Nathalie Lepage

Estimating Glomerular Filtration Rate in Kidney Transplantation: A Comparison between Serum Creatinine and Cystatin C–Based Methods

Accurate measurement of GFR is critical for the evaluation of new therapies and the care of renal transplant recipients. Although not accurate in renal transplantation, GFR is often estimated using creatinine-based equations. Cystatin C is a marker of GFR that seems to be more accurate than creatinine. Equations to predict GFR based on the serum cystatin C concentration have been developed, but their accuracy in transplantation is unknown. GFR was estimated using four equations (Filler, Le Bricon, Larsson, and Hoek) that are based on serum cystatin C and seven equations that are based on serum creatinine in 117 adult renal transplant recipients. GFR was measured using radiolabeled diethylenetriaminepentaacetic acid (99mTc-DTPA), and the bias, precision, and accuracy of each equation were determined. The mean (99m)Tc-DTPA GFR was 58 +/- 23 ml/min per 1.73 m(2). The cystatin C-based equations of Filler and Le Bricon had the lowest bias (-1.7 and -3.8 ml/min per 1.73 m2), greatest precision (11.4 and 11.8 ml/min per 1.73 m2), and highest accuracy (87 and 89% within 30% of measured GFR, respectively). The cystatin C equations remained accurate even when the measured GFR was >60 ml/min per 1.73 m2. The creatinine-based equations were not as accurate, with only 53 to 80% of estimates within 30% of measured GFR. Cystatin C-based equations are more accurate at predicting GFR in renal transplant recipients than traditional creatinine-based equations. Further prospective studies with repetitive measurement of cystatin C are needed to determine whether cystatin C-based estimates of GFR will be sufficiently accurate to monitor long-term allograft function.

Universal approach to pharmacokinetic monitoring of immunosuppressive agents in children

Abstract: Current data indicate that pharmacokinetic (PK) monitoring of cyclosporin microemulsion (CsA) should be performed using the 2‐h concentration (C2), that tacrolimus (Tac) is commonly monitored using the trough level, and mycophenolate mofetil (MMF) should be monitored using the 1‐h (C1), 2‐h (C2) and 6‐h (C6) concentrations. The three differing time‐point requirements are cumbersome, and we aimed to develop universal guidelines for all three drugs using a large number of full PK profiles in children. One‐hundred and twenty two stable pediatric patients, receiving either CsA (165 PK profiles, 69 patients, 24 with concomitant MMF) or Tac (122 PK profiles, 53 patients, 18 with MMF) were analyzed retrospectively. Pearson r for the CsA C2 was 0.90 [95% confidence interval(CI): 0.86–0.92], for Tac C2 r was 0.86 (95% CI: 0.80–0.90), and for MPA C2 r was 0.77 (95% CI: 0.68–0.83), respectively. For MPA, at least three time‐points are required to accurately estimate the area under the concentration–time curve (AUC), and C1, C2 and C6 serve as best markers. Excellent AUC estimations could be obtained from a limited sampling strategy from C1, C2 and C6 or C0, C1, C2 and C4 with clinically acceptable errors for all three drugs. The AUC can be estimated with great precision by using an identical approach for all three drugs. Target AUCs for a given time‐point after transplantation remain to be established.

Cystatin-C and beta trace protein as markers of renal function in pregnancy

Objective  To assess the validity of Cystatin‐C (Cys‐C) and beta trace protein (BTP) as clinical markers of glomerular filtration rate (GFR) in pregnant women.

Effect of cyclosporine on Mycophenolic acid area under the concentration-time curve in pediatric kidney transplant recipients

Mycophenolic acid (MPA), the active compound of mycophenolate mofetil (MMF), shows substantial interindividual and intraindividual variability. It was recently shown that in vitro calcineurin inhibitors alter the bioavailability of MPA by dose-dependent inhibition of MPA glucuronidation. The authors retrospectively analyzed full 10-point profiles for both MPA and cyclosporine (CyA) in 23 pediatric patients receiving MMF and cyclosporine microemulsion (Neoral; Novartis Pharmaceuticals Canada; Dorval, Quebec, Canada). Mycophenolic acid was measured using a commercially available EMIT (Novartis Pharmaceuticals, Canada) assay. As the majority of patients were treated with low doses of cyclosporine after adding MMF, the area under the concentration–time curve (AUC) for cyclosporine showed a wide scatter ranging from 296 to 6400 ng × h/mL. The mean cyclosporine dose was 100 ± 76 mg/m 2 per day (range: 28 to 331). There was no correlation between MPA AUC and MPA dose, and there was substantial interindividual variation. However, there was a significant negative correlation between dose-normalized MPA AUC and cyclosporine AUC (r 2 = 0.23, p < 0.0220). When dividing the MPA profiles into two groups (11 and 12 patients) with a CyA AUC less than or greater than 1600 ng × h/mL, there was a significantly higher 8-hour concentration in the patients with the lower CyA AUC, secondary to a higher second peak. The data demonstrate that the cyclosporine AUC is a determining factor for the MPA AUC and that MPA dose should be reduced when cyclosporine dose is reduced to achieve the same MPA AUC. The significantly higher peak in the group with a lower CyA profile supports the concept of a dose-dependent cyclosporine-induced inhibition of MPA glucuronidation.

Estimating GFR using serum beta trace protein: accuracy and validation in kidney transplant and pediatric populations

The limitations of estimates of glomerular filtration rate (GFR) based only on serum creatinine measurements have spurred an interest in more sensitive markers of GFR. Beta-trace protein (BTP), a low-molecular-weight glycoprotein freely filtered through the glomerular basement membrane and with minimal non-renal elimination, may be such a marker. We have recently derived two GFR estimation equations based on BTP. To validate these equations, we measured BTP and the plasma clearance of (99)mTc-DTPA in 92 adult kidney transplant recipients and 54 pediatric patients with impaired kidney function. GFR was estimated using the serum creatinine-based Modification of Diet in Renal Disease (MDRD) Study equation for adults, the Schwartz and updated Schwartz equations in children, and 4 novel BTP-derived equations (our 2 equations and 2 proposed by Poge). In adults, our BTP-based equations had low median bias and high accuracy such that 89-90% of estimates were within 30% of measured GFR. In children, the median bias of our 2 equations was low and accuracy was high such that 78-83% of estimates were within 30% of measured GFR. These results were an improvement compared to the MDRD and Schwartz equations, both of which had high median bias and reduced accuracy. The updated Schwartz equation also performed well.

Preliminary reference intervals for cystatin C and beta-trace protein in preterm and term neonates ☆

OBJECTIVE To determine the reference intervals for serum cystatin C (CysC) and beta-trace protein (BTP) as markers of renal function in preterm and term neonates. DESIGN AND METHODS Blood samples of 128 neonates (34% female) admitted to the NICU were analyzed to determine the levels of serum creatinine (enzymatically), CysC and BTP (nephelometric, Siemens Health Care). RESULTS The reference intervals, categorized by age, were reported for the 128 neonates. Median (lower/upper limit) BTP were 1.85 (0.57/3.16) and 1.27 (0.51/2.07) mg/L on days 1 and 3. In keeping with maturation of renal function after birth, CysC and BTP fell from days one to day three after birth, whereas creatinine did not. CONCLUSION Our data provides reference intervals for the levels of creatinine, CysC, and BTP in neonates on days 1 and 3 after birth and demonstrates that CysC and BTP reflect neonatal renal function, whereas creatinine reflects maternal renal function.

Intra-individual variation of cystatin C and creatinine in pediatric solid organ transplant recipients

Abstract:  There is controversy about the feasibility of cystatin C (CysC) as a marker of glomerular filtration rate (GFR) post‐transplant (Tx). We studied intra‐patient variability of CysC in comparison with serum creatinine (SCr) in 20 children (11 males, mean age 11.5 ± 6.4 yr) with solid organ transplants (14 kidney, four liver, and two combined liver + kidney transplants). The mean age at Tx was 7.0 ± 5.6 yr. A total of 178 simultaneous SCr and CysC measurements (median 8 per patient) were analyzed. In addition, GFR was calculated using the Schwartz and a novel CysC‐based formula. Intra‐individual coefficient of variations (CV) was calculated as ratio of standard deviation over mean. The mean CV was significantly lower for SCr (7.71 ± 4.16%) when compared with CysC (10.27 ± 4.87, p = 0.04), but was no longer significantly different when excluding patients with a bladder augment. The CV of the GFR estimated by Schwartz formula (7.44 ± 3.77) was significantly lower than GFR calculated from CysC (12.52 ± 7.37), p = 0.001. The mean ratio between the Schwartz GFR and the GFR calculated from CysC was 102.6 ± 12.8%, not significantly different from 100% (p = 0.3796). The only potential confounding factors to explain increased CV after Tx were gender and bladder augmentation, whereas calcineurin inhibitors or steroids did not influence CV. With the limitation of a small number of subjects, our data suggest that the CysC and the CysC‐calculated GFR is equivalent but not better than SCr and Schwartz formula. We therefore conclude that measurement of CysC can be used for longitudinal intra‐individual follow‐up of renal function post‐Tx.

Effect of Clinical Variables and Immunosuppression on Serum Cystatin C and Beta-Trace Protein in Kidney Transplant Recipients

BACKGROUND Cystatin C and beta-trace protein (BTP) are low-molecular-weight proteins that have generated interest as alternative endogenous markers of glomerular filtration rate (GFR). Studies examining the effect of demographic, biometric, clinical, and biochemical variables on cystatin C levels have yielded conflicting results, perhaps because of the reliance on inferior methods of GFR determination. The aim of this study is to examine the independent effect of various clinical parameters on serum concentrations of creatinine, cystatin C, and BTP in kidney transplant recipients. STUDY DESIGN Cross-sectional study. SETTING & PARTICIPANTS 207 kidney transplant recipients with stable kidney function. PREDICTORS GFR, age, race, sex, body mass index, albumin level, proteinuria, smoking status, prednisone, and calcineurin inhibitor and mycophenolate mofetil use. OUTCOMES & MEASUREMENTS Multiple linear regression analysis was used to examine the relationship between predictor variables and cystatin C, BTP, and creatinine levels. GFR was measured by using technetium 99m-radiolabeled diethylenetriaminepentaacetic acid clearance. RESULTS After adjusting for GFR, cystatin C and BTP levels were significantly lower in women compared with men. Greater albumin concentration was associated with significantly lower cystatin C and BTP concentrations. There was a statistically significant, but clinically small, association between body mass index and cystatin C level, but no association between the other demographic variables or medications analyzed. LIMITATIONS Predominantly white population; results may not be applicable to other racial groups. CONCLUSION Important nonrenal factors can influence BTP and cystatin C concentrations and need to be considered when interpreting BTP and cystatin C values in kidney transplant patients.

Adding Sirolimus to Tacrolimus-Based Immunosuppression in Pediatric Renal Transplant Recipients Reduces Tacrolimus Exposure

In adult renal recipients, coadministration of tacrolimus (TAC) and sirolimus (SIR) results in reduced exposure to TAC at SIR doses of 2 mg/day. Eight pediatric renal recipients (median age at transplant 2.0 years, range: 1.2–12.9 years) were converted to TAC‐ and SIR‐based immunosuppression as a rescue therapy. All patients had biopsy‐proven chronic allograft nephropathy. TAC levels were measured using a commercially available EMIT assay and SIR levels with a newly developed assay based on the LC‐MS MS technology. SIR was started at 0.13 ± 0.05 mg/kg/day (3.51 ± 1.26 mg/m2/day) in two divided doses. TAC was given at 0.14 ± 0.09 mg/kg/day, resulting in a trough level of 6.3 ± 2.5 ng/mL. After the addition of SIR, the median dose required to keep TAC blood trough concentrations within the target range increased by 71.2% (range: 21.9–245.4%), dose‐normalized TAC exposure (AUC) decreased to 67.1% and the dose‐normalized Cmax, a surrogate for absorption rate, to 53.8% (both geometric means) while terminal half‐life (t1/2), a pharmacokinetic parameter characterizing systemic elimination, remained unchanged (p < 0.93). Adding SIR to TAC‐based immunosuppression in young pediatric renal transplant recipients results in a significant decrease of TAC exposure. TAC trough levels should be monitored frequently.

Outcome validation of the Beckman Coulter access analyzer in a second-trimester Down syndrome serum screening application.

BACKGROUND Mid-trimester maternal serum alpha-fetoprotein (AFP) and unconjugated estriol (uE3) are 30% lower and human chorionic gonadotropin (hCG) is twofold higher in Down syndrome pregnancies compared with unaffected pregnancies. In maternal serum screening, patient-specific risks are calculated using published gaussian frequency distribution parameters for these three markers obtained with previously available immunoassays. New immunoassays must generate similar distribution parameters if the accuracy of assigned risks and overall performance of prenatal screening are to be maintained. METHODS Agreement between the Beckman Coulter Access and the Bayer Immuno 1 assays for AFP and hCG and the Amersham Amerlex-M RIA for uE3 was assessed in 558 fresh sera. Precision was measured over 6 weeks. Median concentrations were calculated by regression of 568 Caucasian singleton pregnancy samples against gestational age in days. Frozen mid-trimester sera from 44 confirmed Down syndrome singleton pregnancies (cases) were selected without conscious bias for reanalysis, and each case was matched with five control specimens from unaffected pregnancies. Serum markers were expressed as the multiple of the median (MoM) concentration derived from the control samples, corrected for maternal weight and converted to their log-equivalent values. Normality was assessed using probability plots and the Shapiro-Wilk W-test. Gaussian distribution parameters were compared with established values, and Down syndrome risk calculations were assessed with a commonly used risk algorithm. RESULTS The Access AFP and hCG assays had consistent proportional agreement with the established assays, whereas agreement between the uE3 methods was less consistent. Analytical imprecision was 3-6% at mid-trimester concentrations. Normal distributions were obtained for the log MoM values of all three markers in both the Down syndrome and unaffected populations, and their gaussian distribution parameters compared well with established values. The performance of the Access assays in an established trivariate risk algorithm for Down syndrome was equal to the performance exhibited by traditional methods. CONCLUSION The Beckman Coulter Access analyzer provides valid mid-trimester serum AFP, uE3, and hCG results and risk assessments when applied in a prenatal Down syndrome screening service.