How Do I Assess Renal Function for My Nursing Home Residents?

How Do I Assess Renal Function for My Nursing Home Residents?

By Barbara J. Zarowitz, PharmD, MSW, BCGP, BCPS, FASCP, FPALTC

Assessment of renal function in older adults is important to identify and stage chronic kidney disease (CKD) and adjust doses of over 50% of medications (that undergo significant renal elimination), approved for use in the United States. CKD affects about 38% of adults aged 70 years and older, and the prevalence increases with age.¹

Looking solely at elevations in serum creatinine as a marker of kidney disease is inadequate because creatinine is a breakdown product of muscle metabolism. Serum creatinine is affected by age, sex, muscle mass, physical activity, protein consumption, and nutritional status,² resulting in spuriously low serum creatinine concentrations and underreporting of CKD in many older adults.  

Clinicians may be confronted with differing estimates of renal function. Today, the majority of laboratories report an estimated glomerular filtration rate (eGFR) in mL/min/1.73 m2 calculated by a race-free equation developed by the Chronic Kidney Disease Epidemiology Collaborative called CKD-EPI.³  Reported eGFR estimates are typically based on serum creatinine and not both serum creatinine and cystatin C as recommended by the National Kidney Foundation because cystatin C assays are not available in most commercial laboratories and Medicare covers cystatin  C under a very restricted basis, thereby preventing a possibly more accurate estimate of eGFR in older adults.^4,5

Additionally, consultant pharmacists typically calculate creatinine clearance using the Cockcroft-Gault (C-G) formula, developed in the 1970s in a small cadre of men.⁶ Between 2009 and 2024, renal dosage optimization for U.S. Food and Drug Administration (FDA)-approved medications was based on estimated creatinine clearance (eCrCl) using the C-G equation, some variant of weight (actual, ideal, or adjusted), and serum creatinine.⁷ The discordance rate (the disagreement) between the recommended dose based on C-G eCrCl (and manufacturer’s label) and eGFR, for drug dosing, can be as high as 32%.⁸.

Gabapentin Example

Let’s take an example of an 82-year-old female resident with serum creatinine of 0.8 mg/dL, weight 54 kg (118.8 lb.), height 160 cm (5’3”), and body surface area of 1.6 m2 in whom you are prescribing an initial dose of gabapentin. If her renal function is consistent with a creatinine clearance of ≥ 60 mL/min, the manufacturer’s recommended starting dose is 900 to 3600 mg/day in divided doses. For creatinine clearance of between 30 and 59 mL/min, the recommended starting dose of gabapentin is 400 to 1400 mg/day in divided doses.⁹ The table shows renal function estimated by eGFR and eCrCl for this resident and the resulting recommended daily starting dose.

Perhaps most clinicians would err on the side of caution and prescribe the lower dose in an 82- year-old woman, but use of eGFR could lead to prescription of a gabapentin dose more than double that calculated by eCrCl and more likely to be associated with adverse effects such as a decline in cognitive and functional status, and falls.¹⁰ 

In 2024, a National Kidney Foundation Workgroup released a consensus statement recommending moving away from the C-G equation for eCrCl and toward the race-free eGFR-based equation (CKD-EPI) standardized for body surface area for medication dosing and medical decision-making in adults.4 Also, in 2024, the FDA changed its guidance to the pharmaceutical industry to favor eGFR over eCrCl for renal dose adjustments during drug development.¹¹ The CKD-EPI equation was developed with an insufficient number of older adults, and to date, no studies have evaluated the accuracy and precision of it in older adults representative of the U.S. population or nursing home residents.¹²  Studies in Europe tend to reflect more homogeneous populations of mostly white, non-obese older adults that may not be generalizable to the United States.

So, how should clinicians estimate renal function, particularly in adults aged 85 years and older and those in nursing homes? While we wait for definitive evidence, here are some suggested considerations.

• Investigate how your laboratory estimates eGFR. Do they use CKD-EPI?  Do they use the patient’s actual BSA, or is eGFR still indexed to 1.73 m2? It is better to use the patient’s actual BSA to index eGFR.
• Where coverage is available, consider ordering cystatin C and calculating eGFR using both serum creatinine and cystatin C. Currently, the Centers for Medicare & Medicaid Services is reimbursing claims for cystatin C if medical record documentation supports that the cystatin C test was performed on an adult patient with creatinine-based eGFR 45–59 ml/min/1.73 m2 who does not have markers of kidney damage.  Note that eGFR calculators with and without cystatin C are available online at https://www.kidney.org/professionals/gfr_calculator.

• Consider calculating eCrCl using the C-G equation for older adults with eGFR < 60 mL/min/1.73 m², where:  CrCl (mL/min) = 140- age x weight x 0.85 (if female).

                                          72 x SCr 

• If there is disagreement between the resulting estimates, assess the relative efficacy/toxicity profile of the medication. Sometimes it may make more sense to round the dose down for medications with greater potential toxicity (e.g., chemotherapy, gabapentin) and round the dose up (e.g., beta-lactam antibiotics) where the risk of treatment failure with undertreatment outweighs the chance of creating drug toxicity with a higher dose.
• In older adults whose renal function is considered normal (≥ 60 mL/min), eGFR may be the best estimating method for drug dosing. However, older adults with CKD have not been studied adequately to recommend one estimating method over another.
• When the discrepancy between estimating methods is large, consider identifying an alternate medication that does not rely as significantly on renal clearance. 

Heterogeneity due to increasing age, diversity in body mass index, and declining renal function preclude recommending one estimating method over another. The blanket recommendation to estimate renal function in adults using eGFR does not account for the variances introduced in older adults. Caution is recommended when estimating renal function until studies in older adults provide evidence to support the selection of a more accurate and precise method.

Find the references for this article here.

Barbara J. Zarowitz is editor-in-chief of Caring for the Ages and senior advisor for the Peter Lamy Center on Drug Therapy and Aging, University of Maryland School of Pharmacy.