Estimated Glomerular Filtration Rate Reporting From Whole Blood Creatinine: A Proficiency Testing Survey

To the Editor.—Assessment of impaired renal function in adults is commonly determined by calculation of the estimated glomerular filtration rate (eGFR) based on creatinine, and historically has included a race adjustment factor.1,2 To remove bias, an eGFR equation based on creatinine that eliminated the race adjustment has become available as the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) 2021 creatinine equation.3 Guidance calling for adoption and implementation of this updated, race-free equation has been publicized4,5 and we have previously shown rapid adoption of the CKD-EPI 2021 equation by clinical laboratories.6,7However, not all assessments of eGFR are calculated with creatinine results using assays performed on high-throughput clinical laboratory instruments. As the presence of impaired renal function may have implications for procedures or interventions (eg, radiocontrast), point-of-care testing (POCT) for creatinine may have advantages. The College of American Pathologists (CAP) Whole Blood Creatinine proficiency testing (PT) program is intended for a specific POCT device (Nova StatSensor). This PT program was used as a surrogate for general POCT practices owing to feasibility of survey administration and the POC device’s ability to calculate eGFR.In November 2023 the CAP included a survey in the Whole Blood Creatinine PT program to assess participating laboratories’ practices for calculating and reporting eGFR in adults. Surveys with at least 1 response were returned from 182 of 236 participating laboratories (77.1% response rate), of which 173 (95.1%) were from US locations. Of the 182 survey responses, 135 respondents (74.2%) reported providing a clinically reported eGFR result for adult patients along with the whole blood creatinine measured by the StatSensor. Academic (75 of 94; 79.8%) and nonacademic (55 of 74; 74.3%) hospitals/medical centers reported eGFR at higher rates than nonhospital sites (5 of 14; 35.7% (logistic regression P = .005 and P = .02, respectively, with Bonferroni correction) (Table).For those reporting an eGFR, the calculation methods used by 128 respondents were CKD-EPI 2021 (69; 53.9%), MDRD (Modification of Diet in Renal Disease) (54; 42.2%), and CKD-EPI 2009 (5; 3.9%). The prevalence of CKD-EPI 2021 reporting from POCT in domestic laboratories (69 of 122; 56.6%) was lower than the rate reported in a March 2023 survey of laboratories performing testing on higher-volume chemistry instruments (2332 of 3542; 65.8%; χ2 test, P = .03).7In this November 2023 POCT survey, the calculation of eGFR based on the creatinine measurement was performed via the Nova StatSensor (100; 74.1%), laboratory information system (25; 18.5%), electronic health record (9; 6.7%), or middleware (1; 0.7%), according to 135 responses.Taken together, the high portion of respondents reporting the use of CKD-EPI 2021 was somewhat surprising as, at the time of the survey, the outward-facing information from the product website did not list the CKD-EPI 2021 equation as a reporting option for calculation on the Nova StatSensor itself. However, subsequent communications with the manufacturer indicated the software had been updated to include the CKD-EPI 2021 as an option, which is concordant with our survey responses.Even so, 16.4% (21 of 128 respondents) indicated that the eGFR reported based on the StatSensor creatinine measurement was not the same eGFR calculation based on creatinine used for the rest of the institution (ie, main laboratory eGFR method). The lack of harmonization of eGFR calculation and reporting within an institution has the potential for diagnostic and treatment differences and may be an area of practice optimization across medical groups.Overall, the survey results were reassuring and suggest substantial adoption of the updated CKD-EPI 2021 eGFR equation in a near-patient setting, echoing the rates seen for general laboratories.7 One limitation of our survey was not directly determining which eGFR equation was used by the main laboratory/medical institution. Nevertheless, it is likely that consistency of health system–wide eGFR calculations (as with any calculation) is an important consideration for many laboratories in harmonizing practice, regardless of where or on what instrument a particular measurement is made.The authors would like to thank Hilda Murphy, MS, for assistance in the preparation and administration of survey questions.