The estimated glomerular filtration rate (eGFR) is a crucial measure for assessing kidney function, but there are certain conditions and factors where it may be unreliable:
- Extremes of Body Size: eGFR calculations are based on average body sizes and may be less accurate in individuals who are very muscular, have amputations, are extremely overweight or underweight, or have unusually short or tall stature.
- Changing Kidney Function: eGFR is less accurate when kidney function is changing rapidly, such as with acute kidney injury or in the setting of certain medications that affect kidney function.
- Dietary Influences: A diet very high in meat can increase serum creatinine and may lead to an overestimation of the severity of renal impairment. Conversely, vegetarian diets can lead to lower serum creatinine levels.
- Certain Populations: eGFR equations may be less accurate in certain racial and ethnic groups, including individuals of African descent, where a correction factor is often used. They may also be less accurate in the elderly, children, and pregnant women.
- Substances that Interfere with Creatinine Testing: Some substances, such as certain drugs (trimethoprim, fenofibrate) or supplements (creatine), can interfere with the assay for serum creatinine, leading to inaccurate eGFR readings.
- Advanced Liver Disease: Liver disease can lower creatinine production, which may lead to an overestimation of kidney function.
- Certain Chronic Illnesses: Conditions like severe heart failure or other illnesses that affect the blood flow to the kidneys can cause fluctuations in creatinine clearance.
When eGFR may be unreliable, additional tests, such as a 24-hour urine collection for creatinine clearance or direct measures of glomerular filtration (like the inulin clearance test), may be considered to more accurately assess kidney function. It is also important to look at trends in eGFR over time, rather than relying on a single value.