTIVES

The ratio of tubular maximum reabsorption of phosphate to glomerular filtration rate (TmP/GFR) is used to evaluate renal phosphate transport. TmP/GFR is most probably calculated using the formula described by Kenny and Glen or obtained from the nomogram described by Walton and Bijvoet. Even though the calculation itself is well described, no attention has been given to its measurement uncertainty (MU). The aim of this study is to provide a procedure for evaluating the MU of the Kenny and Glen formula; a procedure which is based on the Evaluation of measurement data - Guide to the expression of uncertainty in measurement (GUM).

METHODS

TmP/GFR is a quantity value calculated from the input of measured values for serum (plasma) phosphate and creatinine, plus measured values of urine phosphate and creatinine. Given the measurement uncertainty associated with these input quantities, the GUM describes the mathematical procedures required to determine the uncertainty of the calculated TmP/GFR. From a medical laboratory perspective, these input uncertainties are the standard deviations of the respective internal quality control estimates for serum and urine phosphate, plus serum and urine creatinine.

RESULTS

Based on representative measurements for the input quantities and their associated standard uncertainties, the expanded relative uncertainty for a calculated TmP/GFR is approximately 3.0-4.5 %.

CONCLUSIONS

With the continued relevance of the TmP/GFR procedure and the use of creatinine clearance as an estimate of GFR, the addition of an uncertainty estimate is important as an adjunct to this diagnostic procedure.

Low bone mineralization in phenylketonuria may be due to undiagnosed metabolic acidosis

Background

Dietary intervention is to date the mainstay treatment to prevent toxic phenylalanine (Phe) accumulation in PKU patients. Despite success preventing central nervous system damage, there is increasing evidence of possible other unfavorable outcomes affecting other systems, e.g. kidney and bone; underlying mechanisms are yet to be fully elucidated.

Methods

This observational, cross-sectional and descriptive study investigated 20 adult with PKU evaluating biochemical parameters, BMD measurements and extrapolating data from 3-days food records and protein substitutes (PS) and special low protein foods (SLPF) composition.

Results

Blood gas venous analysis (VBG) indices were indicative of metabolic acidosis in 60% of PKU patients and VBG pH significantly correlated with BMD's Z-score (p-value = 0.022) even if its overall mean was in range (-1.29). Low bone mineral density for chronological age (Z-score < - 2.0) was found in 4 patients (20%). Indices of kidney function were not impaired. All used PS had a moderate excess of acidity, while SLPF were alkalizing and type/variety of consumed vegetables did not determine significant changes in acid-base equilibrium. Total intakes of potassium and magnesium were lower than expected.

Discussion

PKU patients seem to be at risk of metabolic acidosis, directly linked to possible low bone mineralization. This may be related to the acidic composition of PS, potentially capable of acidifying the entire diet. Reported low intakes of potassium and magnesium may be relevant to these observations. Further studies are needed to better address these topics.

Recommendations for living donor kidney transplantation

This Guide for Living Donor Kidney Transplantation (LDKT) has been prepared with the sponsorship of the Spanish Society of Nephrology (SEN), the Spanish Transplant Society (SET), and the Spanish National Transplant Organization (ONT). It updates evidence to offer the best chronic renal failure treatment when a potential living donor is available. The core aim of this Guide is to supply clinicians who evaluate living donors and transplant recipients with the best decision-making tools, to optimise their outcomes. Moreover, the role of living donors in the current KT context should recover the level of importance it had until recently. To this end the new forms of incompatible HLA and/or ABO donation, as well as the paired donation which is possible in several hospitals with experience in LDKT, offer additional ways to treat renal patients with an incompatible donor. Good results in terms of patient and graft survival have expanded the range of circumstances under which living renal donors are accepted. Older donors are now accepted, as are others with factors that affect the decision, such as a borderline clinical history or alterations, which when evaluated may lead to an additional number of transplantations. This Guide does not forget that LDKT may lead to risk for the donor. Pre-donation evaluation has to centre on the problems which may arise over the short or long-term, and these have to be described to the potential donor so that they are able take them into account. Experience over recent years has led to progress in risk analysis, to protect donors' health. This aspect always has to be taken into account by LDKT programmes when evaluating potential donors. Finally, this Guide has been designed to aid decision-making, with recommendations and suggestions when uncertainties arise in pre-donation studies. Its overarching aim is to ensure that informed consent is based on high quality studies and information supplied to donors and recipients, offering the strongest possible guarantees.

Natural History of Bone Disease following Kidney Transplantation

BACKGROUND

Knowledge of the effect of kidney transplantation on bone is limited and fragmentary. The aim of this study was to characterize the evolution of bone disease in the first post-transplant year.

METHODS

We performed a prospective, observational cohort study in patients referred for kidney transplantation under a steroid-sparing immunosuppressive protocol. Bone phenotyping was done before, or at the time of, kidney transplantation, and repeated at 12 months post-transplant. The phenotyping included bone histomorphometry, bone densitometry by dual-energy x-ray absorptiometry, and biochemical parameters of bone and mineral metabolism.

RESULTS

Paired data were obtained for 97 patients (median age 55 years; 72% male; 21% of patients had diabetes). Bone turnover remained normal or improved in the majority of patients (65%). Bone histomorphometry revealed decreases in bone resorption (eroded perimeter, mean 4.6% pre- to 2.3% post-transplant; P<0.001) and disordered bone formation (fibrosis, 27% pre- versus 2% post-transplant; P<0.001). Whereas bone mineralization was normal in all but one patient pretransplant, delayed mineralization was seen in 15% of patients at 1 year post-transplant. Hypophosphatemia was associated with deterioration in histomorphometric parameters of bone mineralization. Changes in bone mineral density were highly variable, ranging from -18% to +17% per year. Cumulative steroid dose was related to bone loss at the hip, whereas resolution of hyperparathyroidism was related to bone gain at both spine and hip.

CONCLUSIONS

Changes in bone turnover, mineralization, and volume post-transplant are related both to steroid exposure and ongoing disturbances of mineral metabolism. Optimal control of mineral metabolism may be key to improving bone quality in kidney transplant recipients.

CLINICAL TRIAL REGISTRY NAME AND REGISTRATION NUMBER

Evolution of Bone Histomorphometry and Vascular Calcification Before and After Renal Transplantation, NCT01886950.

Assessment of Proximal Tubular Function by Tubular Maximum Phosphate Reabsorption Capacity in Heart Failure

BACKGROUND AND OBJECTIVES

The estimated glomerular filtration rate (eGFR) is a crucial parameter in heart failure. Much less is known about the importance of tubular function. We addressed the effect of tubular maximum phosphate reabsorption capacity (TmP/GFR), a parameter of proximal tubular function, in patients with heart failure.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

We established TmP/GFR (Bijvoet formula) in 2085 patients with heart failure and studied its association with deterioration of kidney function (>25% eGFR decrease from baseline) and plasma neutrophil gelatinase-associated lipocalin (NGAL) doubling (baseline to 9 months) using logistic regression analysis and clinical outcomes using Cox proportional hazards regression. Additionally, we evaluated the effect of sodium-glucose transport protein 2 (SGLT2) inhibition by empagliflozin on tubular maximum phosphate reabsorption capacity in 78 patients with acute heart failure using analysis of covariance.

RESULTS

Low TmP/GFR (<0.80 mmol/L) was observed in 1392 (67%) and 21 (27%) patients. Patients with lower TmP/GFR had more advanced heart failure, lower eGFR, and higher levels of tubular damage markers. The main determinant of lower TmP/GFR was higher fractional excretion of urea (P<0.001). Lower TmP/GFR was independently associated with higher risk of plasma NGAL doubling (odds ratio, 2.20; 95% confidence interval, 1.05 to 4.66; P=0.04) but not with deterioration of kidney function. Lower TmP/GFR was associated with higher risk of all-cause mortality (hazard ratio, 2.80; 95% confidence interval, 1.37 to 5.73; P=0.005), heart failure hospitalization (hazard ratio, 2.29; 95% confidence interval, 1.08 to 4.88; P=0.03), and their combination (hazard ratio, 1.89; 95% confidence interval, 1.07 to 3.36; P=0.03) after multivariable adjustment. Empagliflozin significantly increased TmP/GFR compared with placebo after 1 day (P=0.004) but not after adjustment for eGFR change.

CONCLUSIONS

TmP/GFR, a measure of proximal tubular function, is frequently reduced in heart failure, especially in patients with more advanced heart failure. Lower TmP/GFR is furthermore associated with future risk of plasma NGAL doubling and worse clinical outcomes, independent of glomerular function.

TmP/GFR reference values from childhood to adulthood in the era of IDMS-standardized creatinine values

BACKGROUND

The assessment of phosphate homeostasis in clinical practice relies on circulating phosphate levels but also on phosphate tubular reabsorption, ideally assessed using the Tubular maximum Phosphate Reabsorption per Glomerular Filtration Rate (TmP/GFR). TmP/GFR reference values were established before the onset of IDMS-standardized creatinine assays, and thus need to be updated. Our objective is to provide reference values for TmP/GFR from childhood to adulthood, using the gold-standard of GFR assessment and IDMS-standardized creatinine values.

METHODS

We retrospectively analyzed all the inulin and iohexol clearances (mGFR) performed in children and in adults screened for a living-donation in our unit since the beginning of IDMS-creatinine assays. TmP/GFR was calculated on a fasting sample, using the conventional formula without correction for TRP in subjects below 19 years of age.

RESULTS

A total of 2051 subjects (1711 children, 340 adults), aged from 1.9 to 73.4 years with normal GFR, normal phosphate and normal calcium levels, was included for TmP/GFR analysis. As expected, there was a progressive decrease along puberty in both genders of plasma phosphate and TmP/GFR, the decrease occurring earlier in girls. After the age of 19, there was a stabilization of plasma phosphate and TmP/GFR levels until the age of 55, phosphate levels and TmP/GFR being slightly lower in men than in women.

CONCLUSION

We present the largest cohort describing TmP/GFR reference values in the era of IDMS-standardized creatinine assays. We believe that these data will help physicians to better diagnose and manage patients with abnormal phosphate metabolism in daily clinical routine.