Estimation of creatinine clearance in morbidly obese patients

Estimating creatinine clearance: a meta-analysis

STUDY OBJECTIVES

To determine which body weight descriptor most accurately predicts measured creatinine clearance (Cl(cr)) and whether rounding serum creatinine concentration (S(cr)) to 1 mg/dl when it is less than 1 mg/dl accurately predicts measured Cl(cr).

DESIGN

Meta-analysis of 13 English-language trials comparing 24-hour measured Cl(cr) with Cockcroft-Gault estimated Cl(cr) by using various body weights or rounded S(cr) values.

PATIENTS

A total of 1197 patients (mean age 53.3 yrs, 48.7% male) were included in the meta-analysis.

MEASUREMENTS AND MAIN RESULTS

A thorough literature search of the PubMed, MEDLINE, and the Cochrane Library databases was performed (1976-June 2010) to identify relevant clinical trials. Patient population, number of subjects, age, and 24-hour measured and estimated Cl(cr) were extracted independently by two investigators by using standardized data collection forms. Mean difference (MD) between estimated and measured Cl(cr) was assessed. Thirteen studies met all selection criteria. A random-effects model was applied secondary to a high level of heterogeneity among the studies (I(2)>50%). Total body weight in the Cockcroft-Gault equation overestimated measured Cl(cr) (MD 15.91 ml/min, 95% confidence interval [CI] 7.17-24.65 ml/min). Ideal body weight underestimated Cl(cr) (MD-5.15 ml/min, 95% CI -9.92 to -0.38 ml/min). No body weight (i.e., assumes body weight is 72 kg, thus removing the factor of 72 from the denominator of the equation) closely estimated Cl(cr) (MD 0.43 ml/min, 95% CI -5.42-6.27 ml/min). Adjusted body weight with correction factors of 0.3 or 0.4 also closely estimated Cl(cr) (MD 4.55 ml/min, 95% CI -11.41-20.50 ml/min, and MD 19.94 ml/min, 95% CI -9.6-49.49 ml/min, respectively). Total body weight with a rounded S(cr) value closely estimated measured Cl(cr) (MD 3.51 ml/min, 95% CI -17.18-24.20 ml/min). Ideal body weight with a rounded S(cr) value underestimated Cl(cr) (MD -29.45 ml/min, 95% CI -48.46 to -10.43).

CONCLUSION

Using the Cockcroft-Gault equation with no body weight and actual S(cr) value most closely estimated measured Cl(cr). In obese patients, it may be reasonable to use actual body weight with a correction factor of 0.3 or 0.4 and actual S(cr) value in the Cockcroft-Gault equation. Based on this analysis, the use of total body weight, ideal body weight, and a rounded S(cr) value cannot be recommended.

Interpreting different measures of glomerular filtration rate in obesity and weight loss: pitfalls for the clinician

To combat the increasing incidence of obesity, much research has been devoted to devising successful strategies for weight loss, including manipulation of diet and gastric surgery. Obesity itself can be associated with renal dysfunction, and the degree of reversibility of this with weight loss has being studied. However, there are significant limitations and flaws in the methods we have available to measure glomerular filtration rate (GFR) in overweight and obese subjects. Obesity is associated with changes in body composition including lean and fat mass. This has implications for assumptions that underpin creatinine-based measures such as creatinine clearance, estimated GFR and other equations devised for obesity including the Salazar-Corcoran equation. These changes in body composition also affect measures of glomerular filtration such as cystatin C and nuclear medicine isotope scans. This article will review the accuracy of these current measures of renal function in the obese and consider the evidence for adjusting for body surface area or adjusting for lean body mass. Finally, the effect of weight loss itself on serial measurements of renal function in a given individual, independent of a true change in renal function, will be reviewed. Ultimately using the Cockcroft-Gault equation with an adjustment for lean body mass seems to be the best measure for renal function in obesity. No method for measuring renal function in situations of weight loss has been shown to be unequivocally superior.

Use of estimated glomerular filtration rate for drug dosing in the chronic kidney disease patient

PURPOSE OF REVIEW

Assessment of kidney function is necessary to stage chronic kidney disease (CKD) and appropriately dose medications. The Cockcroft-Gault equation provides an estimate of creatinine clearance (eClCr) and is the method commonly referenced in pharmacokinetic studies. The Modification of Diet in Renal Disease (MDRD) and CKD-Epidemiology Collaboration (EPI) equations provide an estimate of glomerular filtration rate (eGFR), with the MDRD eGFR now automatically reported by most clinical laboratories. This review describes the differences in the Cockcroft-Gault, MDRD, and CKD-EPI equations and considerations when applying estimates from these equations for drug dosing.

RECENT FINDINGS

Studies evaluating drug-dosing regimens using eClCr and eGFR differ in their results depending on the population in which the equation is applied, the adjustment factors used to account for body size, and the number of dosing levels for a particular medication. The largest study to evaluate drug regimen design by method concluded that either the eGFR or Cockcroft-Gault estimates could be used for drug dosing. Differences in methodology among studies are a key factor in evaluating these results and will be highlighted in this review.

SUMMARY

The Cockcroft-Gault, MDRD, and CKD-EPI equations provide reasonable estimates of kidney function; however, clinicians must understand the limitations when using these estimates for drug regimen design.

Fat-free weight prediction in morbidly obese females

Purpose

Precise estimation of creatinine clearance in obese individuals relies on the appropriate assessment of lean body weight (LBW). Anthropometric methods of predicting LBW have not been validated in morbidly obese populations.

Patients and methods

Using an existing dataset of anthropometric data for a female cohort with morbid obesity who had undergone measured FFW with dual energy absorptiometry, we evaluated the performance of five previously reported estimating equations for the prediction of LBW. Linear regression was used to derive a new LBW prediction formula and was then compared with the other formulae.

Results

Seventy females (mean [standard deviation] age, weight, and body mass index 43.0 [11.0] years, 128.1 [13.8] kg, and 48.3 [4.8] kg/m², respectively) were identified. LBW as estimated by the method of Garrow and Webster correlated well (r = 0.87) with measured mass while demonstrating the highest accuracy, best precision, and smallest bias (93%, 2.1 kg, and 2.9 kg, respectively; P < 0.0001 for all comparisons). The derived formula further improved bias, precision, and accuracy.

Conclusion

Among females with morbid obesity, most previously reported estimating equations for LBW predicted FFW poorly. These findings have important clinical implications for the assessment of kidney function and for safe and effective drug dosing.

Simplified estimation of aminoglycoside pharmacokinetics in underweight and obese adult patients

Aminoglycosides are an important class of agents that are used in combination antimicrobial regimens to treat bacterial pathogens. Dosing of aminoglycosides is typically based on total body weight. However, the most appropriate alternative body size descriptor for dosing aminoglycosides at the extremes of weight (underweight and obese) is not known. Also, the predictive performance of newer formulas to assess kidney function, such as the modification of diet in renal disease (MDRD) and chronic kidney disease-epidemiology (CKD-EPI) equations compared to the Cockcroft-Gault equation to predict aminoglycoside clearance, is not known. We sought to examine dosing of aminoglycosides across the extremes of weight using a variety of formulas to assess kidney function. Pharmacokinetic data were obtained from a set of prospectively collected data (1982 to 2003) of 2,073 (53.5% male) adult patients that included 497 tobramycin- and 1,576 gentamicin-treated cases. The median (minimum, maximum) age, weight, and body mass index were 66 (18, 98) years, 70.0 (29.7, 206.7) kg, and 24.4 (11.3, 73.8) kg/m(2), respectively. The percentage of underweight, normal-weight, overweight, and obese cases based on the World Health Organization classification were 8.8%, 45.5%, 26.5%, and 19.2%, respectively. The aminoglycoside volume of distribution was normalized to several alternative body size descriptors. Only lean body weight estimated by the method of S. Janmahasatian et al. (Clin. Pharmacokinet. 44:1051-1065, 2005) normalized the volume of distribution for both tobramycin and gentamicin across all weight strata, with the estimate being approximately 0.45 liter/kg. Aminoglycoside dosing can be simplified across all weight strata with the use of lean body weight. The CKD-EPI equation best predicts aminoglycoside clearance.

Use of Recalibrated Serum Creatinine Concentrations for Adjustment of Drug Dosages: Determination of Values Compatible with Conventional Dosing Recommendations

BACKGROUND:

Measurement of serum creatinine concentration is central to evaluation of kidney function. Recent efforts to increase the accuracy of this evaluation have led to recalibration of laboratory instruments. Recalibrated creatinine concentrations often are lower than previously reported.

OBJECTIVES:

To derive a method for converting recalibrated serum creatinine (R-SCr) concentrations into values that are compatible with recommended equations for calculation of estimated creatinine clearance.

METHODS:

Beginning with a proprietary array of recalibrated and corresponding non-recalibrated serum creatinine (NR-SCr) numerical data provided by the instrument manufacturer, relationships were examined with exponential and linear regression analyses. The validity of derived values for NR-SCr obtained through these analyses was tested by comparison of proprietary and derived serum creatinine concentrations and calculated creatinine clearance values.

RESULTS:

Analyses revealed that relationships between R-SCr and NR-SCr creatinine were essentially linear. Rearranging and solving the equation for a straight line described this relationship as x = (y - b)/m, where x is the derived creatinine value, y is the R-SCr concentration, and, for our laboratory instrument, best parameters for m and b equal to 0.987 and −0.07, respectively. Use of these parameters to derive NR-SCr values was shown to significantly decrease positive bias in subsequent creatinine clearance calculations.

CONCLUSIONS:

As compared to R-SCr concentrations, use of derived NR-SCr values can improve the predictive performance of conventional equations used to calculate estimated creatinine clearance.

Potentially inappropriate prescribing of primarily renally cleared medications for older veterans affairs nursing home patients

BACKGROUND

Inappropriate prescribing of primarily renally cleared medications in older patients with kidney disease can lead to adverse outcomes.

OBJECTIVES

To estimate the prevalence of potentially inappropriate prescribing of 21 primarily renally cleared medications based on 2 separate estimates of renal function and to identify factors associated with this form of suboptimal prescribing in older VA nursing home (NH) patients.

DESIGN

Longitudinal study

PARTICIPANTS

Participants were 1304 patients, aged 65 years or older, admitted between January 1, 2004, and June 30, 2005, for 90 days or more to 1 of 133 VA NHs.

MAIN MEASURES

Potentially inappropriate prescribing of primarily renally cleared medications determined by estimating creatinine clearance using the Cockcroft Gault (CG) and Modification of Diet in Renal Disease (MDRD) equations and applying explicit guidelines for contraindicated medications and dosing.

KEY RESULTS

The median estimated creatinine clearance via CG was 67 mL/min, whereas it was 80 mL/min/1.73m(2) with the MDRD. Overall, 11.89% patients via CG and only 5.98% via MDRD had evidence of potentially inappropriate prescribing of at least 1 renally cleared medication. The most commonly involved medications were ranitidine, glyburide, gabapentin, and nitrofurantoin. Factors associated with potentially inappropriate prescribing as per the CG were age older than 85 (adjusted odds ratio [AOR] 4.24, 95% confidence interval [CI] 2.42-7.43), obesity (AOR 0.26, 95% CI 0.14-0.50) and having multiple comorbidities (AOR 1.09 for each unit increase in the Charlson comorbidity index, 95% CI 1.01-1.19).

CONCLUSIONS

Potentially inappropriate prescribing of renally cleared medications is common in older VA NH patients. Intervention studies to improve the prescribing of primarily renally cleared medications in nursing homes are needed.

Availability of information for dosing injectable medications in underweight or obese patients

PURPOSE

Product information and pivotal studies on newly marketed injectable medications were reviewed to determine whether a weight descriptor was included and if information was provided for dosing patients with extremes of body weight.

METHODS

Products with new drug applications approved by the Food and Drug Administration between January 1, 2004, and January 30, 2009, were evaluated. Any information related to weight descriptors or dosing of patients with extremes of weight (body mass index of <18.5 or >40 kg/m(2)) relative to age and height was extracted from the product labeling or pivotal studies. Pharmaceutical companies were contacted if pivotal studies had not been published. The information was evaluated with a dosing usefulness score of 0-3; a score of 2 or greater was considered minimally adequate for dosing patients with extremes of weight.

RESULTS

Of the 84 medications evaluated, some reference to weight descriptors was found for 23 (27%). None had a calculated usefulness score of 2 or above based on information from product information or pivotal trials. conclusion: Information from product labeling and pivotal studies involving newly marketed injectable medications is inadequate for dosing patients with extremes of weight.

Estimating the glomerular filtration rate in obese adult patients for drug dosing

One-third of adult Americans are currently classified as obese. Physiologic changes associated with obesity can potentially alter the clearance of commonly used drugs. Clearance of certain drugs by the kidneys occurs primarily through glomerular filtration and tubular secretion. Obesity has been associated with glomerular hyperfiltration, whereas obesity-related effects on tubular secretion are not well characterized. Estimation of the glomerular filtration rate (GFR) is currently performed using serum creatinine using the Modification of Diet in Renal Disease (MDRD) equation. However, drug dosing guidelines are often based on creatinine clearance (CLcr) using the Cockcroft-Gault equation as a surrogate of GFR. There is a lack of consensus on the most appropriate method for estimation of GFR or CLcr in patients with obesity. The controversy relates to the use of 2 body size descriptors that confound these equations. The Cockcroft-Gault equation relies on total body weight and so overestimates GFR in patients with obesity. The MDRD equation indexes GFR based on a normalized body surface area, that is, mL/min/1.73 m(2). Conversion of MDRD estimated GFR to non-normalized body surface area overestimates GFR in patients with obesity. The current review explores current approaches and controversies to estimation of GFR and CLcr among obese patients in clinical practice. The role of the alternate body size descriptor, lean body weight to estimate CLcr in obese patients is reviewed.

Feasibility of therapy with hypomethylating agents in patients with renal insufficiency

BACKGROUND

To our knowledge, the feasibility of therapy with hypomethylating agents (HAs) in patients with renal insufficiency (RI) has not been examined.

PATIENTS AND METHODS

We reviewed 41 patients with a diagnosis of acute myeloid leukemia (n = 17), myelodysplastic syndromes (n = 15), and chronic myelomonocytic leukemia (n = 9) who had RI and were receiving therapy with azacitidine or decitabine. The median number of administered cycles was 3. Most patients (39; 95%) received a standard dose of the drugs at the initiation of therapy. Nine patients (22%) required treatment interruptions or discontinuation, and 10 patients (24%) required dose reductions.

RESULTS

The overall response rate was 63%, and 4 patients (10%) achieved a complete response. Twenty patients (51%) experienced grade 3 or 4 myelosuppression-related toxicities. Hospitalization was required in 68% of the patients. Among 12 patients with an estimated glomerular filtration rate of 29 mL per minute or less, 6 required dose reductions attributable to myelosuppression (n = 3) or to worsening renal function (n = 3). The overall survival (OS) at 18 months was 12%, and the median OS was 8.6 months.

CONCLUSION

The use of HA in patients with RI is feasible, but is associated with a higher incidence of toxicity. Dose adjustments and the use of growth factor may be necessary for some patients.

Low serum creatinine is associated with type 2 diabetes in morbidly obese women and men: a cross-sectional study

BACKGROUND

Low skeletal muscle mass is associated with insulin resistance and metabolic syndrome. Serum creatinine may serve as a surrogate marker of muscle mass, and a possible relationship between low serum creatinine and type 2 diabetes has recently been demonstrated. We aimed to validate this finding in a population of Caucasian morbidly obese subjects.

METHODS

Cross-sectional study of 1,017 consecutive morbidly obese patients with an estimated glomerular filtration rate >60 ml/min/1.73 m2. Logistic regression (univariate and multiple) was used to assess the association between serum creatinine and prevalent type 2 diabetes, including statistically testing for the possibility of non-linearity in the relationship by implementation of Generalized Additive Models (GAM) and piecewise linear regression. Possible confounding variables such as age, family history of diabetes, waist-to-hip ratio, hypertension, current smoking, serum magnesium, albuminuria and insulin resistance (log HOMA-IR) were adjusted for in three separate multiple logistic regression models.

RESULTS

The unadjusted GAM analysis suggested a piecewise linear relationship between serum creatinine and diabetes. Each 1 mumol/l increase in serum creatinine was associated with 6% (95% CI; 3%-8%) and 7% (95% CI; 2%-13%) lower odds of diabetes below serum creatinine levels of 69 and 72 mumol/l in women and men, respectively. Above these breakpoints the serum creatinine concentrations did not reduce the odds further. Adjustments for non-modifiable and modifiable risk factors left the piecewise effect for both women and men largely unchanged. In the fully adjusted model, which includes serum magnesium, albuminuria and log HOMA-IR, the piecewise effect for men was statistically non-significant, but it remained present for women. Patients with creatinine levels below median had approximately 50% (women) and 75% (men) increased odds of diabetes.

CONCLUSIONS

Low serum creatinine is a predictor of type 2 diabetes in Caucasian morbidly obese patients, independent of age, gender, family history of diabetes, anthropometric measures, hypertension, and current smoking. Longitudinal studies of both obese and non-obese populations are needed to investigate whether serum creatinine may be causally linked with type 2 diabetes, and if so, precisely how they are linked.

Comparison of methods for determination of glomerular filtration rate in hypertensive subjects with normal serum creatinine

BACKGROUND

Glomerular filtration rate (GFR) measured through technetium-99m diethyl triamine penta-acetic acid (Tc(99m)DTPA) renal scintigraphy (rsGFR) was compared with that estimated (eGFR) from 24-h creatinine clearance (CrCl) and, using both the Cockcroft-Gault (CG) and Modification of Diet in Renal Disease (MDRD) formulas, in a population of hypertensive subjects (HTs) with normal serum creatinine (SCr) levels.

PATIENTS AND METHODS

In 200 normoalbuminuric (<30 mg/24 h) HTs 55-75 years old, without diabetes and history of coronary and cerebrovascular diseases, Pearson's correlation assess the relationship between rsGFR and eGFR. The Bland-Altman method was used to assess the agreement between rsGFR and eGFR, separately in subjects with low (<60 ml/min/1.73 m(2)) and normal (≥60 ml/min/1.73 m(2)) rsGFR. The span between -1.96 and +1.96 standard deviations of mean difference (bias) was calculated and used for this purpose.

RESULTS

In 76 subjects, an unknown low renal function was found by Tc(99m)DTPA renal scintigraphy. In subjects with normal rsGFR the Bland-Altman analysis showed that the smallest span between rsGFR and eGFR was evident for ClCr values (26.0 ml/min/1.73 m(2)), whereas higher values were detected with the CG and MDRD formulas (41.0 and 40.4 ml/min/1.73 m(2), respectively). The same results were observed for low rsGFR, where a smaller span was found for ClCr (21.2 ml/min/1.73 m(2)), whereas CG and MDRD methods gave greater results (30.4 and 31.8 ml/min/1.73 m(2) respectively); no differences were found between genders. The degree of agreement for eGFR estimated with the CG and MDRD formulas was wider than that derived from ClCr, reflecting a greater between-methods variability and a considerable discrepancy of rsGFR values in the former than in the latter.

CONCLUSIONS

In HTs with normal SCr values, Tc(99m)DTPA renal scintigraphy discovered un known renal organ damage in 38% of cases. If this methodology is not available, ClCr measurement should be preferred to estimate GFR whereas CG and MDRD formulas are of limited efficacy.

Dosing of medications in morbidly obese patients in the intensive care unit setting

OBJECTIVE

To derive recommendations for the dosing of commonly used medications in the morbidly obese patient in the ICU.

DATA SOURCES

Articles were obtained through computerized searches involving MEDLINE. The bibliographies of retrieved publications and textbooks were reviewed for additional references.

STUDY SELECTION

All studies involving the pharmacokinetics or pharmacodynamics of medications in obese subjects or patients.

DATA EXTRACTION

The emphasis was on studies involving morbidly obese patients but, in the absence of such data, investigations involving lesser forms of obesity were extracted.

DATA SYNTHESIS

There is a paucity of data upon which to make recommendations for dosing commonly used medications in the morbidly obese patient in the ICU, although recommendations were provided based on the available information.

CONCLUSIONS

There is clearly a need for more investigations involving dosing regimens of medications in the morbidly obese population. Until such studies are available, the clinician must try to derive the best dosing regimens for medications based on the limited pharmacokinetic data available for some agents and clinical judgement.