Effects of analytic variations in creatinine measurements on the classification of renal disease using estimated glomerular filtration rate (eGFR)

An improved implementation of metrological traceability concepts is needed to benefit from standardization of laboratory results

Non-harmonization of laboratory results represents a concrete risk for patient safety. To avoid harms, it is agreed that measurements by in vitro diagnostic medical devices (IVD-MD) on clinical samples should be traceable to higher-order references and adjusted to give the same result. However, metrological traceability is not a formal claim and has to be correctly implemented, which in practice does not happen for a non-negligible number of measurands. Stakeholders, such as higher-order reference providers, IVD manufacturers, and External Quality Assessment organizers, have major responsibilities and should improve their contribution by unambiguously and rigorously applying what is described in the International Organization for Standardization 17511:2020 standard and other documents provided by the international scientific bodies, such as Joint Committee on Traceability in Laboratory Medicine and IFCC. For their part, laboratory professionals should take responsibility to abandon non-selective methods and move to IVD-MDs displaying proper selectivity, which is one of the indispensable prerequisites for the correct implementation of metrological traceability. The practicality of metrological traceability concepts is not impossible but relevant education and appropriate training of all involved stakeholders are essential to obtain the expected benefits in terms of standardization.

A Low-cost Creatinine Biosensor by Differential Optical Signal Readout for the Whole Blood Analysis

This study developed a low-cost paper-based biosensor for point-of-care (POC) detection of blood creatinine by using differential optical signal readout. Dual-channel photochemical paper-based test strips were fabricated with stackable multilayer films containing pre-immobilized enzymes and reagents for the identification and conversion of creatinine and creatine. Enzyme-linked reactions generated hydrogen peroxide (H2O2), which formed a blue oxidized condensate with aniline derivatives. The color depth was quantified via the differential optical signal of the two channels and positively correlated with the concentration of the analyte. This method was first proposed to address the issue of endogenous interferences in the enzymatic assay of creatinine, greatly improving the detection accuracy. The proposed biosensor was calibrated with spiked blood samples, and achieved a wide detection range of 31-1483 μmol/L, showing superior detection performance to general enzymatic methods, especially in the low concentration range. Creatine interference testing demonstrated that the biosensor could resist the interference of ≤ 300 μmol/L endogenous creatine. It is believed that the proposed optical differential biosensor for blood creatinine could enable to pave the way for a daily monitoring system for renal diseases.Clinical Relevance- This stackable multilayer paper-based biosensor provides an enzymatic colorimetric assay of creatinine in whole blood, which can be read out by the differential optical signal to exclude interference from endogenous creatine.

Redesigning the surveillance of in vitro diagnostic medical devices and of medical laboratory performance by quality control in the traceability era

IVD manufacturers have total responsibility in terms of the traceability of marketed in vitro diagnostic medical devices (IVD-MD). This includes the provision of a quality control (QC) material as a part of the measuring system, suitable for traceability verification and alignment surveillance by end-users in daily practice. This material [to be used for the internal QC (IQC) component I as described in this paper] should have unbiased target values and an acceptability range corresponding to analytical performance specifications (APS) for suitable (expanded) measurement uncertainty (MU) on clinical samples. On the other hand, medical laboratories (by the IQC component II as described in this paper) should improve the IQC process and its judging criteria to establish a direct link between their performance, estimated as MU of provided results, and APS defined according to recommended models to apply corrective actions if the performance is worsening with the risk to jeopardize the clinical validity of test results. The participation to external quality assessment (EQA) programs that meet specific metrological criteria is also central to the evaluation of performance of IVD-MDs and of medical laboratories in terms of harmonization and clinical suitability of their measurements. In addition to the use of commutable materials, in this type of EQA it is necessary to assign values to them with selected reference procedures and to define and apply maximum allowable APS to substantiate the suitability of laboratory measurements in the clinical setting.

Diffusion tensor imaging in renal artery stenosis: a preliminary report

Objective:

To investigate the diffusion properties in the kidneys affected by renal artery stenosis (RAS) using diffusion tensor imaging (DTI).

Methods:

In this prospective study, 35 patients with RAS and 15 patients without renal abnormalities were enrolled and examined using DTI. Cortical and medullary regions of interest (ROIs) were located to obtain the corresponding values of the apparent diffusion coefficient (ADC) and fractional anisotropy (FA). The cortical and medullary ADC and FA were compared in the kidney affected by variable degrees of stenosis (RAS 50–75% and >75%) vs controls, using the one-way ANOVA and Student’s t-test. The Spearman correlation test was used to correlate the mean ADC and FA values in the cortex and medulla with the estimate glomerular filtration rate (eGFR).

Results:

For the controls, the ADC value was significantly (p = 0.03) higher in the cortex than in the medulla; the FA value was significantly (p = 0.001) higher in the medulla than in the cortex. Compared with the controls, a significant reduction in the cortical ADC was present with a RAS of 50–75% and >75% (p = 0.001 and 0.041, respectively); a significant reduction in the medullary FA was verified only for RAS >75% (p = 0.023). The Spearman correlation test did not show a statistically significant correlation between the cortical and medullary ADC and FA, and the eGFR.

Conclusion:

The alterations of the diffusional parameters caused by RAS can be detected by DTI and could be useful in the diagnostic evaluation of these patients.

Advances in knowledge:

1. Magnetic resonance DTI could provide useful information about renal involvement in RAS. 2. Magnetic resonance DTI allows non-invasive repeatable evaluation of the renal parenchyma, without contrast media.

Conversion methods for modified Jaffe reaction assays of serum creatinine

BACKGROUND

Modifications in Jaffe serum creatinine (sCr) assays question the suitability of the results for direct comparison.

METHODS

sCr in adult in-patients was routinely measured either by SRM 909-standardized/noncompensated (method A) or isotope dilution mass spectrometry traceable/compensated method (reference). We converted values by method A into values by the reference using a formula provided by the manufacturer [Beckman Coulter (BC)] and traditional equating methods.

RESULTS

The BC-based conversion and linear equating resulted in underestimated sCr values, whereas equipercentile equating (EE) provided sCr with not significantly different distribution from the reference values. Proportions of patients with renal impairment did not differ between the reference and EE-converted sCr, as opposed to BC-recalculated values. Three percent of patients were classified into better renal function category when applying BC versus EE conversion.

CONCLUSIONS

Equipercentile equation was a more accurate method for recalculation of sCr obtained from different Jaffe reaction assays than the linear equating or the BC linear formula. This study emphasizes the importance of the derivation sample specificity when applying research results to other real-world populations.

Cystatin C measurement leads to lower metformin dosage in elderly type 2 diabetic patients

The aim of this study was to provide evidence for the hypothesis that estimated glomerular filtration rate from serum Cystatin C (eGFRcys) is better to be determined for all elderly type 2 diabetes mellitus (T2DM) patients based on eGFRcys upward and downward reclassification rate for hypothetical metformin dose reduction by eGFRcys at the GFR decision point of 45 mL/min/1.73 m². A total of 265 consecutive T2DM elderly patients (age range 65‐91 years) from outpatient diabetic clinic were included in the study. The Kidney Disease Improving Global Outcomes (KDIGO) guidelines for metformin dosing were strictly followed. Estimated glomerular filtration rate from serum creatinine (eGFRcrea) led to results of metformin eligibility. Each of the results of eGFRcrea‐based eligibility was further compared to eGFRcys‐based eligibility. Creatinine was measured by enzymatic method standardized against international reference material SRM 967. Cystatin C was determined by method traceable to DA ERM 471 international standard. eGFRcrea and eGFRcys were calculated according to Chronic Kidney Disease Epidemiology Collaboration (CKD‐EPI) equations. A downward reclassification rate was higher than upward reclassification rate (31 vs 3, respectively; P < 0.0001). The median (IQR) eGFRcrea was higher than eGFRcys (73 (58‐85) vs 63 (50‐75) mL/min/1.73 m², respectively; P < 0.0001). eGFRcys reclassified significant proportion of patients with T2DM from metformin eligible CKD stages to less or non‐eligible stages. The downward reclassification was more frequent in patients older than 80 years (P < 0.01). Cystatin C‐based eGFR selects more complicated patients, where lower doses of metformin are possibly advisable. We recommend calculating both eGFRcrea and eGFRcys for metformin dosing in elderly patients with T2DM.

In vivo evaluation of early renal damage in type 2 diabetic patients on 3.0 T MR diffusion tensor imaging

AIM

To investigate the utility of renal diffusion tensor imaging (DTI) to detect early renal damage in patients with type 2 diabetes.

METHODS

Twenty-six diabetic patients (12 with microalbuminuria (MAU), and 14 with normoalbuminuria) and fourteen healthy volunteers were prospectively included in this study. Renal DTI on 3.0 T MR was performed, and estimated glomerular filtration rate (eGFR) was recorded for each subject. Mean cortical and medullary fractional anisotropy (FA) values were calculated by placing multiple representative regions of interest. Mean FA values were statistically compared among groups. Correlations between FA values and eGFR were evaluated.

RESULTS

Both cortical and medullary FA were significantly reduced in diabetic patients compared to healthy controls (0.403 ± 0.064 vs 0.463 ± 0.047, P = 0.004, and 0.556 ± 0.084 vs 0.645 ± 0.076, P = 0.002, respectively). Cortical FA was significantly lower in diabetic patients with NAU than healthy controls (0.412 ± 0.068 vs 0.463 ± 0.047, P = 0.02). Medullary FA in diabetic patients with NAU and healthy controls were similar (0.582 ± 0.096 vs 0.645 ± 0.076, P = 0.06). Both cortical FA and medullary FA correlated with eGFR (r = 0.382, P = 0.015 and r = 0.552, P = 0.000, respectively).

CONCLUSION

FA of renal parenchyma on DTI might serve as a more sensitive biomarker of early diabetic nephropathy than MAU.

Post-standardization of routine creatinine assays: are they suitable for clinical applications

Introduction

Reliable serum creatinine measurements are of vital importance for the correct classification of chronic kidney disease and early identification of kidney injury. The National Kidney Disease Education Programme working group and other groups have defined clinically acceptable analytical limits for creatinine methods. The aim of this study was to re-evaluate the performance of routine creatinine methods in the light of these defined limits so as to assess their suitability for clinical practice.

Method

In collaboration with the Dutch External Quality Assurance scheme, six frozen commutable samples, with a creatinine concentration ranging from 80 to 239  μmol/L and traceable to isotope dilution mass spectrometry, were circulated to 91 laboratories in four European countries for creatinine measurement and estimated glomerular filtration rate calculation. Two out of the six samples were spiked with glucose to give high and low final concentrations of glucose.

Results

Results from 89 laboratories were analysed for bias, imprecision (%CV) for each creatinine assay and total error for estimated glomerular filtration rate. The participating laboratories used analytical instruments from four manufacturers; Abbott, Beckman, Roche and Siemens. All enzymatic methods in this study complied with the National Kidney Disease Education Programme working group recommended limits of bias of 5% above a creatinine concentration of 100  μmol/L. They also did not show any evidence of interference from glucose. In addition, they also showed compliance with the clinically recommended %CV of ≤4% across the analytical range. In contrast, the Jaffe methods showed variable performance with regard to the interference of glucose and unsatisfactory bias and precision.

Conclusion

Jaffe-based creatinine methods still exhibit considerable analytical variability in terms of bias, imprecision and lack of specificity, and this variability brings into question their clinical utility. We believe that clinical laboratories and manufacturers should work together to phase out the use of relatively non-specific Jaffe methods and replace them with more specific methods that are enzyme based.

A Risk Assessment of the Jaffe vs Enzymatic Method for Creatinine Measurement in an Outpatient Population

BACKGROUND

The Jaffe and enzymatic methods are the two most common methods for measuring serum creatinine. The Jaffe method is less expensive than the enzymatic method but is also more susceptible to interferences. Interferences can lead to misdiagnosis but interferences may vary by patient population. The overall risk associated with the Jaffe method depends on the probability of misclassification and the consequences of misclassification. This study assessed the risk associated with the Jaffe method in an outpatient population. We analyzed the discordance rate in the estimated glomerular filtration rate based on serum creatinine measurements obtained by the Jaffe and enzymatic method.

METHODS

Method comparison and risk analysis. Five hundred twenty-nine eGFRs obtained by the Jaffe and enzymatic method were compared at four clinical decision limits. We determined the probability of discordance and the consequence of misclassification at each decision limit to evaluate the overall risk.

RESULTS

We obtained 529 paired observations. Of these, 29 (5.5%) were discordant with respect to one of the decision limits (i.e. 15, 30, 45 or 60 ml/min/1.73m2). The magnitude of the differences (Jaffe result minus enzymatic result) were significant relative to analytical variation in 21 of the 29 (72%) of the discordant results. The magnitude of the differences were not significant relative to biological variation. The risk associated with misclassification was greatest at the 60 ml/min/1.73m2 decision limit because the probability of misclassification and the potential for adverse outcomes were greatest at that decision limit.

CONCLUSION

The Jaffe method is subject to bias due to interfering substances (loss of analytical specificity). The risk of misclassification is greatest at the 60 ml/min/1.73m2 decision limit; however, the risk of misclassification due to bias is much less than the risk of misclassification due to biological variation. The Jaffe method may pose low risk in selected populations if eGFR results near the 60 ml/min/1.73m2 decision limit are interpreted with caution.

Routine serum creatinine measurements: how well do we perform?

BACKGROUND

The first aim of the study was to investigate the accuracy and intra-laboratory variation of serum creatinine measurements in clinical laboratories in Flanders. The second purpose was to check the effect of this variation in serum creatinine concentration results on the calculated estimated glomerular filtration rate (eGFR) and the impact on classification of patients into a chronic kidney disease (CKD) stage.

METHODS

26 routine instruments were included, representing 13 different types of analyzers from 6 manufacturers and covering all current methodologies (Jaffe, compensated Jaffe, enzymatic liquid and dry chemistry methods). Target values of five serum pools (creatinine concentrations ranging from 35 to 934 μmol/L) were assigned by the gold standard method (ID-GC/MS).

RESULTS

Intra-run CV (%) (n = 5) and bias (%) from the target values were higher for low creatinine concentrations. Especially Jaffe and enzymatic dry chemistry methods showed a higher error. The calculated eGFR values corresponding with the reported creatinine concentration ranges resulted in a different CKD classification in 47% of cases.

CONCLUSIONS

Although most creatinine assays claim to be traceable to the gold standard (ID-GC/MS), large inter-assay differences still exist. The inaccuracy in the lower concentration range is of particular concern and may lead to clinical misinterpretation when the creatinine-based eGFR of the patient is used for CKD staging. Further research to improve harmonization between methods is required.

Calibration and precision of serum creatinine and plasma cystatin C measurement: impact on the estimation of glomerular filtration rate

Serum creatinine (SCr) is the main variable for estimating glomerular filtration rate (GFR). Due to inter-assay differences, the prevalence of chronic kidney disease (CKD) varies according to the assay used, and calibration standardization is necessary. For SCr, isotope dilution mass spectrometry (IDMS) is the gold standard. Systematic differences are observed between Jaffe and enzymatic methods. Manufacturers subtract 0.30 mg/dl from Jaffe results to match enzymatic results ('compensated Jaffe method'). The analytical performance of enzymatic methods is superior to that of Jaffe methods. In the original Modification of Diet in Renal Disease (MDRD) equation, SCr was measured by a Jaffe Beckman assay, which was later recalibrated. A limitation of this equation was an underestimation of GFR in the high range. The Chronic Kidney Disease Epidemiology (CKD-EPI) consortium proposed an equation using calibrated and IDMS traceable SCr. The gain in performance was due to improving the bias whereas the precision was comparable. The CKD-EPI equation performs better at high GFR levels (GFR >60 ml/min/1.73 m(2)). Analytical limitations have led to the recommendation to give a grade (>60 ml/min/1.73 m(2)) rather than an absolute value with the MDRD equation. By using both enzymatic and calibrated methods, this cutoff-grade could be increased to 90 ml/min/1.73 m(2) (with MDRD) and 120 ml/min/1.73 m(2) (with CKD-EPI). The superiority of the CKD-EPI equation over MDRD is analytical, but the precision gain is limited. IDMS traceable enzymatic methods have been used in the development of the Lund-Malmö (in CKD populations) and Berlin Initiative Study equations (in the elderly). The analytical errors for cystatin C are grossly comparable to issues found with SCr. Standardization is available since 2011. A reference method for cystatin C is still lacking. Equations based on standardized cystatin C or cystatin C and creatinine have been proposed. The better performance of these equations (especially the combined CKD-EPI equation) has been demonstrated.

Diffusion tensor imaging and tractography of the kidneys: assessment of chronic parenchymal diseases

OBJECTIVE

To assess renal dysfunction in chronic kidney diseases using diffusion tensor imaging (DTI).

METHODS

Forty-seven patients with impaired renal function (study group) and 17 patients without renal diseases (control group) were examined using DTI sequences. Cortical and medullary regions of interest (ROIs) were located to obtain the corresponding values of the apparent diffusion coefficient (ADC) and the fractional anisotropy (FA). The mean values of the ADC and FA, for each ROI site, were obtained in each group and were compared. Furthermore, the correlations between the diffusion parameters and the estimated glomerular filtration rate (eGFR) were determined.

RESULTS

In both the normal and affected kidneys, we obtained the cortico-medullary difference of the ADC and the FA values. The FA value in the medulla was significantly lower (P = 0.0149) in patients with renal function impairment as compared to patients with normal renal function. A direct correlation between DTI parameters and the eGFR was not found. Tractography visualised disruption of the regular arrangement of the tracts in patient with renal function alteration.

CONCLUSION

DTI could be a useful tool in the evaluation of chronic kidney disease and, in particular, the medullary FA value seems to be the main parameter for assessing renal damage.

KEY POINTS

• Magnetic resonance diffusion tensor imaging (MRDTI) provides new information about renal problems. • DTI allows non-invasive repeatable evaluation of the renal parenchyma, without contrast media. • DTI could become useful in the management of chronic parenchymal disease. • DTI seems more appropriate for renal evaluation than diffusion-weighted imaging.

Performance characteristics of laboratory testing and clinical outcomes

In order to demonstrate the relationship between performance characteristics of laboratory tests and clinical outcomes, diabetes seems to represent a paradigmatic disease: diagnosis, monitoring of therapeutic efficacy and prognosis are adequately achieved by means of laboratory testing. Starting from a simple molecule, glucose, used for the diagnosis of diabetes, continuing with creatinine, used for monitoring renal function in diabetic patients and concluding with cardiac troponins, a recognised gold standard for the diagnosis and risk stratification of cardiovascular diseases, several criticisms may be stressed considering the current methodological state-of-the art. Finally, an often overlooked aspect of performance, the analytical interferences, being responsible of unexpected results, that in turn depend from unknown or undisclosed factors will be discussed, concerning in particular, in our paper, the macroprolactin and the heterophilic antibodies aspects.

Enzymatic assays for creatinine: time for action

Accurate serum creatinine measurements in glomerular filtration rate estimation (eGFR) using equations are critical to ongoing global public health efforts to improve the diagnosis and treatment of chronic kidney disease. There is now an ongoing activity to promote worldwide standardization of methods to determine creatinine together with the introduction of a revised eGFR equation appropriate for use with standardized creatinine methods. Standardization of calibration, i.e., implementation of calibration traceability to high-order reference measurement procedures and reference materials, does not, however, correct for analytical interferences of field methods (non-specificity bias). To account for the sensitivity of alkaline picrate-based methods to non-creatinine chromogens, some manufacturers have adjusted the calibration to minimize the pseudo-creatinine contribution of plasma proteins, producing results more closely aligned to the reference method (isotope dilution-mass spectrometry), but this strategy makes an assumption that the non-creatinine chromogen interference is a constant among samples, which is an oversimplification. Thus, analytical non-specificity for substances found in individual patient samples can affect the accuracy of eGFR computed from serum creatinine values for any alkaline picrate method, including the so-called "compensated" Jaffé methods. The use of assays that are more specific for serum creatinine determination, such as those based on enzymatic reactions, may provide more reliable eGFR values. Supporting the choice of more specific assays by clinical laboratories represents one of the main tasks of our profession in order to achieve the ultimate clinical goal, which is to routinely report an accurate eGFR in all the pertinent clinical situations.