Are eGFR equations better than IDMS-traceable serum creatinine in classifying chronic kidney disease?

Relationships and Clinical Utility of Creatinine, Cystatin C, eGFRs, GFRs, and Clearances

BACKGROUND

This review addresses techniques for glomerular filtration rate (GFR), either measured by clearance tests such as with creatinine, iothalamate, inulin, or iohexol [measured GFR (mGFR)] or calculated by equations that determine the estimated GFR (eGFR) from serum measurements of creatinine and/or cystatin C. However, mGFR tests are slow and impractical for routine use. Therefore, calculations of eGFRs have been developed that have advantages over the mGFRs.

CONTENT

The eGFR is a serum creatinine and/or cystatin C adjusted for age, sex, and race, with mathematical manipulations to produce an average numerical agreement with the mGFR. However, all comparisons between eGFR and mGFR show wide scatter that appears to be related to the large variability of the mGFR. Procedures for mGFR often do not agree with each other and have both wide population variation (similar to plasma creatinine and cystatin C) and within-individual variation that is much larger than creatinine or cystatin C. Whether the measured GFR is even equivalent to serum creatinine and/or cystatin C for detecting early clinical changes in chronic kidney disease will be addressed.

SUMMARY

Procedures for measuring GFR are tedious and expensive, and have both wide population variation (similar to plasma creatinine and cystatin C), and within-individual variation that is much larger than either creatinine or cystatin C. Because the normal range for mGFR overlaps considerably with the stages 1 and 2 of chronic kidney disease, mGFR has significant clinical limitations. Instead of trying to mimic mGFRs, the focus should be on using eGFRs on their own clinical merits to detect impaired kidney function.

No role for patient body weight on renal function assessment for drug dosing

Objectives

To evaluate the ability of body-weight-driven renal function assessment (RFA) formulae to predict on-target elimination rate ranges for gentamicin in patients with varying degrees of renal function.

Methods

A 6 year retrospective pharmacokinetic study was conducted at a university teaching hospital.

Results

A total of 85 patients met the inclusion criteria and 127 pharmacokinetic files were analysed from patients on medical-surgical wards (53%) and medical-surgical ICUs (13%) receiving intravenous gentamicin for treatment, as well as those for patients receiving it for surgical prophylaxis (34%). Each RFA formula was examined against standard dosing tables for gentamicin. A table of acceptable elimination rates was generated using a traditional peak of 8 mg/L and trough between 0.5 and 2 mg/L associated with each of the dosing interval extensions. The ability of each RFA formula to select on-target elimination rates was evaluated. The RFA formula assuming a normalized body weight of 72 kg and a modified creatinine reagent adjustment factor of 90% provided the most accurate on-target elimination rate selection. This method was superior to dosing interval selection based on the Modification in Diet Renal Disease (MDRD) formula, Sanford's guide method, as well as the Cockcroft-Gault formulae using total body weight, ideal body weight or lean body weight ( P  <   0.0001).

Conclusions

Based on the use of gentamicin as a surrogate guide for renally adjusted drugs, these results support dosing interval selection based on a normalized body weight method and a formula reagent adjustment factor of 90% within the Cockcroft-Gault formula.

Serum neutrophil gelatinase associated lipocalin (NGAL) and cystatin C as early predictors of contrast-induced acute kidney injury in patients undergoing percutaneous coronary intervention

BACKGROUND

Contrast-induced acute kidney injury (AKI) is diagnosed by estimating serum creatinine at 48-72h after diagnostic or interventional coronary angiography. It is too late for an early intervention. Neutrophil gelatinase associated lipocalin (NGAL) and cystatin C are novel markers of AKI. We determined the optimum cut-off level of NGAL and cystatin C in early diagnosis and prediction of AKI in patients undergoing coronary angiography followed by angioplasty.

METHODS

In a nested case control study, serum NGAL, cystatin C by ELISA and serum creatinine by Jaffe's kinetic method were estimated at 0, 4, 24 and 48h of coronary angiography followed by angioplasty in 30 cases who developed contrast-induced AKI and 30 subjects who did not develop AKI. eGFR was estimated for both cases and controls by the MDRD equation. ROC was used to determine the optimum cut-off.

RESULTS

Serum NGAL increased sharply at 4h after the procedure and then gradually declined to near normal level at 48h in AKI cases. The rise in cystatin C peaked at 24h and then declined but remained high till 48h. In controls, they remained static. The optimum cut-off of serum NGAL and cystatin C was 155.2ng/ml and 0.517mg/l respectively at 4h and 89.5ng/ml and 0.99mg/l respectively at 24h of angiography. Odds ratio for hypertensives to develop AKI was 3.57 (CI: 1.2-11.1, p=0.03).

CONCLUSION

Serum NGAL and cystatin C may act as early markers of contrast-induced AKI in patients undergoing percutaneous coronary intervention. Patients with hypertension are susceptible to develop contrast-induced AKI.

New insights in glomerular filtration rate formulas and chronic kidney disease classification

BACKGROUND

The MDRD Study equation is the most popular equation for estimating the glomerular filtration rate (eGFR) from serum creatinine (Scr), age, sex and race. Many articles deal with ethnic factors, correcting the MDRD Study equation for different populations, with more or less success. The new CKD-EPI equation introduced the concept of a population-normalized Scr in the eGFR equation for white men (Scr/0.90) and white women (Scr/0.70).

METHODS

We introduce alternative mathematical forms for the MDRD Study equation and the CKD-EPI equation, using the concept of a population-normalized Scr, resulting in a more general and mathematically less complicated form for the eGFR equation.

RESULTS

We show that the normalization constant corresponds to the mean Scr-value for the specific healthy population. We compared the established equations with the new alternative forms, and show that the differences are minimal. The sex/race dependency is completely comprehended in the normalization constant, making the alternative eGFR equations independent of sex and race.

CONCLUSION

The age-dependency of eGFR remains and consequently age-dependent cutoff values for the classification of Chronic Kidney Disease (CKD) look more appropriate, contrary to the current classification rules. The population-normalized Scr which is independent of age, sex and race may serve as an alternative for the classification of CKD.