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Letters to the Editor |
Diagnostic Services of Manitoba, University of Manitoba, Winnipeg, Canada
aAddress correspondence to the author at: Room L3020 Department of Laboratory Medicine St Boniface General Hospital 409 Tache Ave Winnipeg, Manitoba, R2H 2A6. e-mail dparry{at}sbgh.mb.ca
To the Editor:
Effective use of equations for calculating estimated glomerular filtration rate (eGFR) requires reliable plasma creatinine measurements (1). Considerable effort has been made to standardize calibration (2)(3), but standardization does not correct for analytical interferences, such as protein interference in Jaffe creatinine assays.
A single-value correction factor is often used to correct for nonspecific protein interference in Jaffe assays, but this approach does not take into account differences in plasma albumin concentrations among samples. Because of the current interest in low creatinine results for calculation of eGFR, we evaluated the validity and clinical impact of using a single compensation value.
We determined the dependence of creatinine results on albumin concentration by adding increasing amounts of human albumin powder to plasma specimens containing low albumin and creatinine concentrations of 50 and 340 µmol/L, respectively. We then measured, in triplicate, creatinine and albumin (by the Roche bromcresol purple method) on a Modular Analytics system that uses a constant –26 µmol/L adjustment of creatinine results. As expected, the reported creatinine results decreased with decreasing albumin concentrations.
We assumed that when protein concentration is within the adult reference interval, the Roche factor of –26 µmol/L provides reliable compensation. Hence, the "true" creatinine results of these samples were estimated at an albumin concentration of 40 g/L. The differences between these true creatinine results and the measured reportable results are attributable to miscompensation at the respective albumin concentrations that differed from 40 g/L. More accurate compensation values specific for each albumin concentration were then calculated and used to derive the following protein compensation formula by regression analysis:
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It is noteworthy that at 0 g albumin/L the intercept is only 4.075 µmol/L, which represents interference from nonalbumin protein. This result is consistent with albumin per se being the main component of nonspecific protein interference in the Jaffe assay.
To evaluate the clinical impact of miscompensation on creatinine results, the laboratory information system database for our health region was searched for a 6-month period (January 1 to June 30, 2006) to obtain 49 824 records of specimens (from 17 347 patients) for which both creatinine and albumin were measured. Test results and patient data were downloaded into Microsoft Excel and the above equation applied to compensate creatinine results for measured albumin concentrations (after adding back the Roche adjustment of 26 µmol/L). The difference between the 2 creatinine results (i.e., single-value compensated vs formula compensated) represents the analytical error introduced by not adjusting the protein compensation factor for variations in albumin concentration.
The dataset was segregated into categories based on patient age and sex, and the reported creatinine results were compared with the upper reference decision limits (URDL) specific for each category used in our laboratory (Table 1
). Of the 49 824 creatinine results reported, 15 686 (or 31.5%) exceeded their respective URDLs. The range of analytical error attributable to miscompensation was greatest in the pediatric age categories 2 (0–1 month) and 3 (1 month to 5 years) (e.g., errors of –29% to +24% in group 2) for which the URDLs are lowest, and albumin concentrations are usually lower than adult values. This magnitude of error invalidates use of the single-value protein compensation method for these age groups. In patient categories 1 and 4, in which URDLs were higher, the range of error observed was still unacceptably high, as indicated by the large proportions of error >3.4% (namely 97% and 38%). In the adult categories 5, 6, 7, and 8 the proportion of error >3.4% were lowest, but the range of error encountered was still too high for ensuring reliable creatinine results for clinical practice.
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The data presented here indicate unacceptable inaccuracy in creatinine results due to variations in albumin concentrations. Hence, the single-value compensated creatinine Jaffe assay cannot achieve the current desired performance goal of 3.4% bias as recommended by the Laboratory Working Group of the National Kidney Disease Program (1), nor the recommended optimum goal of 1.7%. Awareness of this limitation may be helpful in setting clinical guidelines for result interpretation and in making future choices on methodology.
Acknowledgments
Grant/funding Support: None declared.
Financial Disclosures: None declared.
References
The following articles in journals at HighWire Press have cited this article:
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  J. H. Ellis and R. H. Cohan Reducing the Risk of Contrast-Induced Nephropathy: A Perspective on the Controversies Am. J. Roentgenol., June 1, 2009; 192(6): 1544 - 1549. [Abstract] [Full Text] [PDF]
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 C. M. Cobbaert, H. Baadenhuijsen, and C. W. Weykamp Prime Time for Enzymatic Creatinine Methods in Pediatrics Clin. Chem., March 1, 2009; 55(3): 549 - 558. [Abstract] [Full Text] [PDF]
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