Creatinine excretion in continuous peritoneal dialysis: a systematic error of the Cockroft-Gault formula

A Population-Specific Formula Predicting Creatinine Excretion in Continuous Peritoneal Dialysis

Objective

The Cockroft–Gault formula was shown to systematically overestimate the decline in creatinine excretion with age in continuous peritoneal dialysis (CPD) patients and is, therefore, not suitable for studying creatinine excretion. The purpose of the present study was to develop and test a population-specific formula predicting average creatinine excretion in CPD.

Methods

Creatinine excretion in urine plus dialysate was measured in 925 CPD patients. Forty patients were excluded because of evidence of noncompliance. The remaining 885 subjects were randomly grouped into a derivation group ( n = 432) and a validation group ( n = 453). Stepwise multiple linear regression models were used to predict creatinine excretion in the derivation group. The candidate variables, chosen because they were previously shown to be predictors of creatinine excretion in CPD, included weight (W), age (A), gender (G), diabetes (D), and interaction terms between these four variables. Estimates of creatinine excretion from the best-fit regression formula (CrExcr1) and from the Cockroft–Gault formula (CrExcr2) were compared to creatinine excretion (CrExcr) in the validation group.

Results

The best-fit regression model in the derivation group included all four candidate variables (W, A, G, D), but no interaction terms. This model was as follows: CrExcr1 = 302.150 – 4.380A + 171.234G – 39.041D + 11.730W ( r 2 = 0.477, p < 0.001). In the validation set, CrExcr = –15.795 + 0.988CrExcr1 ( r2 = 0.447, p < 0.001), and CrExcr = –303.823 + 0.732CrExcr2 ( r2 = 0.340, p < 0.001). When the differences between measured and predicted creatinine excretion did not take into account the sign of each individual difference, CrExcr – CrExcr1 = 201 ± 156 mg/24 hours, and CrExcr – CrExcr2 = 235 ± 174 mg/24 hr ( p < 0.001) in the validation group. When the sign of the difference was taken into account, CrExcr – CrExcr1 = –28 ± 149 mg/24 hr, and CrExcr – CrExcr2 = 63 ± 295 mg/24 hr ( p < 0.001).

Conclusions

A population-specific formula predicting creatinine excretion in CPD was derived. This formula has greater accuracy than the Cockroft–Gault formula and can be used in studies of creatinine excretion in CPD.

Nutrition Indices in Obese Continuous Peritoneal Dialysis Patients with Inadequate and Adequate Urea Clearance

♦ Objective

To test whether better nutrition is associated more with adequate urea clearance than with inadequate urea clearance in obese patients on continuous peritoneal dialysis (CPD).

♦ Design

Retrospective analysis of clearance and nutrition indices in obese CPD patients. Only obese patients were analyzed. Obesity was defined as a ratio of actual weight to desired weight (W/DW) ≥ 1.2. The dose of dialysis was considered adequate at weekly Kt/V urea ≥ 2.0. Small solute clearances and nutrition indices were compared between patients with weekly Kt/V urea < 2.0 and patients with weekly Kt/V urea ≥ 2.0 at the first clearance study.

♦ Setting

Four university-affiliated and two private dialysis units in Canada and the United States.

♦ Patients

A total of 270 CPD patients with W/DW ≥ 1.2 at the first clearance study.

♦ Results

Among the 270 obese CPD patients, 157 (58.1%) were underdialyzed (weekly Kt/V urea 1.66 ± 0.22) and 113 (41.9%) had adequate dialysis (weekly Kt/V urea 2.51 ± 0.47) at the first clearance study. Creatinine clearance values also differed between the underdialyzed and adequately dialyzed obese groups (55.6 ± 15.2 vs 87.6 ± 29.8 L/1.73 m2 weekly, respectively, p < 0.001). The underdialyzed group contained fewer women (39.5% vs 60.2%, p < 0.001) and more patients with anuria (35.0% vs 8.8%, p < 0.001), and had higher serum urea (20.7 ± 6.9 vs 18.2 ± 5.3 mmol/L, p = 0.001) and serum creatinine (974 ± 283 vs 734 ± 275 μmol/L, p < 0.001), marginally lower serum albumin (35.8 ± 5.2 vs 37.2 ± 6.4 g/L, p = 0.082), lower urea nitrogen excretion (5778 ± 2290 vs 7085 ± 2238 mg/24 hr, p < 0.001) and indices derived from urea nitrogen excretion (protein nitrogen appearance and normalized protein nitrogen appearance), and lower creatinine excretion (1034 ± 349 vs 1217 ± 432 mg/24 hr, p < 0.001) and indices derived from creatinine excretion (lean body mass normalized to actual or desired weight) than the adequately dialyzed group.

♦ Conclusion

Nutrition indices derived from urea nitrogen and creatinine excretion are worse in underdialyzed than in adequately dialyzed obese CPD patients. This finding may have clinical importance, despite the mathematical coupling between small solute clearances and excretion rates in cross-sectional studies, because of evidence from other studies that small solute excretion rate in cross-sectional studies is a robust independent predictor of outcome in CPD.

Estimated glomerular filtration rate -a more stable indicator than creatinine clearance in peritoneal dialysis practice

Objective: The usefulness of estimated glomerular filtration rate may not be restricted to pre-dialysis patients, since we reported that estimated glomerular filtration rate was well correlated with measured total creatinine clearance in peritoneal dialysis patients. To clarify the clinical usefulness of estimated glomerular filtration rate as a parameter for peritoneal dialysis adequacy, we retrospectively surveyed estimated glomerular filtration rate and total creatinine clearance in peritoneal dialysis patients treated at JA Toride Medical Center.

Patients and Methods: A total of 114 data sets of estimated glomerular filtration rate and total creatinine clearance from 21 PD patients treated at JA Toride Medical Center were collected from November 2010 to October 2011. The patients consisted of 15 men and six women with an average age of 66.6 ± 12.6 years (46–95 years old). The average number of samples was 5.4 ± 1.5 (2 to 7) per patient.

Results: The collected data showed less correlation of estimated glomerular filtration rate and total creatinine clearance (r. = 0.435) than that of a previous cross-sectional study (r. = 0.836). As reported in pre-dialysis patients, the differences between estimated glomerular filtration rate and total creatinine clearance were correlated with total creatinine excretion in urine and PD effluent (r. = 0.821). The differences were also correlated with normalized protein catabolic rate, which was one of the main determinant factors for total creatinine excretion (r. = 0.636). A similar tendency was apparently observed in one patient with poor compliance to diet therapy and fluctuating dietary intake. From the analysis of these data, serum creatinine seemed to fluctuate less possibly due to compensatory capacity of the residual renal function in small solute clearance.

Conclusions: Consequently, estimated glomerular filtration rate was turned out to be a more stable parameter than total creatinine clearance, which might be a desirable feature in long-term follow-up of peritoneal dialysis patients.

Sources of variation in estimates of lean body mass by creatinine kinetics and by methods based on body water or body mass index in patients on continuous peritoneal dialysis

OBJECTIVE

We identified factors that account for differences between lean body mass computed from creatinine kinetics (LBM(cr)) and from either body water (LBM(V)) or body mass index (LBM(BMI)) in patients on continuous peritoneal dialysis (CPD).

DESIGN

We compared the LBM(cr) and LBM(V) or LBM(BMI) in hypothetical subjects and actual CPD patients.

PATIENTS

We studied 439 CPD patients in Albuquerque, Pittsburgh, and Toronto, with 925 clearance studies.

INTERVENTION

Creatinine production was estimated using formulas derived in CPD patients. Body water (V) was estimated from anthropometric formulas. We calculated LBM(BMI) from a formula that estimates body composition based on body mass index. In hypothetical subjects, LBM values were calculated by varying the determinants of body composition (gender, diabetic status, age, weight, and height) one at a time, while the other determinants were kept constant. In actual CPD patients, multiple linear regression and logistic regression were used to identify factors associated with differences in the estimates of LBM (LBM(cr)<LBM(V), or LBM(cr)<LBM(BMI)).

MAIN OUTCOME MEASURE

We sought predictors of the differences LBM(V) - LBM(cr) and LBM(BMI) - LBM(cr).

RESULTS

Both LBM(V) (regardless of formula used to estimate V) and LBM(BMI) exceeded LBM(cr) in hypothetical subjects with average body compositions. The sources of differences between LBM estimates in this group involved differences in the coefficients assigned to gender, age, height, weight, presence or absence of diabetes, and serum creatinine concentration. In CPD patients, mean LBM(V) or LBM(BMI) exceeded mean LBM(cr) by 6.2 to 6.9 kg. For example, the LBM(V) obtained from one anthropometric formula was 50.4+/-10.4 kg and the LBM(cr) was 44.1+/-13.6 kg (P < .001), whereas among the 925 clearance studies, only 216 (23.3%) had LBM(cr)>LBM(V). The differences in determinants of body composition between groups with high versus low LBM(cr) were similar in hypothetical and actual CPD patients. Multivariate analysis in actual CPD patients identified serum creatinine, height, age, gender, weight, and body mass index as predictors of the differences LBM(V)-LBM(cr) and LBM(BMI)-LBM(cr).

CONCLUSIONS

Overhydration is not the sole factor accounting for the differences between LBM(cr) and either LBM(V) or LBM(BMI) in CPD patients. These differences also stem from the coefficients assigned to major determinants of body composition by the formulas estimating LBM.

The MDRD equation predicts peritoneal dialysis-delivered creatinine clearances from serum creatinine

BACKGROUND AND OBJECTIVES

The four- and six-variable Modified Diet in Renal Disease equations (MDRDEs) are empiric expressions that estimate GFR in patients with kidney disease. No method currently exists to directly compare MDRDE estimates of clearance by the failing biologic kidney to the clearances achieved by dialytic modalities.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Because the MDRDEs estimate GFR, which in turn determines much of the small molecule clearance by the biologic kidney, the aim of this study was to determine if the MDRDEs would predict the measured creatinine clearances (including residual renal clearance, if any) from the serum creatinine values alone in 454 peritoneal dialysis patients.

RESULTS

For the group, the four-variable MDRDE predicted a creatinine clearance of 6.5 +/- 3.7 ml/min/1.73 m(2) when measured clearance was from home collections of dialysate and urine was 6.8 +/- 3.1 ml/min/1.73 m(2). The presence or volume of residual urine had no effect on accuracy. The six-variable MDRDE was similarly accurate. Creatinine appearance rates were similar to those reported in other dialysis populations.

CONCLUSIONS

Using serum creatinine values in "standard" fashion, MDRDEs can approximate creatinine clearances achieved by peritoneal dialysis with an accuracy similar to that of the MDRDEs in predialysis populations, perhaps because the MRDREs account exponentially for the increasing fractional gut metabolism of daily creatinine production as the serum creatinine increases. "MDRD-like" equations may provide a much needed method of directly comparing dialytic clearances to those achieved by diseased native kidneys and by kidney transplants.

What is toxicology and how does toxicity occur?

Toxicology has matured since it was defined as the 'science of poisons'. Modern toxicology is no longer anthropocentric but takes on different views at various biological systems, including ecosystems. Each will interact specifically when exposed to defined chemical agents, including drugs. Adverse effects during drug therapy or after (accidental) poisoning are the result of some negative interactions between the agent and the exposed biological system. Toxicity is no longer a specific property of drugs and chemicals but an operative term to describe the adverse outcome of a specific drugs-host interaction. Newer developments in toxicology have focused on the host. Toxicogenetics continues to provide answers to variations of host response to xenobiotics, including drugs. Clinically relevant genetic polymorphisms and gene defects have been detected, and their number is rapidly growing. The key to understanding is in the host proteins that interact with the drug and mediate the cellular response. Hence, the proteom, i.e. the complete set of proteins of a cell, an individual or a species, determines how an exposed biological system may interact with the manifold of different xenobiotics. Structure-activity studies try to find out useful predictive parameters for risk and toxicity assessment.

Derived equations are not precise enough to predict the adequacy of creatinine clearance in peritoneal dialysis patients

BACKGROUND

Most renal units assess dialysis adequacy in peritoneal dialysis (PD) patients by formal 24-hour collections of urine and effluent dialysate. We sought a reliable method of predicting dialysis adequacy that allows a decrease in the frequency of these formal and cumbersome measurements.

METHODS

We created a formula for estimating total creatinine clearances, then assessed the clinical utility of this formula and other published formulae in predicting adequate and inadequate dialysis in PD patients. We collected data over a 6-month period in 2001 from 288 PD patients from 9 centers in Scotland. Four out of every 5 patients were selected at random to create a formula for estimating total creatinine clearance per week, and the fifth patient was used to form a validation group. We plotted creatinine excretion against age, and the resultant linear regression equation was transformed to produce a formula for predicting total creatinine clearance per week, based on patient sex, weight, and serum creatinine. We used the data from the validation subgroup to calculate predictive values for our derived formula and data from all of the patients to calculate predictive values for the Cockcroft and Gault, Jones, and Modification of Diet in Renal Disease Study formulae.

RESULTS

Neither our derived formula nor the three published formulae were sufficiently powerful to predict accurately either adequate or inadequate PD clearance. Receiver operator characteristic curves showed that no significant improvement in these predictive values could be achieved by altering either the sensitivity or the specificity.

CONCLUSION

Prediction formulae are not accurate enough to detect underdialysis in PD patients.