Estimating renal function in old people: an in-depth review
The kidneys' ability to handle creatinine is called the creatinine clearance rate, which helps to estimate the glomerular filtration rate (GFR) -- the. GFR calculated by measuring the clearance of inulin by the kidney is considered . The nonlinear relationship between serum creatinine and GFR. .. of BIS2 in the elderly does not translate into a clinical recommendation. A urinary creatinine clearance is more accurate if the urine collection is complete. Isotopic measurement of glomerular filtration rate can be.
Reporting calculated GFR from serum creatinine
It requires a continuous infusion of inulin and timed urine and serum samples. Due to these practical difficulties, studies have investigated the use of radiolabelled substances including 51Cr EDTA and contrast media [ 8 ], to measure GFR precisely from a single injection. Although these agents provide measurements of GFR comparable to inulin clearance, they carry other practical difficulties. For example, radiolabelled substances require specialist licensing, handling and disposal of waste.
Also, for contrast agents, the lack of robust assay technique and the large volume of contrast required have made these methods less favourable [ 9 ]. For this reason, iohexol is now widely used as an alternative gold standard to measure true GFR due to its simplicity and reliability. Serum creatinine and creatinine clearance Historically, serum creatinine has been used as a surrogate of GFR based on the assumption that it is produced, filtered and secreted in a steady state.
The Cockcroft—Gault CG equation was then developed to estimate creatinine clearance, on the presumption that creatinine clearance was a direct measure of GFR, which it is not.
CG uses serum creatinine adjusted for age, weight, serum creatinine and gender to derive creatinine clearance. CG was the most widely used index of renal function [ 5 ] from its introduction in until the development of the MDRD equation, described below.
Glomerular Filtration Rate vs. Creatinine Clearance | Time of Care
The reason for this and the key limitations of using creatinine clearance and serum creatinine as estimates of GFR are that there are actually many factors which can affect the metabolism of creatinine from creatine in the muscles and the rate of secretion of creatinine in the tubules. Most clearly, creatinine is affected by muscle mass, which in turn changes with older age, and also between genders and ethnic groups.
Other factors which can affect the production and secretion of creatinine and which can also differ in older versus younger patients include dietary protein intake, malnutrition and prescribed medication. Studies have also shown that creatinine clearance overestimates GFR due to the secretion of creatinine from the tubules in normal individuals. In patients with CKD, there is increased extra-renal and decreased urinary elimination of creatinine leading to overestimation of GFR from serum creatinine.
A further complicating factor in using serum creatinine alone is the nonlinear relationship between creatinine and GFR [ 12 ]. Historically, nephrologists would interpret an arbitrary percentage change in creatinine as an indicator of true change in renal function rather than reflecting day-to-day variation from factors such as dietary protein. Similarly, studies have used absolute change in creatinine as a marker of risk of adverse events. However, as demonstrated in Fig.
Estimating renal function in old people: an in-depth review
Because the relationship between serum creatinine and GFR is difficult to interpret, particularly at the low creatinine concentrations that correspond to early CKD, the NKDEP recommends that a calculated GFR be reported with all serum creatinine measurements. This strategy will enable identification of patients early in the course of their kidney disease when therapy can be applied to reduce the progression of CKD and delay or prevent end-stage renal failure.
A Laboratory Working Group was formed to address the issues of variability in creatinine measurement and to develop a standardization program to reduce the impact of creatinine variability on the utility of the calculated GFR. The laboratory working group includes global membership and cooperative relationships with professional organizations from other countries. Variability in measurement of creatinine 0. As the GFR decreases, the creatinine concentration increases, and the relative impact of bias and imprecision on the estimated GFR becomes less significant.
The NKDEP Laboratory Working Group has developed a standardization program for serum creatinine measurements to reduce the bias of creatinine measurements that will, in turn, improve the accuracy and consistency of calculated GFR values. All methods for measuring serum creatinine should have calibration traceable to an isotope dilution mass spectrometry IDMS reference measurement procedure.
The report from the Laboratory Working Group includes a figure that represents the necessary combination of bias and imprecision to meet the total error required . The original MDRD equation was developed using creatinine results measured by a routine method that had a small positive bias compared to an IDMS reference measurement procedure.
Since many routine methods have a similar bias, this conventional calibration equation is recommended for creatinine results from methods that have not been calibrated to be traceable to IDMS. It is recommended to report two values for the estimated GFR, one value if the patient is African American and another value if the patient is not African American.
The reason for this recommendation is that race can be difficult to represent reliably in electronic medical records and it is difficult, even if the race is noted, to know if the patient is of a mixed ethnic background. If the calculated GFR value is less than or equal to 60, it is recommended to report the value rounded to a whole number e. The reason not to report numeric values greater than 60 is because the impact of variability in the creatinine measurement has a progressively greater impact on the variability of the calculated GFR value as the creatinine value becomes smaller corresponding to more normal renal function, and the accuracy of the estimated GFR is poorer at higher GFRs .
Laboratories need to communicate to clinical providers and to pharmacists the clinical issues associated with a creatinine method that is calibrated to be traceable to IDMS. The critical clinical issues are the change in reference range, and the impact on using creatinine and calculated GFR to adjust dosage of nephrotoxic drugs. The algorithms used to adjust drug dosages are usually based on the Cockcroft-Gault equation or the absolute creatinine value.
Renal function - Wikipedia
Because the product labeling for drugs is based on one of these approaches, pharmacists and providers are obliged to use those algorithms. Consequently, the laboratory must provide information on the magnitude of difference between an IDMS-traceable creatinine result and a result by the former method used by the laboratory.
There are two principal approaches to establish traceability to a RMP. One approach is to measure native clinical samples, using the routine method and using the RMP. The product calibrator s for the routine method is then value assigned to produce results for the native clinical samples that are equivalent to those from the RMP.
The other approach to establish the values for the routine method product calibrator s is to use a reference material that is commutable with the native clinical samples between the RMP and the routine method, and which has its value assigned by a RMP. IVD manufacturers may also need to address imprecision and non-specificity of creatinine methods. IVD manufacturers should provide information to customers regarding the relationship between results from a creatinine method calibrated to be traceable to IDMS and previous conventionally calibrated methods.