Interpreting the estimated glomerular filtration rate in primary care: Benefits and pitfalls
ABSTRACTAs several equations have been developed for estimating the glomerular filtration rate (GFR), many laboratories are now reporting the GFR automatically, and primary care providers are left trying to interpret the results and put them into the context of patient care. Therefore, it is important that health care professionals understand how to interpret the estimated GFR value and how to recognize when the estimate may not be accurate.
KEY POINTS
- Chronic kidney disease must be detected in its early stages so that measures can be taken to detect its complications and to delay its progression to kidney failure.
- The creatinine concentration is an imperfect marker of renal function and should not be used by itself in assessing renal function.
- Formulas for estimating the GFR from the serum creatinine level along with other easily obtained variables continue to be refined.
- Primary care physicians and nephrologists need to collaborate to provide the optimal care for patients with chronic kidney disease.
Concerns about the definition
This definition has not been without controversy.
An unintended consequence of not reporting estimated GFR values above 60 mL/min/1.73 m2 in absolute numbers is that providers may ignore changes in serum creatinine at estimated GFRs in this range, as they assume that the kidney function is “normal.” This may change in the future if the CKD-EPI equation is used, which produces less bias at slightly higher GFRs.
Providers may also tend to focus solely on the estimated GFR criterion and ignore other evidence of chronic kidney disease, such as abnormalities in urinalysis or imaging studies. For example, proteinuria has been shown to be more important than absolute GFR values in predicting progression of renal dysfunction and cardiovascular risk.14 Proteinuria, especially in the setting of an estimated GFR above 60 mL/min/1.73 m2, can be missed if not screened for and underappreciated once found.
Moreover, in elderly patients, the current GFR equations underperform at borderline GFR values and can yield depressed values even at impressively “normal” serum creatinine levels. As a result, there is concern that chronic kidney disease is being overdiagnosed under the current system. This is especially worrisome in elderly white women without risk factors for chronic kidney disease (eg, as in case 1).
In addition, the question arises whether the arbitrary cutoff for chronic kidney disease— 60 mL/min/1.73 m2—applies to all populations.15,16 The utility of classifying someone as having chronic kidney disease who has an estimated GFR of 55 mL/min/1.73 m2 and no risk factors for chronic kidney disease (Table 2) should be questioned if the risk of progressing to end-stage renal disease or suffering a cardiovascular event is only minimally higher than in patients with a higher estimated GFR.6,17 If the true purpose of developing the chronic kidney disease classification system is to improve patient care and outcomes, then it is of no benefit to overclassify such patients. Indeed, the stress induced by the diagnosis and the negative implications on insurance coverage and health care costs may outweigh any benefits.18
Nevertheless, these concerns do not invalidate the entire chronic kidney disease definition system, but have stimulated current efforts to improve it based on outcomes research.19
CASES REVISITED
Case 1: Problems with estimating GFR in a small woman
Case 1 has several points to note.
The patient’s small body size reflects low-level creatinine production. It is not atypical to find serum creatinine levels of 0.5 mg/dL in such patients. Thus, her serum creatinine level of 1.1 mg/dL may be abnormal. The fact that the MDRD equation “normalizes” the result to 1.73 m2 of body surface area in patients with very low muscle mass will lead to an overestimation of GFR. However, she has no risk factors for chronic kidney disease.
Additionally, in up to two-thirds of patients kidney function declines with age.20 Whether or not this is “normal aging” of the kidney, it is not clear that this decline in GFR reflects an underlying pathologic process.
Finally, since the patient is an older white woman, the estimated GFR tends to underestimate the true GFR. So while her body size may predispose to an overestimation of GFR, her age, race, and sex predispose to an underestimation of GFR. Many nephrologists would simply order urinalysis and ultrasonography to rule out other evidence of renal dysfunction, then recommend routine monitoring of kidney function in this case.
Case 2: Proteinuria is not normal
In case 2, because the patient is African American, young, and male, his creatinine level yields a higher estimated GFR than in case 1, despite having the same value. However, his estimated GFR still underestimates his true GFR because of his greater creatinine production due to his muscular physique.
This patient subsequently underwent iothalamate GFR testing, which yielded a GFR of 115 mL/min/1.73 m2. However, he has dipstick-positive proteinuria, which, if confirmed on further testing, would meet the criteria for chronic kidney disease and put him at a higher risk of cardiovascular events and progression to lower kidney function than the patient in case 1. He also needs to be screened for undiagnosed hypertension and underlying glomerular disease.
REFERRAL TO (AND COLLABORATION WITH) A NEPHROLOGIST
Effective co-management with a nephrologist is essential for the overall health of the patient with chronic kidney disease, as well as slowing the progression to end-stage renal disease. Exactly when and to what extent the care of a patient with chronic kidney disease should be transferred to a nephrologist depends largely on the individual nephrologist and the comfort level of the primary care provider. When the referral does occur, effective communication between providers and a mutual understanding of the goals of care (eg, the blood pressure target) are essential to optimize patient care.
