Estimating GFR from serum creatinine concentration: Pitfalls of GFR-estimating equations

Epidemiology of Cardiorenal Syndrome

Cardiorenal syndrome is a spectrum of disorders that emphasizes the bidirectional nature of cardiac and kidney injury. Observational and retrospective studies have helped us to understand the prevalence and burden of each of the 5 types of cardiorenal syndromes. Cardiorenal syndrome type 1 is the most common. The nature of epidemiologic data limits clear delineation between cardiorenal syndrome types 2 and 4. Overall, the presence of cardiac or renal dysfunction strongly predicts a poor outcome of the contrary organ.

The description of a method for accurately estimating creatinine clearance in acute kidney injury

BACKGROUND

Acute kidney injury (AKI) is a common and serious condition encountered in hospitalized patients. The severity of kidney injury is defined by the RIFLE, AKIN, and KDIGO criteria which attempt to establish the degree of renal impairment. The KDIGO guidelines state that the creatinine clearance should be measured whenever possible in AKI and that the serum creatinine concentration and creatinine clearance remain the best clinical indicators of renal function. Neither the RIFLE, AKIN, nor KDIGO criteria estimate actual creatinine clearance. Furthermore there are no accepted methods for accurately estimating creatinine clearance (K) in AKI.

STUDY DESIGN

The present study describes a unique method for estimating K in AKI using urine creatinine excretion over an established time interval (E), an estimate of creatinine production over the same time interval (P), and the estimated static glomerular filtration rate (sGFR), at time zero, utilizing the CKD-EPI formula. Using these variables estimated creatinine clearance (Ke)=E/P * sGFR.

SETTING AND PARTICIPANTS

The method was tested for validity using simulated patients where actual creatinine clearance (Ka) was compared to Ke in several patients, both male and female, and of various ages, body weights, and degrees of renal impairment. These measurements were made at several serum creatinine concentrations in an attempt to determine the accuracy of this method in the non-steady state. In addition E/P and Ke was calculated in hospitalized patients, with AKI, and seen in nephrology consultation by the author. In these patients the accuracy of the method was determined by looking at the following metrics; E/P>1, E/P<1, E=P in an attempt to predict progressive azotemia, recovering azotemia, or stabilization in the level of azotemia respectively. In addition it was determined whether Ke<10 ml/min agreed with Ka and whether patients with AKI on renal replacement therapy could safely terminate dialysis if Ke was greater than 5 ml/min.

OUTCOMES AND RESULTS

In the simulated patients there were 96 measurements in six different patients where Ka was compared to Ke. The estimated proportion of Ke within 30% of Ka was 0.907 with 95% exact binomial proportion confidence limits. The predictive accuracy of E/P in the study patients was also reported as a proportion and the associated 95% confidence limits: 0.848 (0.800, 0.896) for E/P<1; 0.939 (0.904, 0.974) for E/P>1 and 0.907 (0.841, 0.973) for 0.9<E/P<1.1. Ke<10 ml/min correlated very well with Ka, while Ke>5 ml/min accurately predicted the ability to terminate renal replacement therapy in AKI.

LIMITATIONS

Include the need to measure urine volume accurately. Furthermore the precision of the method requires accurate estimates of sGFR, while a reasonable measure of P is crucial to estimating Ke.

CONCLUSIONS

The present study provides the practitioner with a new tool to estimate real time K in AKI with enough precision to predict the severity of the renal injury, including progression, stabilization, or improvement in azotemia. It is the author's belief that this simple method improves on RIFLE, AKIN, and KDIGO for estimating the degree of renal impairment in AKI and allows a more accurate estimate of K in AKI.

LC-MS/MS Method for Serum Creatinine: Comparison with Enzymatic Method and Jaffe Method

Accurate quantification of creatinine (Cre) is important to estimate glomerular filtration rate (GFR). Differences among various methods of Cre quantification were previously noted. This study aims to develop a liquid chromatography tandem mass spectrometry (LC-MS/MS) method for serum Cre and compare this method with clinical routine methods. LC-MS/MS analysis was performed on API 4000 triple quadrupole mass spectrometer coupled with an Agilent 1200 liquid chromatography system. After adding isotope-labeled Cre-d3 as internal standard, serum samples were prepared via a one-step protein precipitation with methanol. The LC-MS/MS method was compared with frequently used enzymatic method and Jaffe method. This developed method, with a total run time of 3 min, had a lower limit of quantification of 4.4 μmol/L, a total imprecision of 1.15%–3.84%, and an average bias of 1.06%. No significant matrix effect, carryover, and interference were observed for the LC-MS/MS method. The reference intervals of serum Cre measured by LC-MS/MS assay were 41–79 μmol/L for adult women, and 46–101 μmol/L for adult men. Using LC-MS/MS as a reference, the enzymatic method showed an average bias of -2.1% and the Jaffe method showed a substantial average bias of 11.7%. Compared with the LC-MS/MS method, significant negative bias was observed for the enzymatic and Jaffe methods in hemolytic and lipimic samples. We developed a simple, specific, and accurate LC-MS/MS method to analyze serum Cre. Discordance existed among different methods.

[Diagnosis and therapy of renal anemia]

The cardiovascular state and life quality of patients suffering from chronic renal insufficiency is primarily determined by their haemostatic status. Renal anemia can positively be diagnosed if the glomerular filtration rate diminishes significantly (<60 ml/min/1,73 m 2 ). Other causes of anemia besides renal insufficiency can be excluded in these instances. The primary aim of erythropoietin treatment is to abolish the transfusion demand of patients suffering from renal insufficiency as this could lead to antibody formation and the transduction of viral infections. In case the existence of renal anemia is proved, the target values must be determined. A target value of >11 g/dl hemoglobin should be achieved for at least 85% of the patients in order to get an average hemoglobin level of 12-12,5 g/dl for the whole patient population. During the treatment of renal anemia regulating the iron metabolism of patients is of primary importance. A >5% rate of the hypochromic red blood cells in the blood circulation implies iron deficiency; but a value above 10% positively indicates iron deficiency. The transferric saturation values under 20% indicate functional iron deficiency and this indicator is a good means of following iron treatment. In the case of patients receiving dialysis parenteral input is advised because of poor iron absorption. In national clinical practice several erythropoietin products are available (erythropoietin-alpha, erythropoietin-beta, alpha-darbepoetin and continuous erythropoietin receptor activator, a new product now being introduced). When selecting the appropriate treatment strategy for each patient, the application method, the effect range and cost efficiency of the selected erythropoietin product must be taken into consideration.

Bias and precision of estimated glomerular filtration rate in children

Determining true glomerular filtration rate (GFR) using an exogenous marker is time-consuming and cumbersome. Therefore, creatinine-based estimates of GFR are used. Recent papers using new population-specific/local parameters in their prediction equations, standardizing creatinine determination or adding other endogenous surrogate markers of GFR, like cystatin C, could demonstrate an improvement of bias inherent in the results of the prediction equations. Precision, however, is still poor. Currently, we have to accept a precision (as defined in the so-called Bland-Altman plot) of +/-20% in adults and +/-30-40% in children. This problem of poor precision/uncertainty is especially bothering in the higher, near normal GFR range. Caution should be exercised when applying prediction equations in individuals in need of an accurate GFR determination. In that case, a real clearance procedure has to be performed. In the long run, the true clearance procedure should be simplified using new exogenous GFR markers and developing new devices, allowing GFR measurements to be performed, for example, transcutaneously. Such a procedure would be more acceptable for both patients and physicians.

Kidney function estimating equations: where do we stand?

PURPOSE OF REVIEW

Estimation of the glomerular filtration rate (GFR) is central to the diagnosis, evaluation and management of chronic kidney disease. This review summarizes recent data on the performance of equations using serum creatinine to estimate the GFR, particularly the Modification of Diet in Renal Disease (MDRD) Study equation.

RECENT FINDINGS

During 2005 GFR estimation has received substantial attention with a focus on comparing the MDRD Study equation with the Cockcroft-Gault equation. Several large studies (n>500) have appeared. Most studies discuss creatinine calibration but few were able to standardize measurements. Studies that did calibrate the creatinine had improved performance. Overall, the MDRD Study equation performed well in populations with a low range of GFR and often outperformed the Cockcroft-Gault equation. Both equations have lower precision in high GFR populations, and the MDRD equation under-estimated the GFR in a number of studies. Efforts are underway to develop improved prediction equations by pooling data across many study populations.

SUMMARY

Equations for estimating the GFR from serum creatinine are useful for systematic staging of chronic kidney disease. The MDRD Study equation and systematic creatinine assay calibration improve the level of precision and accuracy in many settings. GFR estimates are less useful in the normal range of GFR, however, and are sensitive to the population under study.