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The Best Way to Test Kidney Function

The kidneys are responsible for removing excess fluid and wastes from the body in the form of urine. When they fail to perform this function, fluid retention and the build-up of waste products can lead to dangerous changes within the body that in some cases become life-threatening. A number of tests are available to test kidney function. The best of these is a procedure that calculates the kidneys' glomerular filtration rate, or GFR.

  1. The Basics

    • GFR is a measure of your kidneys' efficiency in removing wastes from your blood. According to the National Kidney Foundation (NKF), it is the single best indicator of potential malfunctions and kidney disease. A standard GFR requires the injection of a testing substance into the bloodstream, followed by a 24-hour period in which the presence of the substance is measured in the urine. While this method of testing is highly accurate, it is also relatively difficult to perform, and requires the use of skilled personnel. However, a modern technique---called an eGFR---obtains similar results through a simple blood test. While an eGFR is somewhat less accurate than a traditional GFR, it is still considered sufficient for proper diagnosis.

    Testing Procedures and Results

    • In an eGFR, your filtration rate is not measured directly. Instead, a blood sample is used to measure a substance called serum creatinine. Creatinine is a natural byproduct of muscle function in your body, and during normal activity it is gathered in your bloodstream and excreted in your urine. However, if your kidneys are not functioning properly, creatinine will build up in your body, where it is easily detected. Different laboratories calculate creatinine levels in slightly different ways. A normal level of creatinine is roughly 0.6mg to 1.2mg per deciliter (mg/dL) of blood. In many labs, a creatinine level of 1.7mg/dL in men or 1.4mg/dL in women indicates a drop in kidney function of roughly 50 percent.

      Your doctor will determine the significance of your creatinine level and how it applies to further treatment. Part of this determination will include factors based on your age, sex and racial background. Taking all factors into account, your doctor will calculate your GFR. If your GFR score is less than 30, you will likely be sent to a kidney specialist for further examination. If your GFR is less than 15, you will almost certainly be treated for kidney failure. Common treatment options for this condition include dialysis (artificial cleansing of your blood) and kidney transplant.

      You may have your serum creatinine measured apart from an eGFR test. If you would like to use the results of your creatinine test to measure eGFR, your doctor can use a special calculator provided on the NKF website. Consult your doctor for a full explanation of this procedure.

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