Creatinine is a waste product that comes from the digestion of protein in your food and the normal breakdown of muscle tissue. It is removed from your body through the kidneys. A “normal” creatinine level in the blood is hard to define because it can change depending on your age, sex, body size, and other factors. For this test, a lower number is better. High creatinine levels can be a sign of acute kidney injury and/or chronic kidney disease.
Cystatin C is a protein that is produced by the cells in your body. Like creatinine, it is also removed from the body through the kidneys. So, it is another option to use to calculate your eGFR, if needed. This test is not as common as the creatinine test and can be more expensive. For this test, a lower number is better.
Estimated glomerular filtration rate (eGFR)
The estimated glomerular filtration rate (eGFR) is an estimate of how well your kidneys are removing waste products from the blood. It is calculated using your serum creatinine level, age, and sex. It can also be calculated using your cystatin C level. A “normal” eGFR varies according to age – it decreases as you get older. For this test, a higher number is better. In general, an eGFR value lower than 60 is a sign that the kidneys may not be working properly. An eGFR lower than 15 is a marker of kidney failure.
In less common situations where a more accurate measure of your kidney function is needed, your healthcare provider may order a measured GFR (mGFR).
Blood urea nitrogen (BUN)
Urea nitrogen is a waste product in your blood that comes from the breakdown of protein in the foods you eat. It is removed from the body through the kidneys. A “normal” BUN level varies, and usually increases as you get older. Checking your BUN level is usually not very helpful by itself. So, your healthcare provider will likely compare your BUN level to your creatinine and eGFR levels when evaluating your kidney health.