Evaluation of methods to estimate glomerular filtration rate versus actual drug clearance in patients with chronic spinal cord injury.

STUDY

DESIGN

A retrospective chart review.

OBJECTIVES

To evaluate different methods of estimating renal function compared with patient-specific vancomycin and aminoglycoside (AG) clearance (CL(DRUG)) in patients with spinal cord injury (SCI), and to develop a new equation to more accurately estimate glomerular filtration rate (GFR) in SCI patients in order to optimize dosing for vancomycin and AG.

SETTING

Veterans Affairs medical center in California, United States of America, tertiary care facility with the largest inpatient SCI center in the VA system.

METHODS

Retrospective data collection from patient records. Pharmacokinetic analysis was performed to obtain actual CL(DRUG,) which is compared with different methods of estimating GFR.A total of 310 patients were initially assessed; however, only 141 patients met the inclusion criteria, had a diagnosis of chronic SCI, and received vancomycin or AG with at least one drug level at steady state from January to December of 2008.

RESULTS

All four equations evaluated to estimate GFR significantly overestimated CL(DRUG): the Modification of Diet in Renal Disease equation by 141%, Cockcroft-Gault equation by 83%, Chronic Kidney Disease Epidemiology Collaboration equation by 82% and 24-h endogenous creatinine clearance by 71% (P<0.001). The modified Cockcroft-Gault equation (CL(M)) showed improvement, however, still overestimated CL(DRUG) by 39% (P<0.001). Thus, a new equation for SCI (CL(SCI)) was developed which underestimated CL(DRUG) by <5% (P=0.16).

CONCLUSION

Compared with different methods of estimating GFR, CL(SCI)=2.3 × x (0.7) (x equals CL(M) in ml min(-1)) more accurately estimates CL(DRUG) in chronic SCI patients.